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14: Module 11: Homeostasis - Biology

14: Module 11: Homeostasis - Biology


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14: Module 11: Homeostasis

14: Module 11: Homeostasis - Biology

By the end of this section, you will be able to:

  • Define homeostasis
  • Describe the factors affecting homeostasis
  • Discuss positive and negative feedback mechanisms used in homeostasis
  • Describe thermoregulation of endothermic and ectothermic animals

Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.


A physiologist's view of homeostasis

Homeostasis is a core concept necessary for understanding the many regulatory mechanisms in physiology. Claude Bernard originally proposed the concept of the constancy of the “milieu interieur,” but his discussion was rather abstract. Walter Cannon introduced the term “homeostasis” and expanded Bernard's notion of 𠇌onstancy” of the internal environment in an explicit and concrete way. In the 1960s, homeostatic regulatory mechanisms in physiology began to be described as discrete processes following the application of engineering control system analysis to physiological systems. Unfortunately, many undergraduate texts continue to highlight abstract aspects of the concept rather than emphasizing a general model that can be specifically and comprehensively applied to all homeostatic mechanisms. As a result, students and instructors alike often fail to develop a clear, concise model with which to think about such systems. In this article, we present a standard model for homeostatic mechanisms to be used at the undergraduate level. We discuss common sources of confusion (“sticky points”) that arise from inconsistencies in vocabulary and illustrations found in popular undergraduate texts. Finally, we propose a simplified model and vocabulary set for helping undergraduate students build effective mental models of homeostatic regulation in physiological systems.

in 2007, a group of 21 biologists from a wide range of disciplines agreed that “homeostasis” was one of eight core concepts in biology (14). Two years later, the American Association of Medical Colleges and Howard Hughes Medical Institute in its report (1) on the scientific foundations for future physicians similarly identified the ability to apply knowledge about “homeostasis” as one of the core competencies (competency M1).

From our perspective as physiologists, it is clear that homeostasis is a core concept of our discipline. When we asked physiology instructors from a broad range of educational institutions what they thought the 𠇋ig ideas” (concepts) of physiology were, we found that they too identified “homeostasis” and �ll membranes” as the two most important big ideas in physiology (15). In a subsequent survey (16), physiology instructors ranked homeostasis as one of the core concepts critical to understanding physiology.

If, as these surveys indicate, the concept of homeostasis is central to understanding physiological mechanisms, one would expect that instructors and textbooks would present a consistent model of the concept. However, an examination of 11 commonly used undergraduate physiology and biology textbooks revealed that this is not necessarily the case (17). Explanations of the concept of homeostasis and subsequent references to the concept suffer from a number of shortcomings. Although these texts define some terms related to homeostatic regulatory systems, many authors do not use these terms consistently. Moreover, they do not always use consistent visual representations of the concept. In addition, the explanation of the concept often conflicts with the current understanding of homeostatic regulatory mechanisms. These limitations of textbooks most likely carry over to classroom instruction, thereby weakening the power of the concept as a unifying idea for understanding physiology.

The goals of this article are to develop a correct description and visual representation of a general homeostatic mechanism that can serve as a learning tool for faculty members and students. We will limit our discussion to homeostatic mechanisms found in organismal systems that maintain a constant extracellular compartment and will not consider other types of homeostasis. Although this tool can be useful at any academic level, our primary focus is its application at the undergraduate level when students are first introduced to the concept. We will also briefly discuss the history of the concept and then address the “sticky points” that may lead to confusion for faculty members and students alike when attempting to apply the concept to mammalian, organismal physiology. We conclude with suggestions for improving instruction on homeostasis and its applications.

History of the Concept of Homeostasis

Claude Bernard asserted that complex organisms are able to maintain their internal environment [extracellular fluid (ECF)] fairly constant in the face of challenges from the external world (8). He went on to say that 𠇊 free and independent existence is possible only because of the stability of the internal milieu” (3). Walter Cannon coined the term “homeostasis” with the intent of providing a term that would convey the general idea proposed some 50 yr earlier by Bernard (8). Cannon's view focused on maintaining a steady state within an organism regardless of whether the mechanisms involved were passive (e.g., water movement between capillaries and the interstitium reflecting a balance between hydrostatic and osmotic forces) or active (e.g., storage and release of intracellular glucose) (6). While we recognize the validity of both passive and active mechanisms of homeostasis, our consideration will focus exclusively on the active regulatory processes involved in maintaining homeostasis.

Early physiology textbooks reflected this broad definition by briefly mentioning Bernard's concept of the constancy of the internal milieu, but the term “homeostasis” was not used in discussions of specific regulatory mechanisms (9, 11, 4).

This situation began to change in the mid-1960s, when a branch of biomedical engineering emerged that focused on applying engineering control systems analysis to physiological systems (18, 19, 2, 20). Arthur Guyton was the first major physiology textbook author to include a control systems theory approach in his textbook, and his book included detailed attention to the body's many regulatory mechanisms (10). Hence, Guyton introduced many students to the concept of homeostasis as an active regulatory mechanism that tended to minimize disturbances to the internal environment.

Engineering control systems theory describes a variety of other mechanisms to maintain the stability of a system. Although many of these mechanisms may be found in biological systems (7), not all of them are components of homeostatic mechanisms. For instance, the ballistic system used by the nervous system for throwing a ball simply calculates in advance the pattern of commands needed to achieve some particular outcome based on previous experience (7). Here, there is no element involved that regulates the internal environment.

Homeostatic mechanisms originated to keep a regulated variable in the internal environment within a range of values compatible with life and, as has been more recently suggested, to reduce noise during information transfer in physiological systems (22). To emphasize the stabilizing process, we distinguish between a “regulated (sensed) variable” and a “nonregulated (controlled) variable” (5, 23). A regulated (sensed) variable is one for which a sensor exists within the system and that is kept within a limited range by physiological mechanisms (5). For example, blood pressure and body temperature are sensed variables. Baroreceptors and thermoreceptors exist within the system and provide the value of the pressure or temperature to the regulatory mechanism. We call variables that can be changed by the system, but for which sensors do not exist within the system, nonregulated (controlled) variables. Nonregulated variables are manipulated or modulated to achieve regulation of the variable being held constant. For example, heart rate can be changed by the autonomic nervous system to regulate blood pressure, but there are no sensors in the system that measure heart rate directly. Hence, heart rate is a nonregulated variable.

A simple model illustrating the fundamental engineering control system concepts relevant to homeostatic regulatory mechanisms is shown in Fig. 1 .

Diagram of a generic homeostatic regulatory system. If the value of the regulated variable is disturbed, this system functions to restore it toward its set point value and, hence, is also referred to as a negative feedback system.

This model, some version of which appears in many current physiology texts, includes the following five critical components that a regulatory system must contain to maintain homeostasis:

1. It must contain a sensor that measures the value of the regulated variable.

2. It must contain a mechanism for establishing the “normal range” of values for the regulated variable. In the model shown in Fig. 1 , this mechanism is represented by the “set point,” although this term is not meant to imply that this normal range is actually a “point” or that it has a fixed value. In the next section, we further discuss the notion of a set point.

3. It must contain an 𠇎rror detector” that compares the signal being transmitted by the sensor (representing the actual value of the regulated variable) with the set point. The result of this comparison is an error signal that is interpreted by the controller.

4. The controller interprets the error signal and determines the value of the outputs of the effectors.

5. The effectors are those elements that determine the value of the regulated variable.

Such a system operates in way that causes any change to the regulated variable, a disturbance, to be countered by a change in the effector output to restore the regulated variable toward its set point value. Systems that behave in this way are said to be negative feedback systems.

While the model shown in Fig. 1 is a relatively simple one, there is a great deal of information that can be packed into each of the boxes that constitute the model. Homeostasis can also be described as a hierarchically arranged set of statements, a conceptual framework, that contains whatever breath and depth of information is appropriate for a particular set of students in a course. We have developed and described such an “unpacking” of the core concept of homeostasis (12, 13). The model and the conceptual framework provide students with different tools for thinking about homeostasis.

Topics That Cause Confusion for Students and Instructors: Sticky Points

A sticky point is any conceptual difficulty that makes one's mental model of any phenomenon inaccurate and, hence, less useful. There are a number of factors that contribute to the generation of sticky points for both instructors and students:

The phenomenon in question is a complex one.

There are aspects of the phenomenon that are counterintuitive.

The language or terminology used to describe the phenomenon or concept is inconsistent.

The discipline's understanding of the phenomenon is uncertain or incomplete.

In this section, we will describe some sticky points about homeostatic regulatory mechanisms that we have uncovered as we have interacted with instructors and students about their understanding of homeostasis. We will address these sticky points in the form of a series of questions and answers.

What environment is regulated by organismal homeostasis?

Organismal homeostasis, as originally defined by Cannon (6), refers to physiological mechanisms that maintain relatively constant the variables related to the internal milieu of the organism. This includes variables related to the entire ECF compartment or to its subcompartments (e.g., the plasma). We will not be discussing intracellular homeostatic mechanisms.

Are all negative feedback systems homeostatic?

Although negative feedback is an essential element of homeostatic regulatory mechanisms, the presence of negative feedback in a system does not mean that the system is homeostatic in function. Negative feedback exists in many systems that do not involve homeostatic regulation. For example, negative feedback plays a role in the muscle stretch reflex, but this reflex is not involved with maintaining the constancy of the internal environment. In other cases, the presence of negative feedback may minimize oscillation of a variable, even though that variable itself is not maintained relatively constant (i.e., it is not a regulated variable). Control of the blood levels of cortisol is an example of the oscillating damping effects of negative feedback (see further discussion below).

Can other types of control mechanisms (e.g., feedforward) maintain homeostasis?

Feedforward or anticipatory control mechanisms permit the body to predict a change in the physiology of the organism and initiate a response that can reduce the movement of a regulated variable out of its normal range (7, 23). Thus, feedforward mechanisms may help minimize the effects of a perturbation and can help maintain homeostasis. For example, anticipatory increases in breathing frequency will reduce the time course of the response to exercise-induced hypoxia. Because of this, attempts have been made to broaden the definition of homeostasis to include a range of anticipatory mechanisms (23).

However, we have decided to limit our generic model of a homeostatic regulatory system ( Fig. 1 ) to one that illustrates negative feedback and demonstrates the minimization of an error signal. We have done this because our model is intended to help faculty members teach and students learn the core concept of homeostasis in introductory physiology (12, 13). There are additional complex features found in feedback systems that are not included here because our intention is to first help students make sense of the foundational concept of homeostatic regulation. As situations are encountered where this basic model is no longer adequate to predict system behavior (7, 23), additional elements like feedforward mechanisms can be added to the model.

What is a set point?

Understanding the concept of a set point is central to understanding the function of a homeostatic mechanism. The set point in an engineering control system is easily defined and understood it is the value of the regulated variable that the designer or operator of the system wants as the output of the system. The cruise control mechanism in an automobile is an example of a system with an easy to understand set point. The driver determines the desired speed for the car (the set point). The regulatory mechanism uses available effectors (the throttle actuators) and a negative feedback system to hold the speed constant in the face of changes in terrain and wind conditions. In such a system, we can envision an electronic circuit located in the engine control module that compares the actual ground speed with the set speed programmed by the driver and uses the error signal to control the throttle actuator appropriately.

In physiological systems, the set point is conceptually similar. However, one source of difficulty is that, in most cases, we do not know the molecular or cellular mechanisms that generate a signal of a particular magnitude. What is clear is that certain physiological systems behave as though there is a set point signal that is used to regulate a physiological variable (23).

Another challenge to our understanding of set points arises from the fact that set points are clearly changeable, either physiologically or as the result of a pathological change in the system (23). The mechanisms that cause variations in a set point can operate temporarily, permanently, or cyclically. Physiologically, this can occur as a result of discrete physiological phenomena (e.g., fever), the operation of hierarchical homeostats (e.g., regulation of ECF P co 2) (see Ref. 7), or through the influence of biological clocks (e.g., circadian or diurnal rhythms of body temperature). The observation that set points can be changed adds complexity to our understanding of homeostatic regulation and can lead to confusion about whether the measured change in a regulated variable results from a change in the physiological stimulus or from a changing set point (23). In these cases, it is important to make such distinctions between a change in the stimulus and the modulation of the set point to arrive at an accurate picture of how a particular homeostatically regulated system operates.

Do homeostatic mechanisms operate like an on/off switch?

Control signals are ALWAYS present, and they continuously determine the output of the effectors. Changes in the control signals alter effector outputs and therefore change the regulated variable. The amplitude of these control signals vary when there is an error signal (i.e., when the regulated variable is not the same as the set point). Thus, homeostatic regulation is a constant, continuous process and does not ordinarily operate as an on/off switch that results in an all-or-none response.

What is the difference between an effector and a physiological response?

Textbook diagrams and narratives can blur the distinction between the effector and a response generated by the effector, making it difficult for students to build a correct mental model. This problem can occur if, when a visual representation of a homeostatic mechanism is presented (see Fig. 1 ), a physiological response is placed in the same 𠇌oncept” box as the effector. For example, “increased secretion by sweat glands” and “vasodilation of blood vessels in the skin” might be identified as effectors in the control system for thermoregulation. However, only “sweat glands” and 𠇋lood vessels” are effectors, whereas “increased secretion” and “vasodilation” are the responses of the effectors. Comprehensive understanding of homeostatic mechanisms requires that we, and students, make clear distinctions between effectors and responses. The term �tor” should only be applied to a physical entity such as a cell, tissue, or organ, whereas responses such as secretion and vasodilation are actions, not physical entities.

Students may also be confused if only the change in the regulated variable is thought of as being the response of the effector. The change in the regulated variable is typically a consequence of changes in function caused by effectors that determine the value of the regulated variable. By applying the term “response” to only the change in the regulated variable, the intermediary steps between the action of the effector and the change in the regulated variable are not acknowledged explicitly. Under these circumstances, it would reasonable for students to conclude that the intermediary steps are, in some way, aspects of the effector rather than the effect of actions of the effectors. This practice may also reflect a lack of understanding of the difference between the regulated variable, e.g., body temperature, and all of the nonregulated variables that are modified (e.g., arteriole diameter and rate of sweat production) in the steps between the action of the effector and the change in the regulated variable.

What does “relatively constant over time” mean?

In the above sections, we emphasized that homeostatic mechanisms operate to keep a regulated variable in the internal environment “relatively constant.” This is a common phrase used to describe what normally happens to the value of the regulated variable over time. A potential sticky point arises from the use of this phrase. How much change can occur to a regulated variable that is held relatively constant? Three points of clarification need to be made. By saying relatively constant, we mean that:

1. Regulated variables are held within a narrower range of values than if they were not regulated.

2. The regulated value is maintained within a range that is consistent with the viability of the organism.

3. There are differences in the range of values permitted for different regulated variables.

The second point is key to understanding the range over which regulated variables can change homeostatic mechanisms operate to prevent a potentially lethal change in the internal environment. Indeed, as it is often used, relatively constant essentially serves as a surrogate phrase for within the range compatible with an organism's viability. For some regulated variables, the range is quite narrow (e.g., extracellular H + concentration or extracellular osmolarity). For other variables, the range can be broad under some circumstances (e.g., blood glucose concentration during the fed state) and narrow in other situations (e.g., blood glucose during the fasting state). The factors that contribute to the normal range or, in our model, the set point, of a particular variable are undoubtedly complex and, in most cases, have not been elucidated.

What physiological variables are homeostatically regulated?

To identify specific variables that may be homeostatically regulated, the five critical components illustrated in the model shown in Fig. 1 must be present. That is, a regulatory system for that variable must exist that contains the five critical components described in Fig. 1 . Based on this test, we have generated a partial list of the physiological variables that are homeostatically regulated ( Table 1 ). The list of widely recognized and clearly established regulated variables in humans includes a number of inorganic ions (e.g., H + , Ca 2+ , K + , and Na + ), blood-borne nutrients (e.g., glucose), blood pressure, blood volume, blood osmolarity, and core body temperature.

Table 1.

Homeostatically regulated variables typically found in undergraduate human physiology textbooks

Regulated VariableNormal Range or ValueSensor (Location If Known)Control Center (Location)EffectorsEffector Response
Arterial P o 275� mmHgChemosensors (carotid bodies and aortic body)Brain stemDiaphragm and respiratory musclesChange breathing frequency and tidal volume
Arterial P co 234� mmHgChemosensors (carotid bodies, aortic body, and the medulla)Brain stemDiaphragm and respiratory musclesChange breathing frequency and tidal volume
K + concentration3.5𠄵.0 meq/lChemosensors (adrenal cortex)Adrenal cortexKidneysAlter reabsorption/secretion of K +
Ca 2+ concentration4.3𠄵.3 meq/l (ionized)Chemosensors (parathyroid gland)Parathyroid glandBone, kidney, and intestineAlter reabsorption of Ca 2+ , alter resorption/building of bone, and alter absorption of Ca 2+
H + concentration (pH)35� nM (pH 7.35𠄷.45)Chemosensors (carotid bodies, aortic body, and floor of the fourth ventricle)Brain stemDiaphragm and respiratory musclesChange breathing frequency and tidal volume and change secretion/reabsorption of H + /bicarbonate ions
Chemosensors (kidney)KidneyKidney
Blood glucose concentration70� mg/dlFed state: chemosensors (pancreas)PancreasLiver, adipose tissue, and skeletal muscleAlter storage/metabolism/release of glucose and its related compounds
Fasting state: chemosensors (hypothalamus, pancreas)Hypothalamus
Core body temperature98.6ଏThermosensors (hypothalamus, skin)HypothalamusBlood vessels and sweat glands in the skin as well as skeletal musclesChange peripheral resistance, rate of sweat secretion rate, and shivering
Alter heat gains/losses
Mean arterial pressure93 mmHgMechanosensors (carotid sinus and aortic arch)MedullaHeart and blood vesselsAlter heart rate, peripheral resistance, inotropic state of the heart, and venomotor tone
Blood volume (effective circulating volume)5 litersMechanosensorsMedullaHeartAlter heart rate, peripheral resistance, and inotropic state of the heart
(Blood vessels: carotid bodies)HypothalamusBlood vesselsAlter Na + and water reabsorption
(Heart: atria and ventricle)AtriaKidneysAlter water absorption
(Kidney: juxtaglomerular apparatus and renal afferent arterioles)KidneyIntestine
Blood osmolality280� mosM/kgOsmosensors (hypothalamus)HypothalamusKidneysAlter water reabsorption

This table includes commonly found components of control systems involved in physiological regulation (i.e., homeostasis). This is not meant to be an exhaustive list but rather reflect the current understanding of homeostatically regulated variables that undergraduate physiology students should understand and be able to apply to problems (e.g., making predictions about responses to perturbations or explaining symptoms of disease).

A potential sticky point occurs when textbooks identify variables as homeostatically regulated even though the system involved does not have all of the required components. The proposition that certain metabolic waste products (e.g., nitrogenous wastes, bilirubin, and creatinine) are homeostatically regulated illustrates such a failure. We are not suggesting that the levels of these substances are not kept relatively constant by steady-state processes in the body. Rather, the concentrations of these substances are not maintained by a system that meets the definition of a homeostatic mechanism listed above. The body does not possess a physiological sensor for detecting these substances in the ECF and therefore cannot homeostatically regulate the ECF concentration of these substances.

Conversely, some mechanisms for controlling the level of a physiological variable include one component of the model (e.g., negative feedback) and may give the appearance of homeostatic regulation but, in the final analysis, do not meet all criteria and should not be considered homeostatic. For example, textbook diagrams illustrating control of blood cortisol levels show several negative feedback loops. This can cause students to think that cortisol is a regulated variable. However, the sensed variable(s) in this system is(are) the variables (e.g., blood glucose or “stress”) whose values are processed by the higher brain centers or hypothalamus and result in the release of corticotropin-releasing hormone. The result of the negative feedback loops involving adrenocorticotropic hormone and cortisol is a modulation of the release rate of the respective hormones. Therefore, corticotropin-releasing hormone, adrenocorticotropic hormone, and cortisol should not be considered homeostatically regulated variables. They are signaling elements controlling the effectors that determine the value of the regulated variable(s).

Another possible source of confusion about the identification of regulated variables arises when a physiological variable is regulated under one set of circumstances but behaves as a controlled variable under other circumstances. This can happen if a regulated variable is under the control of two different homeostatic systems or if a regulated variable can be 𠇌oopted” by another homeostatic system. This often happens if a physiological variable plays a role in more than one function in the body.

It is here that the concept of nested homeostasis or hierarchies of homeostats can be helpful. Carpenter (7) has pointed out that there are circumstances in which the maintenance of one regulated variable at its set point value is more important for continued viability of the organism than the simultaneous regulation of another variable.

One example of this is provided by the value of P co 2 in the ECF. As a variable in the internal environment that affects cell viability, P co 2 meets all of the criteria for a homeostatically regulated variable. P co 2 in the ECF depends on the action of respiratory muscles that alter the rate and depth of ventilation. As such, P co 2 in the ECF is maintained within defined limits by a regulatory system that senses P co 2 and operates by negative feedback. However, as any student of acid-base physiology knows, P co 2 in the ECF is not maintained relatively constant during compensatory adjustments in the acid-base balance of the body. From the perspective of H + homeostasis, P co 2 functions as a controlled variable.

At this point, some of our students might ask “Which is it? Is P co 2 a regulated variable or is it a controlled variable?” Our answer is that P co 2 is 𠇋oth,” and we can explain this using the idea of nested homeostatic mechanisms. There are circumstance in which it is more important to maintain arterial H + concentration (pH) in the normal range that maintaining a constant P co 2, perhaps because of the particular impact of the H + concentration on cell survival. Therefore, effective regulation of the H + concentration of the ECF can only be achieved by allowing P co 2 to dramatically vary from its normal range during acid-base disturbances. By introducing the concept of nested homeostatic mechanisms, we have refined how we view P co 2 as a homeostatically regulated variable, and we have offered another way to resolve other, “sticky” situations where the authenticity of a homeostatically regulated variable might be called into question.

Best Practices in Teaching Homeostasis

Given the centrality of the concept of homeostasis (15, 16), one would expect that both instructional resources and instructors would provide a consistent model of the concept and apply this model to appropriate systems in which variables are sensed and maintained relatively constant.

However, examination of undergraduate textbooks revealed that this is not the case (17). The problems found include, but were not limited to, inconsistent language used to describe the phenomenon and incomplete or inadequate pictorial representations of the model. In addition, texts often define homeostasis early in the narrative but fail to reinforce application of the model when specific regulatory mechanisms are discussed (17).

Furthermore, our work focusing on developing a concept inventory for homeostatic regulation (12, 13) revealed considerable confusion among faculty members regarding the concept. We think this confusion may stem, in part, from the level of faculty uncertainty about the concept and degree of complexity of homeostatic regulatory mechanisms. Our discussion of the sticky points associated with homeostasis is an attempt to suggest potential sources of this confusion and to indicate ways that instructors can work through these difficulties.

How do we ameliorate this situation? We propose five strategies that will help in approaching the problem.

1. Faculty members members should adopt a standard set of terms associated with the model. There is inconsistency within and among textbooks with respect to the names for critical components of the model. We propose the terminology shown in Table 2 to be used when discussing homeostatic regulatory mechanisms.

Table 2.

Definitions of terms for homeostasis paper

Term
Control center (or integrator)The control center consists of an error detector and controller. It receives signals (information) from sensors, compares information (value of regulated variable) with the set point, integrates information from all sensors, and sends output signals (sends instructions or commands) to increase or decrease the activity of effectors. The control center determines and initiates the appropriate physiological response to any change or disturbance of the internal environment
ControllerThe component of the control center that receives signals (information) from the error detector and sends output signals (instructions or commands) to increase or decrease the activity of effectors. The controller initiates the appropriate physiological response to an error signal resulting from a change or disturbance of the regulated (sensed) variable.
EffectorA component whose activity or action contributes to determining the value of any variable the system. In this model, the effectors determine the value of the regulated (sensed) variable.
Error detectorThe component in the control center that determines (calculates) the difference between the set point value and the actual value of the regulated (sensed) variable. The error detector generates the error signal that is used to determine the output of the control center.
Error signalA signal that represents the difference between the set point value and the actual value of the regulated variable. The error signal is one of the input signals to the controller.
External environmentThe world outside of the body and its “state.” The state or conditions in the outside world can determine the state of many internal properties of the organism.
IntegratorThis is another term for the control center. The integrator processes information from the sensor and those components that determine the set point, determines any error signal present, and sends output signals (instructions or commands) to increase or decrease the activity of effectors.
Internal environmentThe internal environment is the extracellular fluid compartment. This is the environment in which the body's cells live. It is what Bernard meant by the “internal milieu.”
HomeostasisThe maintenance of a relatively stable internal environment by an organism in the face of a changing external environment and varying internal activity using negative feedback mechanisms to minimize an error signal.
Negative feedbackA control mechanism where the action of the effector (response) opposes a change in the regulated variable and returns it back toward the set point value.
Nonregulated variable (controlled variable)A variable whose value changes in response to effector activity but whose value is not directly sensed by the system. Controlled variables contribute to determination of the regulated variable. For example, heart rate and stroke volume (controlled variables) contribute to determining cardiac output (another controlled variable) that contributes to arterial blood pressure (a regulated variable).
Perturbation (disturbance)Any change in the internal or external environment that causes a change to a homeostatically regulated variable. Physiologically induced changes in the set point would not be considered a perturbation.
Regulated variable (sensed variable)Any variable for which sensors are present in the system and the value of which is kept within limits by a negative feedback system in the face of perturbations in the system. A regulated variable is any property or condition of the extracellular fluid that is kept relatively constant in the internal environment in order to ensure the viability (survival) of the organism.
ResponseThe change in the function or action of an effector.
Sensor (Receptor)A �vice” that measures the magnitude of some variable by generating an output signal (neural or hormonal) that is proportional to the magnitude of the stimulus. A sensor is a measuring �vice.” For some regulated variables, sensors are specialized sensory cells or “sensory receptors,” e.g., thermoreceptors, baroreceptors, or osmoreceptors. For other regulated variables, sensors are cellular components, e.g., the Ca 2+ -sensing receptor (a G protein-coupled receptor that senses blood Ca 2+ in the parathyroid gland).
Set pointThe range of values (range of magnitudes) of the regulated variable that the system attempts to maintain. Set point refers to the �sired value.” The set point is generally not a single value it is a range of values.

A glossary of terms used in discussing the core concept of homeostasis. The components of a homeostatically regulated system ( Fig. 1 ) are defined here as are some other terms that occur in teaching this concept.

2. A standard standard pictorial representation of the model should be adopted when initially explaining homeostasis, and it should be used to frame the discussion of the specific system being considered. Figure 1 shows such a diagram.

The argument could be made that this diagram may be difficult for undergraduate students to understand. This may be the rationale for presenting the much-simplified diagrams found in most undergraduate texts (17). However, because these simple diagrams do not explicitly include all components of a homeostatic regulatory system (e.g., a set point), they may be a source of the misconceptions discussed as sticky points. As a result, students may not recognize that an essential feature of homeostatic regulatory systems is minimizing an error signal. A simplified representation of the model that includes the critical components of the regulatory system is shown in Fig. 2 . Depending on the course content and level of the student, this model can be expanded to add more levels of complexity as are required.

Simplified representation of a homeostatic regulatory system. Several components shown in Fig. 1 are combined in this representation. The reader should refer to Table 1 to find correspondence between components of physiologically significant homeostatic regulatory systems and this simplified representation. For example, chemosensors in the carotid bodies and aortic body are “sensors,” the brain stem is the 𠇌ontrol center,” and the diaphragm and other respiratory muscles are �tors” in the homeostatic regulatory system for arterial P o 2.

3. Faculty members should introduce the concept of homeostatic regulation early in the course and continue to apply and hence reinforce the model as each new homeostatic system is encountered. It is important to continue to use the standard terminology and visual representation as recommended in the first and second points above. Students tend to neither spontaneously or readily generalize their use of core concepts. It is therefore incumbent on the instructor to create a learning environment where this kind of transfer behavior is promoted. Faculty members can facilitate this by providing multiple opportunities for students to test and refine their understanding of the core concept of homeostatic regulation.

One way to reinforce the broad application of the model of homeostasis and help students demonstrate that they understand any particular homeostatic mechanism is to have them ask (and answer) a series of questions about each of homeostatically regulated systems they encounter (see Table 3 ). In doing so, they demonstrate that they can determine the essential components of the mental model needed to define the homeostatic system. The effort to thoroughly and accurately answer these questions will help students uncover gaps in their understanding and will reveal uncertainties in the resource information that they are using.

Table 3.

Questions students should ask about any homeostatically regulated system

What is the homeostatically regulated variable? Is it a property or condition of the extracellular fluid?
What and where is the sensor?
What and where is the control center?
What and where is the effector(s)? How do they alter their activities so as to produce a response?
Does the response lead to a change in the regulated variable/stimulus consistent with error signal reduction (negative feedback)?

4. Faculty members should use care when they select and explain the physiological examples or analogical models they chose to introduce and illustrate homeostasis in the classroom. In particular, instructors should ensure that the representative examples they use do not introduce additional misconceptions into student thinking. This is especially so when thermoregulation may be considered as an example of homeostatic regulation.

An informal survey of physiology textbooks indicated that thermoregulation is almost universally used as an example of a homeostatic mechanism. The most likely reasons for this selection are that 1) there is an everyday, seemingly easy to understand process involving the regulation of air temperature in room or building (i.e., the operation of a furnace and an air conditioner) and 2) the body's physiological responses are commonly and obviously observable and/or experienced by the learner (sweating, shivering, and changes in skin coloration). However, based on our description of the typical homeostatic regulatory system, there are compelling reasons to recommend that caution be taken if thermoregulation is used as the initial and representative example of homeostasis.

Most concerning, the typical home heating and cooling system operates in a manner that is distinctly different from mechanisms of human thermoregulation. The effectors in most houses, the furnace and air conditioner, operate in a full-on/full-off manner. For example, when the temperature at the thermostat falls below the value that has been dialed in (the set point temperature), the furnace turns on and stays on at maximum output until the temperature returns to the set point value. However, this is not how the human thermoregulatory system functions or how other homeostatic mechanisms operate. One potential consequence of using this model system to illustrate a homeostatic system is the creation of a common student misconception that homeostatic mechanisms operate in an on/off manner (12, 24), a sticky point we have addressed above. Faculty members need to help students overcome this problem area if they chose to use thermoregulation as a representative example of homeostasis.

What alternatives might be recommended? We suggest the automobile cruise control as a helpful nonbiological analog for homeostasis. The use of cruise control is not an uncommon activity for students, and, as we have described previously, the operation of a cruise control is theoretically easy to understand. What about a physiological example to represent homeostasis? A review of Table 1 would suggest the insulin-mediated system for blood glucose regulation during the fed state has much to recommend it. Students are generally familiar with the particulars of the system from either previous coursework or from personal experience. Other systems are likely to be less accessible to the beginning student of physiology.

However, faculty members should be aware that blood glucose regulation is not without its downsides as a representative example of homeostatic regulation. It is not easy to identify or explain the operation of the glucose sensor, the set point, and the controller involved in glucose homeostasis. Furthermore, there is probably no widely understood analog to glucose regulation that can be easily drawn from everyday life. Neither cruise controls, navigation systems on airplanes, autofocuses on cameras or other common, nor everyday examples of servomechanisms fully correspond to the operation of the feedback system involved in regulating blood glucose during the fed state. This points out the tradeoffs that must be made when any particular example or model is adopted to represent homeostatic regulation. Recognizing this, the use of a physiological control system such as glucose regulation during the fed state, where the effectors operate continuously, seems preferable to thermoregulation as a representative example for teaching the concept of homeostatic regulation.

5. When discussing discussing organismal physiology, restrict the use of the term “homeostatic regulation” to mechanisms related to maintaining consistency of the internal environment (i.e., the ECF).

Adopting these five strategies will provide students with a consistent framework for building their own mental models of specific homeostatic mechanisms and will help them recognize the functional similarities among different homeostatic regulatory systems at the organismal level. Because of its widespread application to different systems in organismal biology, homeostasis is one of the most important unifying ideas in physiology (15, 16). To construct a robust and enduring understanding of this concept, students need the proper tools. By giving them a precise and consistent terminology and encouraging them to use a standardized pictorial representation of the homeostatic model, we enable them to build a proper foundation for comprehending homeostatic systems. By making students aware of the potential sources of confusion surrounding the concept of homeostasis, i.e., the sticky points, we help prevent their thinking from becoming misguided or out of square. By doing so, we set the stage for our students to develop an accurate understanding of a wide range of physiological phenomena and to arrive at an integrated sense of the “wisdom of the body.”


Learning objectives by module

  • Module 1: How to Succeed in Anatomy and Physiology
    • Discover and execute the metacognitive cycle as you move through CC-OLI Anatomy and Physiology.

    Unit 2: Introduction to Anatomy and Physiology

    • Module 2: Anatomy and Physiology Introduction
      • Classify individual body system functions based on their contribution to vital human functions.
      • Describe how the “Big Ideas” in Anatomy and Physiology to develop a context that facilitates deep understanding of key concepts, connections and interdependencies.
      • Use a thematic framework to make sense of the different components of anatomy and physiology.
      • Use body planes and directional orientation to describe the locations of body structures.
      • Describe the cardiovascular system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • Describe the digestive system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • Describe the endocrine system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • Describe the integumentary system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • Describe the lymphatic system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • Describe the muscular system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • Describe the nervous system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • Describe the respiratory system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • Describe the skeletal system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • Describe the urinary system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
      • List the organ systems of the human body, and identify the main organs for each.

      Unit 3: Levels of Organization

      • Module 4: Levels of Organization Introduction
        • Describe, in order from simplest tomost complex, the major levels of organization in the human organism.
        • Define an acid and a base and locateeach on a pH scale.
        • Define atoms, molecules andmacromolecules and list their hierarchical assembly.
        • Describe how amphipathicstructures of lipids lead to compartmentalization. Describe how lipids are distributed in acell membrane.
        • Describe the basic structure ofamino acids.
        • Describe the four levels ofprotein structure and discuss the importance of protein structure in function.
        • Describe the structure and differenttypes of carbohydrates.
        • Describe the structure of DNA andits role in protein synthesis.
        • Discuss how atoms combine via ionic and covalent bonds to form molecules.
        • Discuss how control of DNAexpression is related to cell properties.
        • Discuss the physiologicallyimportant properties of water and how these properties are functions of the molecularstructure.
        • Identify biologically relevantatoms and use atomic information to calculate molecular weight.
        • List and describedifferent protein functions within different cell types.
        • List and explain the threemajor functions of carbohydrates.
        • List the four essentialmacromolecules in physiology. Explain how properties of the substructure relate to thefunction of the macromolecule.
        • Define and describe the functions of major cellularorganelles in human cells.
        • Define filtration andprovide examples of molecules that move across membranes via filtration.
        • Define the term cell and describe how cellstructures relate to function. Identify and briefly describe the three main parts of acell.
        • Describe endocytosis andexocytosis as a means of moving materials across the membrane.
        • Describe the componenets and structure of a cellmembrane.
        • Describe the environment in which diffusion will occur. Provide examples ofsubstances that move by simple diffusion and facilitated diffusion
        • Describe the process of activetransport, its energy requirements, and list examples of substances that useit.
        • Describe the process ofosmosis and explain the effects of hypertonic, isotonic, and hypotonic conditions on cellsand water shifts in the human body.
        • Identify and describe the stagesof somatic cell division including interphase and mitosis. Describe how cell number ismaintained and the processes associated with cell death: apoptosis and necrosis.
        • Define organ. Discuss the organ levelwithin the larger hierarchy of human physiology.
        • Define tissue. List the four majortissue types and organ systems associated with each.
        • Discuss how the organ systems worktogether in the whole body and how the body interacts with the environment to impactphysiology.
        • Module 10: Integration of Systems
          • Explain how different organ systems relate to one another to maintain homeostasis.
          • Compare and contrast positive and negative feedback in terms of the relationship between stimulus and response.
          • Define homeostasis and describe the multiple levels of homeostatic maintenance in physiology.
          • List the components of a feedback loop and explain the function of each.
          • Provide an example of a negative feedback loop. Describe the specific structures (organs, cells or molecules) included in the feedback loop.
          • Provide an example of a positive feedback loop. Describe the specific structures (organs, cells or molecules) included in the feedback loop.
          • Describe how abnormalities in homeostatic feedback loops lead to disease states.
          • Predict the types of problems that would occur if homeostasis could not be maintained.

          Unit 5: Skeletal System

          • Module 11: Skeletal System Introduction
            • Explore examples of homeostasis in the skeletal system.
            • Identify and discuss components of the skeletalsystem.
            • Review common facts and myths about the skeletalsystem.
            • Compare and contrast Compact(cortical) bone with Spongy (cancellous) bone.
            • Compare and contrast the bones andthe functions of the Axial and Appendicular divisions of the skeleton.
            • Describe the classificationof bone based on shape.
            • Describe the functionalclassification of articulations, based on degree of movement allowed—synarthrotic,amphiarthrotic, and diarthrotic—and provide examples of each type.
            • Describe the three maincomponents of a long bone.
            • Identify and label the bones of theaxial skeletal system.
            • Identify and label the bones ofthe appendicular skeleton.
            • Integrate the functions of thedifferent skeletal system components to the system functions.
            • Compare intramembranous andendochondral (intracartilaginous) bone formation.
            • Describe the effects of afracture and the most common types of fractures.
            • Describe the functions of theinorganic extracellular matrix components in osseous (bone) tissue.
            • Describe the functions of theorganic extracellular matrix components in osseous (bone) tissue.
            • Describe the mechanics of bonerepair and aging.
            • Integrate the levels of organizationin the skeletal system and their functional interconnections.
            • List the cell types andextracellular matrix components in the osseous (bone) and describe theirfunction.
            • Visually identify microscopicand macroscopic bone structures.
            • Describe the cellular andextracellular matrix reorganization that occurs in response to stress (force) onbones.
            • Describe the passive and activeresponses that occur in cartilage when stress (force) on the tissue changes.
            • Describe two disorders of the skeletal system that can result from calcium deficiency.
            • Explain how the skeletal systemand endocrine system interact for calcium homeostasis.
            • Discuss ways in which other bodysystems integrate with the skeletal system.

            Unit 6: Muscular System

            • Module 16: Muscular System Introduction
              • Explore examples of homeostasis in the muscular system.
              • Identify and discuss components of the muscular system.
              • Review common facts and myths about the muscular system.
              • Define the terms muscle tone, hypotonia and hypertonia.
              • Define the terms prime mover (or agonist), antagonist, synergist and fixator and provide an example of each.
              • Define the terms: aponeuroses, tendons, bursae.
              • Describe different fiber organization (parallel, convergent, pinnate, sphincter) and how the organization is related to functions.
              • Describe the composition of the connective tissue layer that surrounds each cell, fascicle, muscle and group of muscles.
              • Describe the difference between isometric and isotonic contractions of muscle.
              • Differentiate among the three classes of levers in terms of the relative position of fulcrum, effort and load, as well as in terms of the relative power and range of motion.
              • Compare and contrast the structure, location in the body and function of skeletal muscle, cardiac muscle and smooth muscle.
              • Define and describe the functions of major cellularorganelles in human cells.
              • Define the term cell and describe how cellstructures relate to function. Identify and briefly describe the three main parts of acell.
              • Describe how the parallel organization of a sarcomere relates to force generation predict what effect changes in filament overlap would have on muscle function.
              • Describe myoblast fusion to generate multinucleated skeletal muscle cells and tissue structure.
              • Describe the anatomy of the neuromuscular junction.
              • Describe the difference between tetanus and treppe.
              • Describe the difference in distribution of cell/fiber types in different specific body muscles.
              • Describe the different structural levels of skeletal muscle organization.
              • Describe the interaction of actin and myosin in force generation. Identify which cofactors (ions and proteins) regulate actin-myosin force generation.
              • Describe the mechanisms that muscle fibers use to generate ATP for muscle contraction.
              • Describe the process of activetransport, its energy requirements, and list examples of substances that useit.
              • Describe the sequence of events involved in the contraction cycle of skeletal muscle.
              • Describe the specialized structures of muscle cells.
              • Explain how an electrical signal from the nervous system is communicated to muscle cells.
              • Explain how the cellular organization of fused skeletal muscle cells allows muscle tissue to contract properly.
              • Explain the effects of summation and recruitment on muscle contraction.
              • Explain the three phases twitch undergoes as viewed on a myogram.
              • Identify muscle tissue as being a mixture of SO, FG, and FO cells/fibers.
              • Identify skeletal, cardiac and smooth muscle cells by anatomical features.
              • List the anatomical and metabolic characteristics of fast, slow, and intermediate muscle fibers.
              • List the sources of energy used in muscle contraction.
              • Describe how an improperly functioning skeletal muscular system would affect other systems.
              • Describe several factors that can affect the endurance of muscles.
              • Explain the role of the muscular system in maintaining temperature homeostasis.
              • Provide examples of factors that can affect muscle size.
              • Describe how an improperly functioning skeletal muscular system would affect other systems.
              • Describe how muscle tissue within the cardiovascular system contributes to proper function.
              • Describe how muscle tissue within the digestive system contributes to proper function.

              Unit 7: Integumentary System

              • Module 21: Integumentary System Introduction
                • Explore common facts and myths about the integumentarysystem.
                • Explore examples of homeostasis in the integumentary system.
                • Identify and discuss components of the integumentarysystem.
                • Describe the main function of each layer of the integumentary system.
                • Contrast the structure and function ofeccrine(merocrine)glands, apocrine glands, and sebaceous glands.
                • Define and describe the functions of major cellularorganelles in human cells.
                • Describe four functions of hair.
                • Describe how the distribution of adipose tissue differs based on gender,age, diet and exercise.
                • Describe how the molecular assembly of keratinsprovidesstrength to integumentary tissues.
                • Describe inorder,from simplest to most complex, the major levels of organization of the integumentary system.
                • Describe sunscreen and UVA and UVBradiation.
                • Describe the complementaryfunctioningof the cells of the epidermis.
                • Describe the differences between the three categories of skin cancer.
                • Describe the four levels ofprotein structure and discuss the importance of protein structure in function.
                • Describe the function of melanin and discuss the consequences of reduced melanin.
                • Describe the functions of melanocytes.
                • Describe the functions ofthe epidermis.
                • Describe the production and function of vitamin D and discuss the consequences of reduced vitamin D.
                • Describe the role of melanocytes inproducing skin pigmentation and also protecting mitotic cells inthestratum basale from UV damage.
                • Describe the structure and function offingernailsand toenails.
                • Describe the structure and roles of the accessory structures of the integumentary system.
                • Describe the structure of hair andof ahair follicle.
                • Describe the three stages of hair growth.
                • Describe the two different layers of the dermis.
                • Explain common causes of hair loss.
                • Explain how wrinkles and stretch marks are related to the collagen and elastin fibers in the dermis.
                • Explain why the histology of the dermisiswell-suitedfor its functions.
                • Explain why the histology of theepidermis iswell-suitedfor its functions.
                • Explain why the histology of thesubcutaneous layer iswell-suitedfor its functions.
                • Identify and describe the layers ofthe epidermis, indicating which are found in thin skin and which are found in thickskin.
                • Identify and describe the subcutaneoustissue, including the tissue types making up subcutaneous tissue.
                • Identify the cells of the epidermis, dermis and hypodermis.
                • Identify the cells of theepidermis based on their location and anatomic structure (stem cells ofthestratum basale, keratinocytes, melanocytes, Langerhans cells, Merkelcells).
                • Identify the tissue type makingup the epidermis.
                • List and describedifferent protein functions within different cell types.
                • Compare homeostasis of cell numberand mitotic rate in the epidermis with pathologicalconditions,including psoriasis and skin cancer.
                • Comparethermoregulationby the integumentary systemas it pertainstosubcutaneous fat, hair,sweatand blood flow.
                • Describe different sensory receptors located in the integumentary system.
                • Describe how a medical professional can use changes in the appearance of the skin to predict certain medical conditions.
                • Describe the cells involved in repairing damaged skin.
                • Describe the effect ofscarringon regeneration of accessory structures.
                • Explain changes in the integumentary system that occur because of puberty and advanced age.
                • Predict issues related to loss ofskin in burn victims forfirst-,second-andthird-degreeburns.

                Unit 8: Endocrine System

                • Module 25: Endocrine Structures and Functions
                  • Define homeostasis and describe the multiple levels of homeostatic maintenance in physiology.
                  • Describe the major functions of the endocrinesystem.
                  • Identify major diseases associated with the endocrine systemand their causes.
                  • Compare the hormones secreted from organs with secondaryendocrine function and the primary function of these organs.
                  • Compare the production of hormones in the thyroid to otherendocrine glands.
                  • Define “hormone” and list the three hormone types.
                  • Define G-protein-coupled hormone receptors and describe howthey are messengers for signal transduction.
                  • Define intracellular and plasma membrane hormone receptors and describe howthey impact cellular gene expression.
                  • Describe how hormones are involved in loops of homeostasisincluding positive feedback and negative feedback.
                  • Describe the endocrine glands and hormones involved in thereproductive system.
                  • Describe the four levels ofprotein structure and discuss the importance of protein structure in function.
                  • Describe the precursor molecules of amino acidhormones.
                  • Describe the precursor molecules of lipid-derived hormones and identify howlipid-derived hormones are transported.
                  • Describe the precursor molecules of peptide-derivedhormones.
                  • Describe the structure of the adrenal cortex and whathormones it produces.
                  • Describe the structure of the adrenal medulla and whathormones it produces.
                  • Describe the structure of the anterior pituitary and whathormones it produces.
                  • Describe the structure of the pancreas and whathormones it produces.
                  • Describe the structure of the parathyroid glands and whathormones these glands produce.
                  • Describe the structure of the pineal gland and whathormones it produces.
                  • Describe the structure of the posterior pituitary and whathormones it produces.
                  • Describe the structure of the thyroid gland and whathormones it produces.
                  • Discuss how hormone receptors maintain specificity inendocrine regulation.
                  • List and compare the mechanisms of hormonalstimulation.
                  • List and describedifferent protein functions within different cell types.
                  • List the different locations for hormone receptors.
                  • List the endocrine glands, identify their locations within the body,and name the primary hormones that they secrete.
                  • List the hormones secreted by the hypothalamus and describe thefunctions that these hormones regulate.
                  • Provide an example of a negative feedback loop. Describe the specific structures (organs, cells or molecules) included in the feedback loop.
                  • Provide an example of a positive feedback loop. Describe the specific structures (organs, cells or molecules) included in the feedback loop.
                  • Use anatomical terms to describe the location of the adrenal glandsand the layers of the adrenal glands.
                  • Describe differences between short-term and long-term stressresponses.
                  • Describe how endocrine function regulates growth and list thehormones involved in the process.
                  • Describe how endocrine function regulates the femalereproductive system and list the hormones involved in the process.
                  • Describe how endocrine function regulates the homeostasis ofcalcium levels in the body and list the hormones involved in the process.
                  • Describe how endocrine function regulates the homeostasis ofglucose and list the hormones involved in the process. Compare the roles of insulin andthyroid hormones.
                  • Describe how endocrine function regulates the homeostasis ofwater in the body and list the hormones involved in the process.
                  • Describe how endocrine function regulates the male reproductivesystem and list the hormones involved in the process.
                  • Describe how endocrine function regulates the production of milk andlist the hormones involved in the process.
                  • Describe how endocrine function regulates the reproductive system.List the hormones that are common to males and females.
                  • Identify how endocrine function regulates the homeostasis ofdifferent organ systems in the body.
                  • Predict factors or situations affecting the endocrine system thatcould disrupt homeostasis.

                  Unit 9: Digestive System

                  • Module 28: Digestive System Introduction
                    • Assess your skills relativeto stated objectives.
                    • Explore connections between the Big Ideas of Anatomy & Physiology and the Digestive System.
                    • List the organ systems of the human body, and identify the main organs for each.
                    • Use a thematic framework to make sense of the different components of anatomy and physiology.
                    • Describe how material moves through the digestive system.
                    • Describe the digestive system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
                    • Describe the major functions of the digestivesystem.
                    • Describe the process of activetransport, its energy requirements, and list examples of substances that useit.
                    • Describe the process ofosmosis and explain the effects of hypertonic, isotonic, and hypotonic conditions on cellsand water shifts in the human body.
                    • Explain how mechanical and chemical digestion work togetherto produce absorbable nutrients.
                    • Explain how nutrients are absorbed in the digestivesystem.
                    • Identify and discuss the histology and functions of theplicae circulares, villi, and microvilli.
                    • Identify organs of the digestive system based on positionand structure identify the general function of each.
                    • Define an acid and a base and locateeach on a pH scale.
                    • Describe how amphipathicstructures of lipids lead to compartmentalization. Describe how lipids are distributed in acell membrane.
                    • Describe how secretions from the GI tract, salivary glands,pancreas and the liver work together to digest nutritive molecules in food.
                    • Describe how the stomach uses a combination of mechanical force and chemicals todigest food.
                    • Describe the basic structure ofamino acids.
                    • Describe the defecation reflex and the function of theinternal and external anal sphincters.
                    • Describe the functions of the different regions of the smallintestine.
                    • Describe the location of the parotid, submandibular, andsublingual glands and their respective ducts.
                    • Describe the pathway of the bolus from mouth to stomach,identifying major structures and describing their role in facilitating the process ofdeglutition (swallowing).
                    • Describe the pathway of the chyme through the stomach, identifying majorstructures and describing their adaptations and role in the various digestiveactivities.
                    • Describe the process of enzymatic hydrolysis for nutritiveorganic compounds
                    • Describe the role of bacteria living in the largeintestine.
                    • Discuss how atoms combine via ionic and covalent bonds to form molecules.
                    • Explain how different organ systems relate to one another to maintain homeostasis.
                    • Explain the process of deglutition, including the changes inposition of the glottis and larynx that prevent aspiration and peristalsis.
                    • Explain the regulation of gastric secretion in thecephalic phase, the gastric phase and the intestinal phase.
                    • Identify and describe the histological structure and function of each of the four layers of the GI tract wall.
                    • Identify and discuss the functions of the gall bladder.
                    • Identify and discuss the functions of the large intestine andits structures.
                    • Identify and discuss the functions of the liver and its structures.
                    • Identify and discuss the functions of the pancreas and its structures.
                    • Integrate the levels of organization in the digestive system andtheir functional interconnections.
                    • List the organs and specific structures involved in theabsorption of each nutrient.
                    • Relate the regional cell-level specializations withinthe digestive system to changing functions along the length of the GI tract.
                    • Explain how the digestive system relates to other bodysystems to maintain homeostasis.

                    Unit 10: Cardiovascular System

                    • Module 33: Cardiovascular System Introduction
                      • Explain how the cardiovascular system performs the function of moving material through the body.
                      • Identify major diseases associated with the cardiovascular system and their causes.
                      • Contrast the vasculature of the systemic and pulmonary circuits.
                      • Define blood pressure (BP) and describe factors that influence blood pressure. Relate blood pressure to mean arterial pressure (MAP) and how MAP is calculated.
                      • Define venous return and describe how skeletal muscles and the respiratory pump help maintain venous return.
                      • Describe the cardiac cycle and all of its phases.
                      • Describe the conduction system of the heart, including the role of the autonomic nervous system in regulating aspects of cardiac conduction.
                      • Describe the different modes of transport that molecules may take during capillary exchange. Define Starling’s Law of the Capillaries and use it to determine the relative rate and direction of fluid exchange in the capillaries. Relate imbalances in capillary exchange to edema.
                      • Identify the relationship between cardiac output (CO), heart rate (HR), and stroke volume (SV) and predict how changes in HR and SV affect CO.
                      • Identify the waveforms in a normal ECG and relate them to the activity of the conduction system of the heart.
                      • Describe common changes of the circulatory system related to aging.
                      • Describe the anatomical structure of the arteries in the body and relate it with their function.
                      • Describe the anatomical structure of the veins in the body and relate it with their function.
                      • Describe the anatomical structures of the heart and major blood vessels entering and leaving the heart. Relate the features of these structures to blood flow into, out of, and through the heart.
                      • Describe the anatomy of the aorta and its major branches and relate it with their functions.
                      • Describe the basic process of hematopoiesis, where it occurs, and the significance of the pluripotent stem cell (hemocytoblast) in the process.
                      • Describe the features of blood that give it the characteristics of a connective tissue.
                      • Describe the functions for each of the five major types of leukocytes as well as the two major subtypes of lymphocytes (T and B).
                      • Describe the overall composition of plasma, including the major types of plasma proteins, their functions, and where in the body they are produced.
                      • Describe the phases of hemostasis.
                      • Describe the properties of the vessel wall layers (tunica interna, media, externa) and associate each with the function of different vessel types.
                      • Describe the structure and function of arterioles, metarterioles, capillaries, and venules.
                      • Describe the structure and function of platelets.
                      • Identify the function of red blood cells and describe the life cycle of red blood cells, including how and where iron and heme are recycled, as well as the resulting breakdown products.
                      • Identify the microscopic features of erythrocytes (red blood cells), the five types of leukocytes (white blood cells), and thrombocytes (platelets).
                      • Identify the three kinds of cells that make up the myocardium and describe the role of each in the physiology of muscle contraction.
                      • Identify the types of cells associated with blood vessels and relate them to the different properties of blood vessels.
                      • Describe factors that could disrupt homeostasis of the cardiovascular system and predict the types of homeostatic imbalances that would occur.
                      • Explain how the cardiovascular system relates to other body systems to maintain homeostasis through autoregulation.
                      • Describe the determinants of blood flow to an organ or tissue.
                      • Explain the role of the sympathetic nervous system in regulation of cardiac output.
                      • Identify the hormones involved in regulating blood volume flow and blood pressure and the role they play in these processes.
                      • Identify unique characteristics of blood flow to the liver, the kidney, and the hypothalamus/pituitary system.

                      Unit 11: Respiratory System

                      • Module 38: Respiratory System Introduction
                        • Describe major functions and processes of the respiratory system
                        • Define, identify, and determine values for the respiratory volumes (IRV, TV, ERV, and RV) and the respiratory capacities (IC, FRC, VC, and TLC).
                        • Describe the mechanisms of pulmonary ventilation.
                        • Describe the structure and function of the the respiratory conducting zone and respiratory zone.
                        • Identify and describe gross & microscopic anatomy of the respiratory tract and related organs.
                        • Describe and analyze the mechanisms of gas exchange in the lungs & tissues.
                        • Describe how the structure of these macromolecules allow the structures of the respiratory system to perform their functions.
                        • Describe the changes in epithelial and connective tissue seen in various portions of the air passageways and relate these changes to function.
                        • Describe the four respiratory processes – ventilation, external respiration (gas exchange at lung), internal respiration (gas exchange at body tissues), and cellular respiration.
                        • Explain the mechanisms of gas transport in the blood.
                        • State Henry’s Law, and relate it to the events of external and internal respiration and to the amounts of oxygen and carbon dioxide dissolved in plasma.
                        • Explain and analyze respiratory homeostatic mechanisms.

                        Unit 12: Urinary System

                        • Module 43: Urinary System Introduction
                          • Describe the major functions of the urinary system.
                          • Define countercurrent multiplication and countercurrent exchange, and describe how this relates to urine formation.
                          • Describe glomerular filtration rate (GFR), state the average value of GFR, and explain how clearance rate can be used to measure GFR.
                          • Describe the last portion of urine transport and collection for elimination.
                          • Describe the micturition reflex and the voluntary and involuntary neural control of micturition.
                          • Describe the process of tubular reabsoption including specific transport mechanisms, including active transport and osmosis.
                          • Describe the process of tubular secretion.
                          • Identify and describe the functional process of urine formation, including filtration, reabsorption, and secretion.
                          • identify gross and microscopic anatomy of the urinary tract.
                          • Define the chemical properties of urine and their functions.
                          • Describe normal urine composition.
                          • Describe the anatomy and the detailed histology of the nephron.
                          • Describe the internal and external structure of the kidney, including its location, support structures and covering.
                          • Identify the major blood vessels associated with the kidney.
                          • Identify, and describe the structure and location of, the ureters, urinary bladder and urethra.
                          • Explain and analyze urinary homeostatic mechanisms.
                          • Identify and describe the factors regulating and altering urine volume and composition, including the renin-angiotensin system and the roles of aldosterone, antidiuretic hormone, and the natriuretic peptides.
                          • Compare the excretory systems of the body

                          Unit 13: Lymphatic System

                          • Module 48: Lymphatic System Introduction
                            • Describe the lymphatic system: list the major organs and structures, describe the major functions, and use anatomical planes and directional terms to identify organs and their relationships to each other.
                            • Explore connections between the Big Ideas of Anatomy & Physiology and the Lymphatic System and Immunity.
                            • Explore some common misconceptions about the lymphatic system.
                            • Recognize Big Ideas seen in the workings of individual components of the Lymphatic System and Immunity.
                            • Compare and contrast interstitial fluid and lymph.
                            • Compare and contrast lymphatic vessels and blood vessels in terms of structure and function. Describe the mechanisms of lymph formation and circulation. Describe the path of lymph circulation.
                            • Describe early events in the history of immunology in relation to current understanding of immunity.
                            • Describe the major functions of the lymphatic system. Define immunity.
                            • Identify dysfunction associated with the lymphatic circulation.
                            • Identify major diseases associated with the lymphatic system and their causes.
                            • Compare and contrast innate defenses with adaptive defenses. Analyze ways in which the innate and adaptive immunity cooperate to enhance the overall resistance to disease.
                            • Compare and contrast interstitial fluid and lymph. Describe the basic structure and cellular composition of lymphatic tissues and correlate them to the overall functions of the lymphatic system.
                            • Define and describe location of antigens and antigen receptors. Discuss the source of antigen receptor diversity.
                            • Define and describe location of major histocompatibility complex (MHC).
                            • Define and describe location of pathogen-associated molecular patterns (PAMPs) and their receptors (PRRs).
                            • Define and describe the functional role of the important cytokines participating in the immune response.
                            • Define and describe the roles of various types of white blood cells in the innate and adaptive immune response and correlate them to the overall functions of the lymphatic system.
                            • Describe antibody structure, list the five classes of antibodies and functional features that distinguish each class.
                            • Describe how histamine, kinins, prostaglandins, leukotrienes and complement contribute to flammation.
                            • Describe the basic structure and function of chemical molecules of the lymphatic system and correlate it to the overall functions of lymphatic system.
                            • Describe the mechanisms of inflammation initiation. Summarize the cells and chemicals involved in the inflammatory process. List and explain the cause of the four cardinal signs of inflammation. Explain the benefits of inflammation.
                            • Describe the origin and roles of various white blood cells in innate immunity.
                            • Describe the steps involved in phagocytosis and give examples of phagocytic cells in the body.
                            • Describe the steps involved in phagocytosis and give examples of phagocytic cells in the body. Describe the mechanisms of inflammation initiation. Summarize the cells and chemicals involved in the inflammatory process. List and explain the cause of the four cardinal signs of inflammation. Explain the benefits of inflammation.
                            • Describe the types of defensive mechanisms of innate immunity such as barriers, phagocytosis, inflammation and fever.
                            • Distinguish between innate and adaptive immunity. Distinguish the various types of lymphocytes including helper T cells, cytotoxic T cells, B cells, plasma cells and memory cells. Distinguish between humoral and cell-mediated immunity. Describe the immunological memory response.
                            • Explain how interferons, complement and tranferrins function as antimicrobial chemicals.
                            • Explain how the kinin-kallikrein and complement systems aid in the inflammatory response. Describe the mechanism and benefits of fever and the role of pyrogens.
                            • Identify the lymphatic organs and correlate them to the overall functions of the of the lymphatic system. Identify and describe the gross anatomical and microscopic anatomy of each organ. Describe the location and function of each organ.
                            • Integrate the levels of organization in the lymphatic system and their functional interconnections.
                            • Name the barriers and describe their anatomic, chemical and microbiological mechanisms of defense.
                            • Name the cells of the adaptive immune response and correlate their function to the overall functions of the adaptive immune response.
                            • Name the cells of the innate immune response and correlate their function to the overall functions of the innate immune response.
                            • Define immunocompetence (maturity) and self tolerance and distinguish between naïve and activated immune cells. Compare and contrast mechanisms of antigen challenge and the clonal selection processes and defense mechanisms.
                            • Describe the basic process of hematopoiesis, where it occurs, and the significance of the pluripotent stem cell (hemocytoblast) in the process.
                            • Explain the role of antigen-presenting cells (APCs). Distinguish and describe the processing of exogenous and endogenous antigens and provide examples of APCs.
                            • Explore the condition and symptoms of B-Cell Chronic Leukemia using concepts and vocabulary from the Lymphatic System unit.
                            • Provide specific examples to demonstrate how the lymphatic system responds to maintain homeostasis in the body, particularly related to the diseases presented in the introduction.
                            • Explain how the lymphatic system relates to other body systems to maintain homeostasis.

                            Unit 14: Nervous System

                            • Module 53: Nervous System Introduction
                              • Describe the major functions of the NervousSystem.
                              • Assign function(s) to each of the cranialnerves.
                              • Compareand contrast the anatomical features of the spinal cord in the cervical, thoracic and lumbarregions.
                              • Contrast the relative position of gray matter and white matter inthe spinal cord with the corresponding arrangement of gray and white matter in thebrain.
                              • Correlate forebrain regions to their majorfunctions(s).
                              • Correlate hindbrain and midbrain regions totheir major function(s).
                              • Describe the basic (overall) structure of the humanbrain.
                              • Describe the gross anatomy of the spinal cord and spinal nervesand specify their location relative to the anatomy of the vertebralcolumn.
                              • Explain the roles of CSF, ventricles, and the blood brainbarrier.
                              • Identify how spinal structures relate to each other: tract,root, ganglion, nerve, ramus, plexus.
                              • Identify the location of major brainregions.
                              • Classify the organs that are part of the nervous system asbelonging to the central nervous system (CNS) or the peripheral nervous system(PNS).
                              • Compare the somatic and autonomic nervoussystems.
                              • Compare the structure of myelinated vs. unmyelinated axons.Distinguish between white matter and gray matter.
                              • Contrast the anatomy of theparasympathetic and sympathetic systems.
                              • Contrast the relative concentrations of ions in bodysolutions inside and outside of a cell (sodium, potassium, calcium and chlorideions).
                              • Describe examples of specific effectors dually innervatedby the autonomic nervous system and explain how each branch influences function in a giveneffector.
                              • Describe major parasympathetic and sympatheticphysiological effects on target organs.
                              • Describe the local organization of each of the sympathetic and parasympathetic systems, including the pattern of innervation of target glands, organs, and tissues.
                              • Describe the transmembrane potential or voltage across thecell membrane and how it is measured.
                              • Explain action potential.
                              • Explain how a local electrical response in aneuron membrane is caused by stimulation.
                              • Explain how a single neurotransmitter may havedifferent effects at different postsynapticcells.
                              • Explain how four factors determine a neuron’sresting membranepotential.
                              • Explain synaptic transmission in terms of the structuraland functional features of electrical and chemical synapses.
                              • Explain temporal and spatial summation of synapticpotentials and discuss how action potentials differ from synaptic potentials.
                              • Explain the role of the autonomic nervous system as amotor division of the nervous system.
                              • Identify neurons based on anatomical features: unipolar,bipolar, multipolar and anaxonic and based on functional properties: sensory, motor,interneuron.
                              • Identify the four classes of neurotransmitters andidentify the most common excitatory and inhibitory neurotransmitters.
                              • Identify the neurotransmitters released bypreganglionic and postganglionic neurons in the sympathetic and parasympathetic nervoussystems and describe their effects.
                              • Identify the presynaptic and postsynaptic cells at asynapse.
                              • Interpret a graph showing the voltage vs. time relationshipof an action potential.
                              • List the four types of CNS glial cells and describe theirfunction.
                              • List the two types of PNS glial cells and describe theirfunction. Describe the anatomical relationship between the glial cells and thePNS.
                              • Name examples of effectors innervated either by onlythe sympathetic branch or by only the parasympathetic branch of the autonomic nervous systemand explain how that branch by itself influences function in a given effector.
                              • Within a neuron, identify the soma, axon and dendrite anddescribe the main function of eachregion.
                              • Classify receptors based onstructure,location relative to the stimulus, and types of signals theytransduce.
                              • Define dermatome and explain how dermatomes can be used inneurological exams for diagnosing nerve damage.
                              • Describe how the various structures of the ear conduct andtransduce sound.
                              • Describe pain in terms of hyperalgesia,analgesia, and receptive field.
                              • Describe reflexes, reflex repsonses, and distinguish types ofreflexes.
                              • Describe the path of nerve impulses from the ear tovarious parts of the brain.
                              • Describe the path of nerve impulses from the gustatoryreceptors to various parts of the brain.
                              • Describe the path of nerve impulses from theolfactory receptors to various parts of the brain.
                              • Describe the structures and functions of the eye.
                              • Describe the types of information (modality)detected by the receptors associated with the somesthetic senses and the phenomenon ofadaptation.
                              • Distinguish between static and dynamic equilibrium,describe the structures involved, and their functions.
                              • Explain how odorants activate olfactoryreceptors.
                              • Explain how the path of light through the eye causesvision.
                              • Explain pain function, nociceptor distribution, and distinguishthe fiber types that carry their signals.
                              • Explain the distribution of receptors involved in providinginformation for our general (somesthetic) senses.
                              • Explain the gustation and describe the structuresinvolved.
                              • Identify and describe the functions of the accessory eyestructures, the tunics, and the optical components of the eye.
                              • Identify the hearing structures of the outer, middleand inner ear and describe their functions.
                              • Identify the muscles that help to coordinate eyemovement.
                              • Predict the types of problems that would occur inthe body if the olfactory system was not functioning normally.
                              • Trace the path of nerve impulses from the retina tovarious parts of the brain.

                              Unit 15: Review and Synthesis

                              • Module 57: Review and Synthesis
                                • Complete homeostasis loops associated with physiology.
                                • Define homeostasis, and identify specifics aspects of physiology involving homeostasis.
                                • Explore connections between the Big Ideas of Anatomy & Physiology between systems.
                                • Identify examples of integrated systems.
                                • Identify examples of structure and function in anatomy.
                                • Identify how different anatomic size scales integrate for physiological function.
                                • Predict dysfunctions associated with altered anatomical structures.
                                • Predict dysfunctions associated with misregulation of homeostasis.
                                • Predict how dysfunction observed in one organ system could reflect dysfunction in other systems.
                                • Predict what occurs at length scales if there is dysfunction at another length scale.

                                Importance of Homeostasis

                                Based from the aforementioned examples, you may probably already have understood how important homeostasis is. Living organisms need to maintain homeostasis constantly in order to properly grow, work, and survive. In general, homeostasis is essential for normal cell function, and overall balance.

                                • In the human body, chemicals like Oxygen (O2), Carbon dioxide (CO2) and digested food enter and exit the cells using the concept called diffusion and osmosis. For this process to function properly, homeostasis helps our body to keep both water and salt balance level.
                                • Enzymes in the cell help in the speedy chemical reactions to order to keep the cells alive but these enzymes need to be in an optimal temperature to function properly. Again, homeostasis plays a crucial role in maintaining a constant body temperature (37C/98.6F) for enzymes to do their jobs.
                                • Mechanisms to attain homeostasis are stable as they need to resist any change that happens within and outside the organism’s environment. These mechanisms vary depending on the individual and may either be positive or negative feedback.

                                It is important to note that homeostasis occurs naturally when a system is stable and functions correctly. This can be achieved by continuously making systems work together in harmony.


                                Urine Observation

                                Students will determine the effects of various fluids on the endocrine system by sampling different beverages and recording data collected from urine samples. Complete the experiment over four days. Students should drink tap water on the first day, carbonated dark soda on the second day, saltwater on the third day and coffee on the final day. Provide litmus paper so a student can test the pH level of urine during each trip to the restroom on these days. Students should also record whether their urine is dark or light. Discuss the results and draw conclusions about the effect each beverage has on the human body and the importance of kidneys in maintaining homeostasis through proper filtration.


                                33.3 Homeostasis

                                By the end of this section, you will be able to do the following:

                                • Define homeostasis
                                • Describe the factors affecting homeostasis
                                • Discuss positive and negative feedback mechanisms used in homeostasis
                                • Describe thermoregulation of endothermic and ectothermic animals

                                Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.

                                Homeostatic Process

                                The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point . While there are normal fluctuations from the set point, the body’s systems will usually attempt to go back to this point. A change in the internal or external environment is called a stimulus and is detected by a receptor the response of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by getting the nutrient into tissues that need it or to store it for later use.

                                Control of Homeostasis

                                When a change occurs in an animal’s environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes) or a gland that secretes. Homeostatsis is maintained by negative feedback loops. Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine system of mammals.

                                Negative Feedback Mechanisms

                                Any homeostatic process that changes the direction of the stimulus is a negative feedback loop . It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback. An example is animal maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise. This is sensed by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in Figure 33.20. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of the feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and, possibly, the breakdown of bone in order to liberate calcium. The effects of PTH are to raise blood levels of the element. Negative feedback loops are the predominant mechanism used in homeostasis.

                                Positive Feedback Loop

                                A positive feedback loop maintains the direction of the stimulus, possibly accelerating it. Few examples of positive feedback loops exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of positive feedback is uterine contractions during childbirth, as illustrated in Figure 33.21. The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth.

                                Visual Connection

                                State whether each of the following processes is regulated by a positive feedback loop or a negative feedback loop.

                                1. A person feels satiated after eating a large meal.
                                2. The blood has plenty of red blood cells. As a result, erythropoietin, a hormone that stimulates the production of new red blood cells, is no longer released from the kidney.

                                Set Point

                                It is possible to adjust a system’s set point. When this happens, the feedback loop works to maintain the new setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. The result is the maintenance of an elevated blood pressure that can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level. This is called a process of alteration of the set point in a feedback loop.

                                Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization . This occurs, for instance, when an animal migrates to a higher altitude than that to which it is accustomed. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to harmful levels.

                                Link to Learning

                                Feedback mechanisms can be understood in terms of driving a race car along a track: watch a short video lesson on positive and negative feedback loops.

                                Homeostasis: Thermoregulation

                                Body temperature affects body activities. Generally, as body temperature rises, enzyme activity rises as well. For every ten degree centigrade rise in temperature, enzyme activity doubles, up to a point. Body proteins, including enzymes, begin to denature and lose their function with high heat (around 50 o C for mammals). Enzyme activity will decrease by half for every ten degree centigrade drop in temperature, to the point of freezing, with a few exceptions. Some fish can withstand freezing solid and return to normal with thawing.

                                Link to Learning

                                Watch this Discovery Channel video on thermoregulation to see illustrations of this process in a variety of animals.

                                Endotherms and Ectotherms

                                Animals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. Animals that rely on external temperatures to set their body temperature are ectotherms. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. In contrast to ectotherms, poikilotherms are animals with constantly varying internal temperatures. An animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm. Endotherms are animals that rely on internal sources for maintenance of relatively constant body temperature in varying environmental temperatures. These animals are able to maintain a level of metabolic activity at cooler temperature, which an ectotherm cannot due to differing enzyme levels of activity. It is worth mentioning that some ectotherms and poikilotherms have relatively constant body temperatures due to the constant environmental temperatures in their habitats. These animals are so-called ectothermic homeotherms, like some deep sea fish species.

                                Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction (Figure 33.22). Radiation is the emission of electromagnetic “heat” waves. Heat comes from the sun in this manner and radiates from dry skin the same way. Heat can be removed with liquid from a surface during evaporation. This occurs when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.

                                Heat Conservation and Dissipation

                                Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example, uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles cause “goose bumps,” causing small hairs to stand up when the individual is cold this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end. Loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.

                                Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals have adaptations to their circulatory system that enable them to transfer heat from arteries to veins, warming blood returning to the heart. This is called a countercurrent heat exchange it prevents the cold venous blood from cooling the heart and other internal organs. This adaptation can be shut down in some animals to prevent overheating the internal organs. The countercurrent adaptation is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears.

                                Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from getting too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity such as the activity of bees to warm a hive to survive winter.

                                Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold elicits a shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.

                                Neural Control of Thermoregulation

                                The nervous system is important to thermoregulation , as illustrated in Figure 33.22. The processes of homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain.

                                Visual Connection

                                When bacteria are destroyed by leuckocytes, pyrogens are released into the blood. Pyrogens reset the body’s thermostat to a higher temperature, resulting in fever. How might pyrogens cause the body temperature to rise?

                                The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat. This allows the body’s temperature to increase in what is commonly called a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal’s enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen. A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism.

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                                  • Book title: Biology 2e
                                  • Publication date: Mar 28, 2018
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                                  • Section URL: https://openstax.org/books/biology-2e/pages/33-3-homeostasis

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                                  Mersaoui, S. Y., Bonnell, E. & Wellinger, R. J. Nuclear import of Cdc13 limits chromosomal capping. Nucleic Acids Res. 46, 2975–2989 (2018).

                                  Puglisi, A., Bianchi, A., Lemmens, L., Damay, P. & Shore, D. Distinct roles for yeast Stn1 in telomere capping and telomerase inhibition. EMBO J. 27, 2328–2339 (2008).

                                  Miyagawa, K. et al. SUMOylation regulates telomere length by targeting the shelterin subunit Tpz1(Tpp1) to modulate shelterin–Stn1 interaction in fission yeast. Proc. Natl Acad. Sci. USA 111, 5950–5955 (2014).

                                  Taggart, A. K., Teng, S. C. & Zakian, V. A. Est1p as a cell cycle-regulated activator of telomere–bound telomerase. Science 297, 1023–1026 (2002).

                                  Petreaca, R. C. et al. Chromosome end protection plasticity revealed by Stn1p and Ten1p bypass of Cdc13p. Nat. Cell Biol. 8, 748–755 (2006).

                                  Bochkareva, E., Korolev, S., Lees-Miller, S. P. & Bochkarev, A. Structure of the RPA trimerization core and its role in the multistep DNA-binding mechanism of RPA. EMBO J. 21, 1855–1863 (2002).

                                  Price, C. M. et al. Evolution of CST function in telomere maintenance. Cell Cycle 9, 3157–3165 (2010).

                                  Wan, B. et al. The Tetrahymena telomerase p75–p45–p19 subcomplex is a unique CST complex. Nat. Struct. Mol. Biol. 22, 1023–1026 (2015).

                                  Jiang, J. et al. Structure of telomerase with telomeric DNA. Cell 173, 1179–1190.e13 (2018).

                                  Lim, C. J. et al. The structure of human CST reveals a decameric assembly bound to telomeric DNA. Science 368, 1081–1085 (2020).

                                  Vodenicharov, M. D., Laterreur, N. & Wellinger, R. J. Telomere capping in non-dividing yeast cells requires Yku and Rap1. EMBO J. 29, 3007–3019 (2010).

                                  Vodenicharov, M. D. & Wellinger, R. J. DNA degradation at unprotected telomeres in yeast is regulated by the CDK1 (Cdc28/Clb) cell-cycle kinase. Mol. Cell 24, 127–137 (2006).

                                  Larrivee, M., LeBel, C. & Wellinger, R. J. The generation of proper constitutive G-tails on yeast telomeres is dependent on the MRX complex. Genes Dev. 18, 1391–1396 (2004).

                                  Lue, N. F., Chan, J., Wright, W. E. & Hurwitz, J. The CDC13-STN1-TEN1 complex stimulates Pol ɑ activity by promoting RNA priming and primase-to-polymerase switch. Nat. Commun. 5, 5762 (2014).

                                  Grossi, S., Puglisi, A., Dmitriev, P. V., Lopes, M. & Shore, D. Pol12, the B subunit of DNA polymerase ɑ, functions in both telomere capping and length regulation. Genes Dev. 18, 992–1006 (2004).

                                  Wang, F. et al. The POT1–TPP1 telomere complex is a telomerase processivity factor. Nature 445, 506–510 (2007).

                                  Hellman, L. M. & Fried, M. G. Electrophoretic mobility shift assay (EMSA) for detecting protein–nucleic acid interactions. Nat. Protoc. 2, 1849–1861 (2007).

                                  Minor, W., Cymborowski, M., Otwinowski, Z. & Chruszcz, M. HKL-3000: the integration of data reduction and structure solution—from diffraction images to an initial model in minutes. Acta Crystallogr. D Biol. Crystallogr. 62, 859–866 (2006).

                                  Kabsch, W. Integration, scaling, space-group assignment and post-refinement. Acta Crystallogr. D Biol. Crystallogr. 66, 133–144 (2010).

                                  Adams, P. D. et al. PHENIX: a comprehensive Python-based system for macromolecular structure solution. Acta Crystallogr. D Biol. Crystallogr. 66, 213–221 (2010).

                                  Emsley, P., Lohkamp, B., Scott, W. G. & Cowtan, K. Features and development of Coot. Acta Crystallogr. D Biol. Crystallogr. 66, 486–501 (2010).

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                                  Wu, Z. F. et al. Rad6–Bre1-mediated H2B ubiquitination regulates telomere replication by promoting telomere-end resection. Nucleic Acids Res. 45, 3308–3322 (2017).

                                  Wu, Z. et al. Rad6–Bre1 mediated histone H2Bub1 protects uncapped telomeres from exonuclease Exo1 in Saccharomyces cerevisiae. DNA Repair (Amst.) 72, 64–76 (2018).

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                                  14: Module 11: Homeostasis - Biology

                                  To complete the full A Level in Biology, students must complete six modules and be assessed on all six at the end of two years of study &ndash i.e. in Year 13. Students will not sit external AS examinations as the A Level is no longer formed of &lsquoAS&rsquo plus &lsquoA2&rsquo. However, students will sit internal end of year assessments at the end of year 12, in &lsquoAS&rsquo style, to assess progress.

                                  Year 12 Content

                                  Module 1: Development of Practical Skills in Biology

                                  The content of this module is taught in the context of the biological content of other modules, because practical skills are developed when learning other topics. This module is designed to develop the skills of planning, implementing practical methods, analysis of results and evaluation. Evaluating methods and interpreting results of practical investigations will feature on exam papers furthermore, practical skills will be assessed by the teacher throughout the course and students receive a pass/fail practical certificate alongside their grade at the end (this is for full A Level Biology only).

                                  Module 2: Foundations in Biology

                                  This module presents the basic units from which all living organisms are formed: biological molecules and cells. Students learn the chemistry of biological systems and develop their understanding of cells far beyond GCSE level.

                                  Module 3: Exchange and Transport

                                  In this module, students learn how animals and plants exchange substances with their environment, both chemicals essential for survival and chemicals that need eliminating as wastes. The module also covers the transport systems of animals and plants, including the fascinating study of the human circulatory system.

                                  Module 4: Biodiversity, Evolution and Disease

                                  Biodiversity refers to the variety of living organisms on our planet and in specific habitats. Students study the importance of biodiversity and how it can be measured, as well as how the millions of species on Earth have evolved from a common ancestor. In the disease section of the module, students learn how communicable diseases are transmitted and how the human body, and indeed plants, can defend themselves against the pathogens that cause them.

                                  Year 13 Content

                                  Module 5: Communications, Homeostasis and Energy

                                  Organisms larger than a single cell must develop systems of communication between different body parts if the system is to be maintained in a steady state. This is the importance of the themes of communication and homeostasis in Biology: both are truly vital for survival. Similarly, systems to produce food using sunlight energy and systems to release the energy stored in foods are essential for life on Earth. Thus students learn the biochemistry of photosynthesis and respiration in this module.

                                  Module 6: Genetics, Evolution and Ecosystems

                                  Students explore the role of genes in producing characteristics in living organisms, and how genes can change, appear or disappear over time, leading to the production of new species and, indeed, extinction of species. This module also delves into the incredible array of genetic manipulation techniques now possible, before moving onto the impact of human beings on ecosystems around the globe.

                                  Assessment

                                  Students sit examinations in their A Level courses at the end of Year 13. All students will take internal end of year examinations at the end of year 12 to determine suitability to continue with the subject in Year 13. Students who do not meet the required pass grade in the Year 12 end of year examinations will not be permitted to progress into Year 13.

                                  There are three written papers to assess A Level Biology. &lsquoBiological Processes&rsquo and &lsquoBiological Diversity&rsquo are each 2 hours 15 minutes, and each account for 37% of the A Level. The former assess modules 1, 2, 3 and 5. The latter assess content in modules 1, 2, 4 and 6. The third exam is called &lsquoUnified Biology&rsquo, and lasts for 1 hour 30 minutes. This accounts for the remaining 26% of the A Level. The practical skills of students are assessed throughout the course, leading to a separate certificate called &lsquoPractical Endorsement in Biology&rsquo &ndash this is simply pass/fail depending on skills shown throughout the course.


                                  14: Module 11: Homeostasis - Biology

                                  School Biology: Homeostasis: Endocrine system, adrenaline & thyroxine hormones

                                  Hormones 1. Introduction to the endocrine system of hormones

                                  e.g. homeostasis and the function of the hormones adrenaline and thyroxine including explaining negative feedback systems

                                  (other hormones are dealt with on other pages)

                                  Doc Brown's school biology revision notes: GCSE biology, IGCSE biology, O level biology,

                                  US grades 8, 9 and 10 school science courses or equivalent for

                                  14-16 year old students of biology

                                  This page will help you answer questions such as . What do endocrine glands do? What do hormone molecules do? What is the role of thyroxine in our metabolism? How does adrenaline prepare us in an 'emergency' situation? What is a negative feedback system? How does a negative feedback system work - its function?

                                  Sub-index for this page

                                  and for plants see Hormone control of plant growth and uses of plant hormones gcse biology revision note s

                                  Hormones are produced in the endocrine gland system and are transported by the blood to their target cells, tissues or organs

                                  Many process within the body are coordinated and controlled by chemical substances called hormones.

                                  You can think of hormones as chemical messengers sent around the bloodstream.

                                  Hormones are often quite large organic molecules.

                                  Hormones and the nervous system send information around the body.

                                  Hormones, being directly released into the blood, are quite rapidly carried to all parts of the body BUT only affect the function of particular cells, tissue or organs - the 'targets''.

                                  Hormones, in acting as 'chemical messages', trigger particular biochemical reactions in various types of tissue and organs.

                                  Hormones control functions in cells, tissue and organs that need constant adjustment and their effect is relatively long-lasting compared to eg the nervous responses of a reflex arc.

                                  The activated cells are called 'target cells' and have a chemical receptor that responds to the hormone - the hormones work on effectors.

                                  You should appreciate that the nervous system and hormones enable us to respond to external changes and also help us to control conditions inside our bodies.

                                  BUT, unlike the nervous system, the hormone response times are slower e.g. minutes or hours, an exception is adrenaline , which is the fastest acting hormone response.

                                  Know that hormones are used in some forms of contraception and in fertility treatments .

                                  (b) Examples of hormones and which gland or organ produces them

                                  Hormones are produced in, and secreted by, various glands called endocrine glands - hence the overall description - the endocrine system - sources and examples of which are described below.

                                  Endocrine glands secrete hormones directly into the bloodstream.

                                  Pituitary gland

                                  The pituitary is a small gland at the base of the brain

                                  The pituitary gland produces many hormones that regulate conditions in the body and growth hormone is important for the development of the body.

                                  Some hormones have a direct effect on the body, but others have an indirect effect by causing other glands to release hormones.

                                  Which is why the pituitary gland is sometimes referred to as the master gland because these hormones act on other glands causing them to release other hormones to bring about changes somewhere in the body.

                                  The pituitary gland produces the hormones FSH and LH which are important control chemicals in the female menstrual cycle . These act on the ovaries and testes to release reproductive hormones which control the release of eggs from the ovaries and the birth of a baby.

                                  FSH = Follicle stimulating hormone and LH = Luteinizing hormone

                                  The pituitary gland secretes the hormone TSH which acts on the thyroid gland - which in turn secretes thyroxine hormone (see next section).

                                  TSH = Thyroid stimulating hormone

                                  It also produces the hormone ACTH which acts on the adrenal gland to secrete the hormone adrenal hormone.

                                  ACTH = Adrenocorticotropic hormone

                                  The pituitary produces the growth hormone STH, which acts on the whole body - if very deficient in STH for a long time you may be of short stature and dwarfism, and, if too much of STH for a long time you have excessive growth in stature, organ enlargement and suffer from functional disorders such as diabetes and heart disease.

                                  STH = somatotropin or somatotropic hormone

                                  ADH is a hormone that is produced in a part of the brain called the hypothalamus. It is then stored and released from the pituitary gland. ADH acts on the kidneys to control the amount of water excreted in the urine.

                                  Thyroid gland

                                  The thyroid gland is attached to the trachea. The thyroid gland produces thyroxine which takes part in regulating functions such as the rate of chemical reactions in metabolism, heart rate and temperature control - its production is triggered by the hormone TSH produced in the pituitary gland.

                                  See also notes on thyroxine and Homeostasis - thermoregulation, control of temperature

                                  The adrenal gland are the top of the kidneys. The adrenal gland produces adrenaline which is used by the body to prepare for 'fight or flight' e.g. helps your body for action if you suffer trauma or find yourself in danger - its production is triggered by the ACTH hormone from the pituitary gland. (see notes on adrenaline )

                                  The pancreas is situated below the stomach.

                                  The pancreas produces insulin which regulates the glucose concentration ('level') in blood.

                                  See Homeostasis - control of blood sugar level - insulin and diabetes

                                  The ovaries (female only)

                                  The ovaries produce the sex hormone oestrogen which is part of the chemistry of the menstrual cycle .

                                  Oestrogen gives girls their female features such as breasts, soft skin, feminine voice and prepares the womb for a baby.

                                  The testes (male only)

                                  The testes produce testosterone, a hormone that controls puberty and sperm production in males.

                                  Testosterone is a sex hormone that gives boys their male features such as deeper voices and more body hair than females.

                                  (c) A comparison of the nervous system and the endocrine hormone system

                                  Hormones effectively act as 'chemical messages' to trigger particular biochemical reactions and their effects are slower than nervous system.

                                  The effect of hormones is more general around the body, but tend to affect particular cells in particular organs, and relatively long-lasting effect compared to eg the fast but short-term nervous impulses and responses of a reflex arc.

                                  Generally speaking, if your body's response to a situation is relatively long lasting (e.g. minutes or hours) its probably a function of the hormone system.

                                  Some hormones like adrenaline , can act quite quickly - see notes further down.

                                  Nerves: Compared to the hormone system of response and control in the body, the nervous system , using nerve signals which are electrical in nature (not chemical).

                                  The nervous system of neurones acts very fast e.g. a short burst of a nerve impulse for a short time, acting from one precise area to another in the body.

                                  Generally speaking, if your body's response is fast, its probably a nervous reaction.

                                  There are situations when information needs to be passed to your effectors quickly!

                                  Examples of when nerve signal information has to be passed to effectors quickly include nerve signals from your retina, pain receptors, taste buds warning of danger etc. must be processed in microseconds, NOT minutes! too late!

                                  Hormones act too slowly to be of use in most dangerous 'split second' decision making situations.

                                  Hormone levels and negative feedback

                                  Your body can control the level of hormones in the blood using a negative feedback system.

                                  If the body detects that a level of a substance X is above or below the normal level it triggers a response to bring the level of substance X back to normal again.

                                  A good example is the way thyroxine regulates metabolism (see thyroxine notes and graph below)

                                  See also examples of homeostasis

                                  Homeostasis - introduction to how it functions (negative feedback systems explained)

                                  Homeostasis - control of blood sugar level - insulin and diabetes gcse biology revision notes

                                  Homeostasis - osmoregulation, ADH, water control, urea and ion concentrations and kidney function

                                  Homeostasis - thermoregulation, control of temperature gcse biology revision notes

                                  (d) The function of the hormone adrenaline

                                  When you suddenly feel in danger or get a shock (physical or mental) your adrenal gland quite rapidly releases the hormone adrenaline into your bloodstream and distributed all around your body.

                                  The adrenal glands are found just above the kidneys.

                                  Adrenaline causes, what is often described as, the 'fight or fight' response - in other words your body is quite rapidly (by hormonal standards) being prepared to deal with a threat of some kind.

                                  This happens when your brain detects fear or stress (dangerous situation, confrontation etc.) and immediately sends nerve impulses to the adrenal glands which then secrete the hormone adrenaline into the bloodstream to prepare your body for action!

                                  The initial stimulation might be visual, physical or mental.

                                  Note the interaction between the nervous system (electrical impulses in nerve fibres - neurones) and the endocrine system (secretion of hormone molecules into the bloodstream).

                                  There are nerve connections between the brain and adrenal gland - a part of the adrenal gland called the adrenal medulla responds to the nerve signal from the brain (CNS) by releasing the hormone adrenaline.

                                  The secreted adrenaline is carried round in the blood and acts on various parts of the body.

                                  The effects of adrenaline on the body are described below,

                                  The surge in adrenaline levels triggers an increase in heart rate and breathing rate to increase the supply of oxygen and glucose to the cells of your brain and muscles.

                                  The increase in respiration releases more thermal energy and your body temperature rises - but, if it becomes too high, the thermoregulatory centre in the brain detects this and the adrenaline secretion is blocked.

                                  Note that the body's volume of blood is fairly constant, so heart rate must increases to pump more blood around the body at a greater rate to carry extra glucose and oxygen to the muscle cells.

                                  The adrenaline molecules do this by binding to specific receptors in the heart causing the heart muscles to contract more frequently and more forcefully - this increases your heart rate and blood pressure, hence more glucose and oxygen to your cells through your bloodstream e.g. it gives the cells of the muscle tissue extra energy to contract and prepare to fight or flee!

                                  Adrenaline also binds to receptors in the liver causing the cells to increase in the rate of breakdown of glycogen (chemical potential energy store) to increase the level of glucose in the bloodstream for respiration - particularly muscle cells (in limbs or heart).

                                  To increase the rate of respiration you also need more glucose, so the hormone adrenaline performs two functions to increase energy output.

                                  Note that the metabolism of glucose is controlled by three hormones, here its adrenaline acting on the liver, but there is also the action of insulin and glucagon in maintaining the balanced level of glucose in the blood.

                                  Footnote - above is not quite the full "fight or flight" story - another hormone comes into play too!

                                  When the brain responds to the initial stimulus and triggers the release of adrenaline, this hormone from the adrenal gland, cannot alone do everything required in a 'fight or flight' situation.

                                  Simultaneously, the brain also signals the pituitary gland to release a hormone (name ?) that acts on a different part of the adrenal gland to release a 2nd hormone called cortisol, and it this steroid hormone that sustains our response to danger - most cells in the body have cortisol receptors.

                                  This is another example of several hormones jointly controlling a situation.

                                  Also note that it is the hypothalamus links the nervous and endocrine systems by way of the pituitary gland - nervous responses working with hormone responses to keep us alive!

                                  (e) The function of the hormone thyroxine

                                  Thyroxine is a hormone made from iodine and amino acids, it is produced in, and released (secreted) by, the thyroid gland in the neck.

                                  Thyroxine has an important role in regulating the basal metabolic rate, the basic rate (speed) at which the chemical reactions of your body occur while your body is at rest.

                                  Thyroxine increases the rate of metabolism of all the body's cells.

                                  e.g. increases the rate of respiration, powering the cell's chemistry and releasing thermal energy

                                  Thyroxine is also important for many other biochemical processes including facilitating protein synthesis - essential for growth and development.

                                  Problems with an underactive thyroid gland - symptoms of thyroxine deficiency

                                  Tiredness, sluggishness, increase in weight, slower heart rate,

                                  If a child has too little thyroxine it leads to slower growth and mental development.

                                  This potentially harmful situation begins in the uterus, continues in the embryo, through infancy and into childhood, if there is insufficient thyroxine

                                  A negative feedback system maintains the thyroxine concentration in the blood at the correct level.

                                  So, how does the negative feedback system regulate the level of thyroxine in the blood?

                                  Both the pituitary gland and hypothalamus (a small region at the base of the brain) control the thyroid and it is the hypothalamus, using TRH (thyrotropin releasing hormone), that alerts the pituitary gland to produce TSH (thyroid stimulating hormone).

                                  Please note, from now on I'll just use the abbreviations TRH and TSH.

                                  Thyroxine is produced in the thyroid gland, in response to the actions of two principal hormones:

                                  The hormone TRH (from the hypothalamus), stimulates the production of TSH which is made and secreted from the pituitary gland into the bloodstream.

                                  In turn, the production of TSH stimulates the thyroid gland to make more thyroxine.

                                  TSH binds to receptors located on the cells of the thyroid gland to stimulate production of thyroxine.

                                  We now put these two hormonal actions, 'forward and reverse' into the negative feedback system.

                                  I've also added a graph to go with the text below.

                                  If your body detects that the level of thyroxine has risen above 'normal', the hypothalamus stops releasing TRH.

                                  This tells the pituitary gland to stop producing TSH (blocks secretion) inhibiting the production of thyroxine in the thyroid.

                                  In reducing the amount of thyroxine secreted from the pituitary gland, the thyroxine level falls down to normal (1st half of graph below) and your metabolic rate is reduced to 'normal' i.e. becomes stabilised again.

                                  Apparently, a higher than normal thyroxine level also reduces the secretion of TSH from the pituitary gland (i.e. without the intervention of the level of TRH from the hypothalamus).

                                  If your body detects the level of thyroxine has fallen below 'normal', the hypothalamus is stimulated to release TRH.

                                  The release of TRH stimulates the pituitary gland to release TSH.

                                  The TSH stimulates the thyroid gland to produce more thyroxine, whose level rises back to normal (2nd half of graph) and your metabolic rate increases to 'normal' i.e. becomes stabilised again.

                                  Note:

                                  If the body temperature falls, the body produces more thyroxine to increase the rate of respiration and release more thermal energy.

                                  But, since the increase in respiration releases more thermal energy and your body temperature rises, if it becomes too high, the thermoregulatory centre in the brain detects this and the adrenaline secretion is blocked.

                                  The negative feedback system is illustrated in the graph below.

                                  General comment on the graph and negative feedback systems

                                  Using a negative feedback system, your body controls the levels of hormones, and other substances in the blood.

                                  When your body detects that the level of a substance X is too high above the 'normal' level, or too low below the 'normal' level, it triggers a response to bring the level of substance X back up/down to its normal level.

                                  Thyroid gland problems

                                  e.g. if you have an underactive thyroid gland, it can cause your body to gain unnecessary weight.

                                  This is because too little thyroxine is produced and your metabolic rate falls.

                                  As a result, less of the glucose from your food intake is used up in respiration, so the excess glucose is converted to, and stored as, fat.

                                  Fortunately, the remedy, in most cases, is to take thyroxine tablets every day.

                                  and for plants see Hormone control of plant growth and uses of plant hormones gcse biology revision notes


                                  Watch the video: What is Homeostasis? Physiology. Biology. FuseSchool (July 2022).


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