Human respiration system model

Human respiration system model

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I'm a physiology student and very interested in the mechanical model of lungs (meaning variation of pressures during respiration cycle, elasticity function etc., and the way they cooperate with each other). What textbooks or articles you can suggest to find out about this topic? Thank you!

16.2: Structure and Function of the Respiratory System

  • Contributed by Suzanne Wakim & Mandeep Grewal
  • Professors (Cell Molecular Biology & Plant Science) at Butte College

Why can you &ldquosee your breath&rdquo on a cold day? The air you exhale through your nose and mouth is warm, like the inside of your body. Exhaled air also contains a lot of water vapor because it passes over moist surfaces from the lungs to the nose or mouth. The water vapor in your breath cools suddenly when it reaches the much colder outside air. This causes the water vapor to condense into a fog of tiny droplets of liquid water. You release water vapor and other gases from your body through the process of respiration.

Figure (PageIndex<1>): Breath on a cold day

Human Respiratory System ACTIVITY Worksheet - FREE (Science, Biology)

This free interactive activity on human respiratory system comes in two-sides foldable format, is fun and will help students label the parts of this human body system. Observing, identifying and labeling human organs by pinpointing their exact location in the human body is a good way to understand the human systems.

: Explore other activity / worksheets, task cards, all-in-1 packs and bundled teaching resources on by visiting the following links:

What does THIS FREE product include?

It is a two-sides foldable activity / worksheet in printable PDF format showing the human respiratory system. It is accompanied with a complete set of instructions to execute the activity using the provided worksheet PDF. A completed sample worksheet / answer key is provided in this product.

The students will:

1. Label the parts of the human respiratory system

2. Have fun coloring each part.

This activity has been used in classrooms and proven to be a useful biology resource for teaching the human respiratory system topic. It can be used as a notebook activity, homework sheet and can be also be utilized for revision by students.

Questions on ‘Human Respiratory System Activity Worksheet’?

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16.3 Circulatory and Respiratory Systems

Animals are complex multicellular organisms that require a mechanism for transporting nutrients throughout their bodies and removing wastes. The human circulatory system has a complex network of blood vessels that reach all parts of the body. This extensive network supplies the cells, tissues, and organs with oxygen and nutrients, and removes carbon dioxide and waste compounds.

The medium for transport of gases and other molecules is the blood, which continually circulates through the system. Pressure differences within the system cause the movement of the blood and are created by the pumping of the heart.

Gas exchange between tissues and the blood is an essential function of the circulatory system. In humans, other mammals, and birds, blood absorbs oxygen and releases carbon dioxide in the lungs. Thus the circulatory and respiratory system, whose function is to obtain oxygen and discharge carbon dioxide, work in tandem.

The Respiratory System

Take a breath in and hold it. Wait several seconds and then let it out. Humans, when they are not exerting themselves, breathe approximately 15 times per minute on average. This equates to about 900 breaths an hour or 21,600 breaths per day. With every inhalation, air fills the lungs, and with every exhalation, it rushes back out. That air is doing more than just inflating and deflating the lungs in the chest cavity. The air contains oxygen that crosses the lung tissue, enters the bloodstream, and travels to organs and tissues. There, oxygen is exchanged for carbon dioxide, which is a cellular waste material. Carbon dioxide exits the cells, enters the bloodstream, travels back to the lungs, and is expired out of the body during exhalation.

Breathing is both a voluntary and an involuntary event. How often a breath is taken and how much air is inhaled or exhaled is regulated by the respiratory center in the brain in response to signals it receives about the carbon dioxide content of the blood. However, it is possible to override this automatic regulation for activities such as speaking, singing and swimming under water.

During inhalation the diaphragm descends creating a negative pressure around the lungs and they begin to inflate, drawing in air from outside the body. The air enters the body through the nasal cavity located just inside the nose (Figure 16.9). As the air passes through the nasal cavity, the air is warmed to body temperature and humidified by moisture from mucous membranes. These processes help equilibrate the air to the body conditions, reducing any damage that cold, dry air can cause. Particulate matter that is floating in the air is removed in the nasal passages by hairs, mucus, and cilia. Air is also chemically sampled by the sense of smell.

From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box) as it makes its way to the trachea (Figure 16.9). The main function of the trachea is to funnel the inhaled air to the lungs and the exhaled air back out of the body. The human trachea is a cylinder, about 25 to 30 cm (9.8–11.8 in) long, which sits in front of the esophagus and extends from the pharynx into the chest cavity to the lungs. It is made of incomplete rings of cartilage and smooth muscle. The cartilage provides strength and support to the trachea to keep the passage open. The trachea is lined with cells that have cilia and secrete mucus. The mucus catches particles that have been inhaled, and the cilia move the particles toward the pharynx.

The end of the trachea divides into two bronchi that enter the right and left lung. Air enters the lungs through the primary bronchi . The primary bronchus divides, creating smaller and smaller diameter bronchi until the passages are under 1 mm (.03 in) in diameter when they are called bronchioles as they split and spread through the lung. Like the trachea, the bronchus and bronchioles are made of cartilage and smooth muscle. Bronchi are innervated by nerves of both the parasympathetic and sympathetic nervous systems that control muscle contraction (parasympathetic) or relaxation (sympathetic) in the bronchi and bronchioles, depending on the nervous system’s cues. The final bronchioles are the respiratory bronchioles. Alveolar ducts are attached to the end of each respiratory bronchiole. At the end of each duct are alveolar sacs, each containing 20 to 30 alveoli . Gas exchange occurs only in the alveoli. The alveoli are thin-walled and look like tiny bubbles within the sacs. The alveoli are in direct contact with capillaries of the circulatory system. Such intimate contact ensures that oxygen will diffuse from the alveoli into the blood. In addition, carbon dioxide will diffuse from the blood into the alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems. Estimates for the surface area of alveoli in the lungs vary around 100 m 2 . This large area is about the area of half a tennis court. This large surface area, combined with the thin-walled nature of the alveolar cells, allows gases to easily diffuse across the cells.

Visual Connection

Which of the following statements about the human respiratory system is false?

Human model systems for human biology

The past years have seen substantial progress in the development and use of human-cell-based model systems for the study of human biology, including both pluripotent and AdSC-based systems.

The introduction of human iPSC technology

The first human PSCs, known as human ESCs, were reported in 1998 (ref. 44 ), nearly two decades after the initial discovery of murine pluripotent cell lines in 1981 (refs 45,46 ). The human stem cell lines were predicted to be useful for the study of human developmental biology, drug discovery and cell therapy — in spite of the technical limitations imposed by the lack of knowledge of appropriate differentiation protocols at that time. As the generation of human PSC lines required the sacrifice of human embryos at the blastocyst stage, strong ethical concerns were raised 47 .

In 2007 the debate around human PSCs was largely circumvented by the introduction of human iPSC technology, which converts an ordinary differentiated fibroblast from an adult human into a pluripotent cell 33 . Reprogramming to a pluripotent state is usually achieved by forced expression of a specific set of transcription factors 33 , and the pluripotent cells can then be differentiated into specific cell types. iPSC technology not only allowed the generation of patient-specific stem cells but also enabled researchers to work with an unlimited supply of human stem cells and stem cell-derived tissues. Furthermore, iPSC technology allows the banking of patient-derived stem cells 48 . There are still technical limitations when using iPSCs — for example, the use of oncogenes for reprogramming, the genetically unstable nature of the reprogramming and the low efficiency of reprogramming. All these technical hurdles made it difficult to obtain error-free iPSC lines from patients 49 . But these limitations have been at least partly overcome by the use of non-integrating vectors and standardized quality control protocols to avoid or screen out unwanted genetic alterations 49 . Line-to-line variability caused by the genetic heterogeneity of humans has also been resolved by the use of isogenic controls generated by genetic engineering using CRISPR–Cas9 technology 50 . These improvements have enabled researchers to use iPSC-derived specialized cell types — for example, neurons, cardiomyocytes, haematopoietic progenitor cells and pancreatic β-cells — in disease modelling and drug screening 51 . However, just over a decade after their introduction, 3D organ culture methods have greatly increased the utility of human iPSCs.

IPSC-derived organoid models

Human PSC-derived organoids are generated by guided differentiation protocols that mimic developmental processes identified through previous work, both in vitro and in vivo (Fig. 2). As our knowledge of human development is very limited, most early studies that aimed at producing organoids with properties similar to those of human tissues were based on parallels drawn from mouse development. In principle, to generate an organoid, the entire process of organ development from PSCs should be faithfully mimicked. In reality, it is nearly impossible in vitro to provide all biochemical cues that drive cell differentiation and 3D tissue assembly and organization at precisely the right times, places and concentrations at which they would occur during embryonic development. Fortunately, cells in vitro tend to follow a semi-autonomous differentiation trajectory, as they do in vivo, and three main types of protocols are utilized to generate functional organoids.

Pluripotent stem cell (PSC)-derived organoids are established following directed differentiation of PSCs, which requires a first step that involves germ-layer specification (endoderm, mesoderm or ectoderm), followed by induction and maturation, by culturing with specific growth and signalling factors to obtain the specific cell types that form the desired organ. Some PSC-derived organoids may contain cells from multiple germ layers to closely mimic the in vivo counterpart. Adult stem cell (AdSC)-derived organoid cultures require isolation of the tissue-specific stem cell population, which can then be embedded into an extracellular matrix (ECM) with defined, tissue-specific combinations of growth factors to allow propagation. AdSC-derived organoids, as shown here, are of epithelial origin and lack a mesenchymal or immune component unless it is added separately. Signalling components that are important for guided differentiation and niche function are shown activated signalling pathways are shown in green, and inhibited ones in red. BMP, bone morphogenetic protein EGF, epidermal growth factor FGF, fibroblast growth factors HGF, hepatocyte growth factor IGF, insulin-like growth factor ROCK, RHO-associated protein kinase TGF, transforming growth factor VEGF, vascular endothelial growth factor.

In the case of brain organoids, human PSCs are initially guided to differentiate into embryoid bodies before further differentiation towards the neuroectodermal lineage. Once the cell aggregates contain the developmental precursors for brain tissue patterning, the rest of the developmental steps occur spontaneously in a spinning bioreactor 5,52 . For the development of liver primordia, guided differentiation to hepatoblasts (hepatocyte precursors) was not sufficient to form a complete set of organ precursors, which requires cells from different lineages. It was known from studies of mouse hepatogenesis that cell-to-cell communication between endothelial, mesenchymal and hepatic endoderm cells was important, so based on this knowledge the first human liver primordia were created by mixing human PSC-derived hepatoblasts, mesenchymal cells and endothelial cells. Through self-condensation and organization, these three cell types assemble in vitro to make an aggregate that mimics the architecture of developing human liver primordia 23,53 . Other organoids require more precise, lengthy protocols in order to acquire the appropriate progenitor cell types for the target epithelium. Many organoids that model endoderm-derived organs (such as oesophagus, stomach, colon, intestine and lung) undergo stepwise differentiation protocols, in which the timing, concentration and combination of specific growth factors and chemical inhibitors for modulating key developmental signalling pathways are crucial to developing the desired epithelial tissue in a manner analogous to fetal development 24,43,54,55,56,57 .

In all cases, the process of organoid formation involves three crucial steps. First, key signalling pathways regulating developmental patterning are activated or inhibited (using commercially available morphogens and signalling inhibitors) in order to establish a correct regional identity during stem cell differentiation. Usually this is achieved through induction of the signalling events that have been identified in the mouse as establishing cell fates in vivo. Second, media formulations that allow proper terminal differentiation of the desired cell types within the organoid are developed, generally following established methods of 2D culture or inspired by the murine developmental process. Finally, cultures are grown in a way that allows their expansion in three dimensions, which is achieved either by aggregating cells into 3D structures or by embedding the cultures into a 3D matrix.

Derivation of the first mouse and human intestinal organoids

AdSCs can be cultured in the presence of niche factors that function to maintain the stem cells in an undifferentiated state while allowing stem cell differentiation. As a result, the cultured stem cells can generate AdSC-derived organoids, which are composed of an epithelial monolayer that mimics the 3D architecture and contains the cell types of the desired organ to be modelled 58 . The first steps in the development of AdSC-derived organoids, however, are to identify and isolate the appropriate population of adult stem cells and to understand their niche requirements. Obtaining the first mouse intestinal organoid cultures required not only the identification of the intestinal stem cell population expressing the selective marker for these cells, the leucine-rich repeat-containing G protein-coupled receptor 5 (Lgr5), but also the essential niche factors to support stem cell activity 59 .

The re-creation of the intestinal stem cell niche in vitro was inspired by mouse genetic studies that had shown that epithelial proliferation and stem cell self-renewal are dependent on epidermal growth factor (EGF) and WNT activity, while differentiation is controlled by bone morphogenetic protein (BMP) signalling. Hence, the first culture medium that was successfully used to obtain mouse intestinal organoids contained EGF as a mitogen, R-spondin to activate the WNT signalling pathway and Noggin to block BMP activity 2 . However, the conditions identified as the minimum niche environment for the maintenance of mouse intestinal stem cells were not directly applicable to human intestinal or colonic organoid cultures. Unlike the Paneth cells that secrete some of the niche factors found in mouse intestinal organoid cultures, human gut organoid cultures are devoid of fully functional Wnt-secreting niche cells (for example, Paneth cells), and so require that exogenous Wnt3a be added to the medium. Moreover, long-term maintenance of human gut organoids through multiple passages requires both inhibition of the TGFβ pathway, by addition of the chemical inhibitor A83-01, and inhibition of the p38 MAPK pathway, by addition of SB202190 (ref. 3 ). After it was discovered that SB202190 prevents secretory lineage differentiation, this inhibitor was replaced by the addition of insulin-like growth factor 1 (IGF1) and fibroblast growth factor 2 (FGF2) 4 this modification enables concurrent multilineage differentiation and self-renewal in human gut organoids. All these changes and refinements to culture conditions, which are the results of another decade of research, improved the quality of human gut organoid cultures, enabling researchers to produce organoids that more closely resemble tissue in vivo. Nevertheless, mouse organoids remain more similar to the in vivo tissue of origin than do human organoids.

Generation of other AdSC-derived organoids

Using the procedure described above, multiple types of organoids from various tissues have been generated by modifying the basic medium that was initially developed for intestinal organoid culture (Fig. 2 Fig. 3). In most cases, a mouse organoid culture system was first established and then adapted to human cells. Mouse colonic organoids can be grown through the addition of Wnt3a 3 . Mouse stomach pyloric organoids and corpus organoids were grown with only slight modifications to the original protocol for mouse intestinal organoids 60,61 . Mouse liver and pancreas ductal organoids were also obtained similarly from injury-activated Lgr5-positive progenitors 62,63 .

The chart provides information on the type (either pluripotent stem cell (PSC)-derived or adult stem cell (AdSC)-derived), biobanking status and use in disease modelling of the human organoid systems reported to date, summarized by organ.

The number of organoids generated from diverse mouse tissues and organs is growing, and for each, previous knowledge of the signalling processes that comprise the organ-specific or tissue-specific stem cell niche environment has been key to developing the appropriate protocols. AdSC-derived human organoids have also become widely available (Fig. 3). They have been generated from almost all endoderm-derived tissues (intestine, colon, stomach, liver, pancreas, lung, bladder and so forth) 3,7,41,64,65,66,67,68,69,70 and from gender-specific tissues (prostate, endometrium, fallopian tube and mammary gland) 8,71,72,73,74,75 . Additional components frequently required for the growth of human cultures, in comparison to the mouse cultures, are a TGFβ pathway inhibitor (such as A83-01) and a p38 MAPK inhibitor (such as SB202190) 9 . As for mouse organoids, human organoids can be derived from minimal amounts of tissue biopsies and can be cultured indefinitely, thus forming the basis for the building of living biobanks, which are an important resource in biomedical research.

Human organoids: model systems for human biology and medicine

The historical reliance of biological research on the use of animal models has sometimes made it challenging to address questions that are specific to the understanding of human biology and disease. But with the advent of human organoids - which are stem cell-derived 3D culture systems - it is now possible to re-create the architecture and physiology of human organs in remarkable detail. Human organoids provide unique opportunities for the study of human disease and complement animal models. Human organoids have been used to study infectious diseases, genetic disorders and cancers through the genetic engineering of human stem cells, as well as directly when organoids are generated from patient biopsy samples. This Review discusses the applications, advantages and disadvantages of human organoids as models of development and disease and outlines the challenges that have to be overcome for organoids to be able to substantially reduce the need for animal experiments.

Conflict of interest statement

The authors declare no competing interests.


Fig. 1. Comparison of organoids with other…

Fig. 1. Comparison of organoids with other model systems.

The most common model organisms that…

Fig. 2. Process for the establishment of…

Fig. 2. Process for the establishment of human PSC-derived and AdSC-derived organoids.

Regulation of Breathing

To understand how breathing is regulated, you first need to understand how breathing occurs.

How Breathing Occurs

Inhaling is an active movement that results from the contraction of a muscle called the diaphragm. The diaphragm is large, sheet-like muscle below the lungs (see Figure below). When the diaphragm contracts, the ribcage expands and the contents of the abdomen move downward. This results in a larger chest volume, which decreases air pressure inside the lungs. With lower air pressure inside than outside the lungs, air rushes into the lungs. When the diaphragm relaxes, the opposite events occur. The volume of the chest cavity decreases, air pressure inside the lungs increases, and air flows out of the lungs, like air rushing out of a balloon.

Breathing depends on contractions of the diaphragm.

Control of Breathing

The regular, rhythmic contractions of the diaphragm are controlled by the brain stem. It sends nerve impulses to the diaphragm through the autonomic nervous system. The brain stem monitors the level of carbon dioxide in the blood. If the level becomes too high, it &ldquotells&rdquo the diaphragm to contract more often. Breathing speeds up, and the excess carbon dioxide is released into the air. The opposite events occur when the level of carbon dioxide in the blood becomes too low. In this way, breathing keeps blood pH within a narrow range.

Process of Respiration in Human Beings (With Diagram)

It means the inflow (inspiration) and outflow (expiration) of air between atmosphere and the alveoli of the lungs. It is affected by the expansion and contraction of lungs. There are mainly two processes by which the lungs are expanded or contracted.

(i) The downward and upward movement of the diaphragm which increases and de­creases the diameter of the thoracic cavity (chest cavity).

(ii) The elevation and depression of the ribs, which lengthens and shortens the thoracic cavity.

It is a process by which fresh air enters the lungs. The diaphragm, intercostal muscles and abdominal muscles play an important role.

The diaphragm becomes flat and gets lowered by the contraction of its muscle fibres thereby increasing the volume of the thoracic cavity in length.

(ii) External intercostal muscles:

They occur between the ribs. These muscles contract and pull the ribs and sternum upward and outward thus increasing the volume of the thoracic cavity.

(iii) Abdominal Muscles:

These muscles relax and allow compression of abdominal organs by the diaphragm. The abdominal muscles play a passive role in inspiration. The muscles of the diaphragm and external intercostal muscles are principal muscles of inspiration.

Movement of Fresh Air into the Lungs:

Thus overall volume of the thoracic cavity increases and as a result there is a decrease of the air pressure in the lungs.

The greater pressure outside the body now causes air to flow rapidly into external nares (nostrils) and through nasal cavities into internal nares.

Thereafter the sequence of air flow is like this:

External nares → Nasal cavities → Internal nares → Pharynx → Glottis → Larynx → trachea → Bronchi → bronchioles → alveolar ducts → alveoli.

From the alveoli oxygen passes into the blood of the capillaries and carbon dioxide diffuses out from the blood to the lumen of the alveoli.

It is a process by which the foul air (carbon dioxide) is expelled out from the lungs. Expiration is a passive process which occurs as follows.

The muscle fibres of the diaphragm relax making it convex, decreas­ing volume of the thoracic cavity.

(ii) Internal Intercostal Muscles:

These muscles contract so that they pull the ribs downward and inward decreasing the size of me thoracic cavity.

(iii) Abdominal Muscles:

Contraction of the abdominal muscles such as external and internal oblique muscles compresses the abdomen and pushes its contents (viscera) to­wards the diaphragm. The internal intercostal and abdominal muscles are muscles of expiration.

Movement of Foul Air out of the lungs:

Thus overall volume of the thoracic cavity decreases and foul air goes outside from the cavities of the alveoli in the following manner:

Alveoli → alveolar ducts → bronchioles → bronchi → trachea → larynx → glottis → pharynx → internal nares → nasal cavities → external nares → outside. The process of expiration is simpler than that of inspiration.

Thoracic Vs. Abdominal Breathing:

In human males, lateral movement of thorax constitutes 25% of breathing while abdomi­nal movement accounts for 75% of breathing. In pregnant women, almost the entire breath­ing is through lateral movement of thorax. Therefore, breathing of women is often regarded as thoracic while that of males as abdominal.

Advantages of Nasal Breathing:

Breathing through nose is healthier because it is a natural process. The air which is inhaled contains dust, bacteria, etc., get filtered in the nose. Thus the air which goes into lungs is cleaner. The conchae of the nose also filter and warm up the air.

Respiratory or Pulmonary Volumes and Capacities:

The quantities of air the lungs can receive, hold or expel under different conditions are called pulmonary (= lung) volumes. Combinations of two or more pulmonary volumes are called pulmonary (= lung) capacities.

Respiratory or Pulmonary Volumes (Lung Volumes):

It is the volume of air inspired or expired during normal breath. This is about 500 mL, i.e., a healthy man can inspire or expire about 6000 to 8000 mL of air per minute. The lowest value is of tidal volume.

2. Inspiratory Reserve Volume (IRV):

It is the extra amount of air that can be inspired forcibly after a normal inspiration. Thus it is forced inspiration. It is about 2500 to 3000 ml. of air.

3. Expiratory Reserve Volume (ERV):

It is the extra amount of air that can be expired forcibly after a normal expiration. Thus it is forced expiration. It is about 1000 to 1100 ml.

It is the volume of air which remains still in the lung after the most forceful expiration. It is about 1100 mL to 1200 ml.

Respiratory or Pulmonary Capacities (Lung Capacities):

1. Inspiratory Capacity (IC):

It is the total volume of air a person can inspire after a normal expiration. It includes tidal volume and inspiratory reserve volume (TV + IRV).

2. Expiratory Capacity (EC):

It is the total volume of air a person can expire after a normal inspiration. This includes tidal volume and expiratory reserve volume (TV + ERV).

3. Functional Residual Capacity (FRC):

Volume of air that will remain in the lungs after a normal expiration is called functional residual capacity. This includes residual volume and the expiratory reserve volume (RV + ERV).

The maximum volume of air a person can breathe in after a forced expiration or the maximum volume of air a person can breathe out after a forced inspiration is called vital capacity. This includes tidal volume, inspiratory reserve volume and expiratory reserve volume (TV + IRV + ERV).

In fact total lung capacity minus residual volume is called vital capacity. VC varies from 3400 mL to 4800 ml. depending upon age, sex and height of the individual. The vital capacity is higher in athletes, mountain dwellers than in plain dwellers, in men than women and in the young ones than in the old persons.

5. Total Lung Capacity (TLC):

It is the total volume of air present in the lungs and the respiratory passage after a maximum inspiration. It includes vital capacity and the residual volume (VC + RV). All pulmonary volumes and capacities are about 20 to 25 per cent less in women than in men and they are greater in tall persons and athletes than in small and asthenic (slight build) people.

Respiratory Quotient (RQ):

Respiratory quotient is the ratio of the volume of carbon dioxide produced to the volume or oxygen consumed over a period of time in respiration.

RQ = Volume of CO2 evolved/Volume of O2 absorbed

Respiratory quotient varies with different foods utilized in respiration. For glucose, RQ (RQ 6CO2/6O2 – 1), for fats it is about 0.7, for proteins it is about 0.9 and for organic acids it is about 1.3 or 1.4.

In anaerobic respiration, there is no consumption of oxygen. Carbon dioxide is produced in most of the cases. Therefore R.Q. is infinity. The respiratory quotient indicates the type of food oxidized in the body of the animal during respiration.

(A) Exchange of gases between alveoli and blood (Fig. 17.9 & 17.11):

The exchange of gases (i.e., oxygen and carbon dioxide) between lung alveoli and pulmonary capillaries is called external respiration.

It occurs as follows:

The wall of the alveoli is very thin and has rich network of blood capillaries. Due to this, the alveolar wall seems to be a sheet of flowing blood and is called respiratory membrane (= alveolar-capillary membrane).

The respiratory membrane (Fig. 17.10) consists mainly of:

(b) Epithelial basement membrane,

(c) A thin interstitial space

(d) Capillary basement membrane and

All these layers form a mem­brane of 0.2 mm thickness. The respiratory membrane has a limit of gaseous exchange between alveoli and pulmonary blood. It is called diffusing capacity. The diffusing capacity is defined as the volume of gas that diffuses through the membrane per minute for a pressure difference of 1 mm Hg.

It is further dependent on the solubility of the diffusing gases. In other words at the particular pressure difference, the diffusion of carbon dioxide is 20 times faster than oxygen and that of oxygen is two times faster than nitrogen.

The partial pressure of oxygen (PO2) in the alveoli is higher (104 mm Hg) than that in the deoxygenated blood in the capillaries of the pulmonary arteries (95 mm Hg.). As the gases diffuse from a higher to a lower concentration, the movement of oxygen is from the alveoli to the blood. The reverse is the case in relation to carbon dioxide.

The partial pressure of carbon dioxide (PCO2) is higher in deoxygenated blood (45 mm Hg) than in alveoli (40 mm Hg), therefore, carbon dioxide passes from the blood to the alveoli. The partial pressure of nitrogen (PN2) is the same (537 mm Hg) in the alveoli as it is in the blood. This condition is maintained because nitrogen as a gas is not used up by the body.

(B) Exchange of gases between blood and tissue cells (Fig. 17.11):

The exchange of gases (i.e., oxygen and carbon dioxide) between tissue blood capillaries and tissue cells is called internal respiration. The partial pressure of oxygen is higher (95mm Hg) than that of the body cells (40 mm Hg) and the partial pressure of carbon dioxide is lesser (40 mm Hg) than that of the body cells (45 mm Hg).

Therefore, oxygen diffuses from the capillary blood to the body cells through tissue fluid and carbon dioxide diffuses from the body cells of the capillary blood via tissue fluid. Now the blood becomes deoxygenated. The latter is carried to the heart and hence to the lungs.

Transport of Gases (Fig. 17.11):

Blood transports oxygen and carbon dioxide.

(A) Transport of Oxygen in the Blood:

Blood carries oxygen from the lungs to the heart and from the heart to various body parts.

Oxygen is transported in the following manners:

(i) As dissolved gas. About 3 per cent of oxygen in the blood is dissolved in the plasma which carries oxygen to the body cells.

(ii) As oxyhaemoglobin. About 97% of oxygen is carried in combination with hae­moglobin of the erythrocytes.

Haemoglobin (Hb) consists of a protein portion called globin and a pigment portion called heme. The heme portion contains four atoms of iron, each capable of combining with a molecule of oxygen. Four molecules of oxygen bind one molecule of haemoglobin. Oxygen and haemoglobin combine in an easily reversible reaction to form oxyhaemoglobin.

Under the high partial pressure, oxygen easily binds with haemoglobin in the pulmonary (lung) blood capillaries. When this oxygenated blood reaches the different tissues, the partial pressure of oxygen declines and the bonds holding oxygen to haemoglobin become unstable. As a result, oxygen is released from the blood capillaries.

A normal person has about 15 grams of haemoglobin per 100 ml of blood. 1 gram of haemoglobin binds about 1.34 ml of O2. Thus on an average 100 ml of blood carries about 20 ml (19.4 ml exactly) of O2 Hence under normal conditions, about 5 ml of oxygen is transported to tissues by 100 ml. of blood.

During exercise or under strenuous conditions, the muscle cells consume more oxygen. The partial pressure of oxygen in the tissue falls, as a result of which, the blood at the tissue level has only 4.4 ml of oxygen/100 ml of blood. Thus about 15 ml. of oxygen is transported by haemoglobin during exercise.

Oxygen-haemoglobin Dissociation curve (=Oxygen Dissociation Curve):

The amount of oxygen that can bind with haemoglobin is determined by oxygen tension. This is expressed as a partial pressure of oxygen (PO2). The percentage of haemoglobin that is bound with O2 is called percentage saturation of haemoglobin.

The rela­tionship between the partial pressure of oxygen (PO2) and percentage satu­ration of the haemoglobin with oxy­gen (O2) is graphically illustrated by a curve called oxygen haemoglobin dissociation curve (also called oxy­gen dissociation curve).

Normal Oxygen Haemoglobin Dissociation Curve:

Under normal conditions, the oxygen haemoglobin dissociation curve is sigmoid shaped or ‘S’ shaped (Fig. 17.12). The lower part of the curve indicates dissocia­tion of oxygen from haemoglobin. The upper part of the curve indicates the acceptance of oxygen by haemoglo­bin.

When the partial pressure of oxy­gen is 25 mm Hg the haemoglobin gets saturated to about 50%. It means the blood contains 50% oxygen. The partial pressure at which the haemoglobin saturation is 50% is called P50. At 40 mm Hg of partial pressure of oxygen, the saturation is 75%. It becomes 95% when the partial pressure of oxygen is 100 mm Hg.

Factors Affecting Oxygen Haemoglobin Dissociation Curve:

The oxygen haemoglo­bin dissociation curve is shifted either to right or left by various factors.

Shift to right indicates dissociation of oxygen from haemoglobin.

The oxygen-haemoglobin curve is shifted to right in the following conditions:

(1) Decrease in partial pressure of oxygen.

(2) Increase in partial pressure of carbon dioxide (Bohr effect).

(3) Increase in hydrogen ion concentration and decrease in pH (acidity).

(4) Increased body temperature.

(5) Excess of 2, 3 diphosphoglycerate (DPG). DPG is a by-product in glyco­lysis. It is present in RBCs.

Shift to left indicates acceptance (association) of oxygen by haemoglobin.

The oxygen haemoglobin dissociation curve is shifted to left in the following conditions:

(1) In the foetal blood, because, foetal haemoglobin has more affinity for oxygen than the adult haemoglobin.

(2) In the low temperature and high pH.

An increase in carbon dioxide in the blood causes oxygen to be displaced from the haemoglobin. This is Bohr effect. This is an important factor increasing oxygen transport. It is named after the Danish physiologist Christian Bohr (1855-1911).

The pres­ence of carbon dioxide decreases the affinity of haemoglobin for oxygen and increases release of oxygen to the tissues. The pH of the blood falls as its CO2 content increases so that when the PCO2 rises the curve shifts to the right and the P50 rises. As stated in the oxygen haemoglobin dissociation curve, the partial pressure at which the haemoglobin satu­ration is 50% is called P50.

Factors Influencing Bohr Effect:

All the factors, which shift the oxygen haemoglobin dissociation curve to the right (mentioned above) increase the Bohr effect.

(B) Transport of Carbon dioxide:

In the oxidation of food, carbon dioxide, water and energy are produced. Carbon dioxide in gaseous form diffuses out of the cells into the capillaries, where it is transported in three ways.

(i) Transport of CO2 in Dissolved Form:

Because of its high solubility, about 7 percent carbon dioxide gets dissolved in the blood plasma and is carried in solution to the lungs. Thus as compared to O2, a much larger volume of CO2 is transported in dissolved form. This is about 7% of all the CO2 transported by blood from tissues to the lungs.

(ii) Transport of CO2 as Bio-carbonate:

The largest fraction of carbon dioxide (about 70%) is converted to bicarbonate ions (HCO3 _ ) and transported in plasma. When carbon dioxide diffuses into the RBCs, it combines with water, forming carbonic acid (H2CO3). H2CO3 is unstable and quickly dissociates into bicarbonate ions and hydrogen ions:

Although this reaction also occurs in plasma, it is thousands of times faster in erythro­cytes because they (and not plasma) contain carbonic anhydrase (karbon’ ik an-hi’ dras), an enzyme that reversibly catalyzes the conversion of carbon dioxide and water to carbonic acid.

Hydrogen ions released during the reaction bind to hemoglobin, triggering the Bohr effect thus, no oxygen release is enhanced by carbon dioxide loading (as HCO3

). Because of the buffering effect of hemoglobin, the liberated hydrogen ions cause little change in pH under resting conditions. Hence, blood becomes only slightly more acidic (the pH declines from 7.4 to 7.34) as it passes through the tissues.

Chloride Shift (= Hamburger’s Phenomenon):

Since the rise in the HCO3 _ content of red cells is much greater than that in plasma as the blood passes through the capillaries, about 70% of the HCO3 formed in the red cells enters the plasma.

The excess HCO3 leaves the red cells in exchange for Cl – (Fig. 17.13). This exchange is called the chloride shift. Because of it, the Cl – content of the red cells in venous blood is, therefore, significantly greater than in arterial blood. The chloride shift occurs rapidly and is essentially complete in 1 second. Consequently, the red cells take up water and increase in size.

(iii) Transport of CO2 as Carbaminohaemoglobin. About 23 per cent CO2 is carried by haemoglobin as carbaminohaemoglobin. In addition to reacting with water, carbon dioxide also reacts directly with amine radicals (NH2) of haemoglobin to form an unstable compound carbaminohaemoglobin (Hb CO2). This is reversible reaction.

CO2 + Hb (Haemoglobin) ⇋ HbCO2 (Carbominohaemoglobin).

Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO2 to the alveoli.

Release of Carbon Dioxide in the Alveoli of Lung:

The pulmonary arteries carry deoxygenated blood to the lungs. This blood contains carbon dioxide as dissolved in blood plasma, as bicarbonate ions and as carbaminohaemoglobin.

(i) CO2 is less soluble in arterial blood than in venous blood. Therefore, some CO2 diffuses from the blood plasma of the lung capillaries into the lung alveoli.

(ii) For the release of CO2 from the bio-carbonate, a series of reverse reactions takes place. When the haemoglobin of the lung blood capillaries takes up O2, the H + is released from it.

Then, the Сl – and HCO3 – ions are released from KC1 in blood, and NaHCO3 in the RBC respectively. After this HCO3 – reacts with H + to form H2CO3 _ As a result H2CO3 splits into carbon dioxide and water in the presence of carbonic anhydrase enzyme and CO2 is released into the alveoli of the lungs.

(iii) High PO2 in the lung blood capillaries due to oxygenation of haemoglobin favours separation of CO2 from carbaminohaemoglobin.

It was proposed by J.S. Haldane, a Scotish physiologist, 1860-1936. Binding of oxygen with haemoglobin tends to displace carbon dioxide from the blood. This is called Haldane effect. It is far more important in promoting carbon dioxide than is the Bohr effect which promotes oxygen transport. The Haldane effect encourages CO2 exchange in both the tissues and lungs.

It is quantitatively far more important in promoting CO2 transport than the Bohr effect in promoting O2 transport. Thus, Haldane effect and Bohr effect complement each other. In the tissues addition of CO2 to the blood facilitates unloading of O2 by Bohr effect. In turn, O2 unloading favours uptake of CO2 by Haldane effect.

As the name indicates it occurs inside the cells. It takes place in all types of living cells. Respiratory substrates are those organic substances which can be catabolized to liberate energy inside the living cells. The most common respiratory substrate is glucose. Fats are used as respiratory substrates by a number of organisms because they contain more energy as compared to carbohydrates.

However, fats are not directly used in respiration. Instead they are first broken to intermediates common to glucose oxidation, viz., acetyl CoA, glyceraldehyde phosphate. Proteins are used rarely in respiration.

Proteins are hydrolysed to form amino acids from which organic acids are produced through deamination. Organic acids enter Krebs cycle, e.g., aspartic acid, glutamic acid. At other times, proteins are employed as reparatory substrates under starvation conditions only when carbohydrates and fats be­come unavailable.

As stated earlier respiration is of two main types: anaerobic and aerobic. In anaerobic respiration food is oxidised without using molecular oxygen. Less energy is produced in anaerobic respiration. In aerobic respiration organic food is completely oxidised with the help of oxygen into carbon dioxide and water. 686 Kcal of energy is also liberated per mole of glucose.

Aerobic respiration consists of four steps:

(i) Glycolysis:

It is a first step which is common to both anaerobic and aerobic modes of respiration. It occurs in cytoplasm and does not require oxygen. Glycolysis consumes ATP molecules. No carbon dioxide is released in glycolysis. Water and ATP molecules are released,

(ii) Krebs Cycle:

It is the second step in respiration. It operates inside mitochondria and uses oxygen and, therefore, occurs only in aerobic respiration. It does not consume ATP but liberates ATP molecules. Water and carbon dioxide are produced during Krebs cycle,

(iii) Electron Transport Chain (ETC):

It is a series of coenzymes and cytochromes that take part in the passage of electrons from a chemical to its ultimate acceptor. The enzymes involved in electron transport chain are components of the inner mitochondrial membrane. Thus ETC occurs in mitochondria. Oxygen is the ultimate acceptor of electrons.

It becomes reactive and combines with protons to form metabolic water 2H + + O 2- → 2H2O]. (iv) Oxidative Phosphorylation. It is the synthesis of energy rich ATP molecules with the help of energy liberated during oxidation of reduced co-enzymes (NADH, FADH2) produced in respiration.

The enzyme required for this synthe­sis is called ATP synthase. ATP synthase is located in F1 or head piece of F1 or elementary particles. The particles are present in the inner mitochondrial membrane.

The net gain from complete oxidation of a molecule of glucose in muscle and nerve cells is 36 ATP molecules. However, in aerobic prokaryotes, heart, liver and kidneys, 38 ATP molecules are produced per glucose molecule oxidised.

Artificial Respiration:

Conditions when artificial respiration is required. It is required when persons have stopped breathing because of (i) drowning, (ii) electric shock, (iii) accidents, (iv) gas poisoning, or (v) anesthesia.

Methods of Artificial Respiration:

Two methods of artificial respiration are (i) manual methods and (ii) mechanical methods.

Manual method of artificial respiration can be applied quickly without waiting for the availability of any mechanical aids. The mouth to mouth breathing is very common.

During the respiratory failure due to paralysis of respiratory muscles or some other cause, the manual method of respira­tion is not useful because in these conditions, the resuscitation should be given for a longer period.

This can be done only by means of mechanical methods which are of two types:

(i) Drinker’s method:

The machine used in this method is called iron lung or Drinker’s respiration or tank respiration invented by Philips Drinker in 1929. By using the tank respirator, the patient can survive for a longer time, even up to the period of one year till the natural respiratory functions are restored

(ii) Ventilation Method:

A rubber tube is introduced into the trachea of the patient through the mouth. When air is pumped, inflation of lungs occurs, when it is stopped expiration occurs, and the cycle is repeated. The apparatus used for this is called ventilator.

Exercise and Respiration:

On the basis of severity, the exercise is classified into three types:

It includes strenuous muscular activity but the severity can be maintained only for short duration. Fast running for a distance of 100 or 4(X) metres is the best example of this type of exercise. Complete exhaustion occurs at the end of severe exercise.

This type of exercise can be performed for a longer period. The examples of this type of exercise are fast walking and slow running. Exhaustion does not occur at the end of moderate exercise.

This is very simple form of exercise like slow walking. So exhaustion does not occur at the end of mild exercise.

After a period of severe muscular exercise the amount of oxygen consumed is enor­mously more. The oxygen required is more than quantity available to muscles.

This much of oxygen is utilized for reversal of the following metabolic processes:

(i) Reformation of glucose from lactic acid accumulated during exercise,

(ii) Re-synthesis of ATP and creatine phosphate, and

(iii) Restoration of amount of oxygen dissociated from haemoglobin and myoglobin.

Regulation of Respiration (= Regulation of Breathing):

Respiration is under both nervous and chemical regulation.

Normal quiet breathing occurs involuntarily. Adult human beings breathe 12 to 14 times per minute, but human infants breathe about 44 times per minute. In each breathe in human beings, inspiration accounts for about two and expiration for about three seconds.

The respiratory centre is composed of groups of neurons located in the medulla oblongata and pons varolii. Hence respiratory centre is divided into the medullary res­piratory centres and pons respiratory centres.

Medullary Respiratory Centres:

(i) Dorsal Respiratory Group (DRG):

It is located in dor­sal portion of the medulla oblongata. The dorsal respiratory group mainly causes inspiration.

(ii) Ventral Respiratory Group (VRG):

It is located in the ventrolateral part of the me­dulla oblongata. The ventral respiratory group can cause ei­ther inspiration or expiration, depending upon which neurons in the group are stimulated.

Pons Respiratory Centres:

It is located in the dorsal part of pons varolii. The function of the pneu­motaxic centre is primarily to limit inspiration.

There is another strange centre called the apneustic centre, located in the lower part of the pons varolii. The function of this centre is not well understood but it is thought that it operates in association with the pneumotaxic centre to control the depth of inspiration. Apneustic centre is considered hypothetical.

2. Chemical Regulation:

The largest number of chemoreceptors is located in the carotid bodies. However, a sizeable number of chemoreceptors are in the aortic bodies.

The carotid bodies are lo­cated bilaterally in the bifurcation of the common carotid arteries and their afferent nerve fibres pass through glossopharyngeal cranial nerves and hence to the dorsal respiratory area of the medulla oblongata. The aortic bodies are located along the arch of the aorta and their afferent nerve fibres pass through the vagi (sing, vagus), cranial nerves and hence to the dorsal respiratory area.

Excess carbon dioxide or hydro­gen ions mainly stimulate the respiratory centre of the brain and increase the inspiratory and expiratory signals to the respiratory muscles.

Increased CO2 lowers the pH resulting acido­sis. However, oxygen does not have a significant direct effect on the respiratory centre of the brain in controlling respiration. Thus carotid and aortic bodies send chemical signals to the respiratory centre in the medulla oblongata.

Functions of Respiration:

The energy required for daily metabolic activities is derived from the oxidation of food going on continuously in the body.

Respiration excretes carbon dioxide, water, etc.

3. Maintenance of Acid-base Balance:

Elimination of CO2 maintains the acid-base balance in the body.

4. Maintenance of Temperature:

A large amount of heat is expelled out during expi­ration which maintains the body temperature.

5. Return of Blood and lymph:

During inspiration the intra-abdominal pressure in­creases and the intrathoracic pressure decreases. This results the return of blood and lymph from the abdomen to the thorax.

Mountain sickness is the condition characterized by the ill effect of hypoxia (shortage of oxygen) in the tissues at high altitude. This is commonly developed in persons going to high altitude for the first time.

In mountain sickness, the symptoms occur mostly in digestive system, respiratory system and nervous system.

Loss of appetite, nausea and vomiting occur because of expan­sion of gases in the gastrointestinal tract.

Breathlessness occurs because of pulmonary oedema. Pulmo­nary oedema develops because of the response of the pulmonary blood vessels to hypoxia.

The symptoms are headache, depression, disorientation, irritability, lack of sleep, weakness and fatigue. These symptoms are developed because of cerebral oedema.

Human Respiratory System and it&rsquos Mechanism (with diagram)

The human respiratory system consists of a pair of lungs and a series of air passages leading to the lungs.

The entire respiratory tract (passage) consists of the nose, pharynx, larynx, trachea, bronchi, and bronchioles.

Air enters the nose through the nostrils. When air passes through the nose, it is warmed, moistened and filtered. The hairs present in the nose filter out particles in the incoming air. The air is moistened by the mucus present in the nose, and it is warmed by the blood flowing through the capillaries in the nose.

The respiratory tract from the nose to the bronchioles is lined by mucous membranes and cilia. The mucus and cilia act as additional filters.

Behind the nose lies the pharynx (throat). There are two passages here—one for food and the other for air. The air passes from the pharynx to the larynx, or the voice box. The opening leading to the larynx is called glottis. It is protected by a lid called epiglottis, which prevents food from entering the passage to the lungs.

From the larynx the air goes to the trachea, or the windpipe. The trachea is about 11 cm long. It is guarded by 16-20 C-shaped cartilage rings, which prevent the trachea from collapsing. The trachea divides into two tubes called bronchi. Each bronchus divides and branches out in the form of thinner tubes called bronchioles.

The bronchioles enter the lungs and divide further into finer branches called alveolar ducts. These open into extremely thin-walled, grape-shaped air sacs called alveoli. Each alveolus is covered by a web of blood capillaries.

The lungs are a pair of spongy organs lying in the chest cavity formed by the ribs. The actual exchange of gases between the air and the body takes place in the capillary-covered alveoli inside the lungs. Here, oxygen from the air in the alveoli goes into the blood, and the carbon dioxide in the blood goes out.

The oxygen binds to the haemoglobin molecules present in the red blood corpuscles and is taken to different parts of the body.

The total surface area through which the exchange of gases can take place increases because of the millions of alveoli in the lungs. Their total surface area can be about a hundred times that of the body. The large surface area allows sufficient oxygen intake needed for releasing the large amount of energy required by us.

Mechanism of Breathing:

There are two main steps in breathing: inspiration and expiration:


Inspiration (inhalation) is the process of breathing in, by which air is brought into the lungs.

Inspiration involves the following steps:

i. The muscles attached to the ribs on their outer side contract. This causes the ribs to be pulled out, expanding the chest cavity.

ii. The muscle wall between the chest cavity and the abdominal cavity, called diaphragm, contracts and moves downwards to further expand the chest cavity.

iii. The abdominal muscles contract.

The expansion of the chest cavity creates a partial vacuum in the chest cavity. This sucks in air into the lungs, and fills the expanded alveoli.


After the exchange of gases in the lungs, the air has to be expelled. Expulsion of the air from the lungs is called expiration. In this process, muscles attached to the ribs on their inner side contract, and the diaphragm and the abdominal muscles relax. This leads to a decrease in the volume of the chest cavity, which increases the pressure on the lungs. The air in the lungs is pushed out and it passes out through the nose.

When we breathe out, not all of the air in the lungs gets expelled. Some of it remains in the lungs. This keeps the lungs from collapsing and allows more time for the exchange of gases.

Transport of Gases:

In very small organisms, there is no need to have a separate transportation system for gases because all its cells are involved directly in the exchange of gases by diffusion. However, a large multicellular organism needs a mechanism for the transport of gases for its different organs and tissues.

Human beings also have a system for transportation of gases. Oxygen is carried by haemoglobin of the red blood cells. Haemoglobin has a great affinity for oxygen—each haemoglobin molecule binds to four molecules of oxygen. The oxygen ‘picked up’ by haemoglobin gets transported with the blood to various tissues.

Carbon dioxide is more soluble in water than oxygen. So, some of it is transported in the dissolved form in our blood. Some carbon dioxide is also transported by haemoglobin. Not all of the carbon dioxide formed is expelled from the body. Some of it reacts with water to form compounds useful for life processes.

Section Summary

Animal respiratory systems are designed to facilitate gas exchange. In mammals, air is warmed and humidified in the nasal cavity. Air then travels down the pharynx and larynx, through the trachea, and into the lungs. In the lungs, air passes through the branching bronchi, reaching the respiratory bronchioles. The respiratory bronchioles open up into the alveolar ducts, alveolar sacs, and alveoli. Because there are so many alveoli and alveolar sacs in the lung, the surface area for gas exchange is very large.

The mammalian circulatory system is a closed system with double circulation passing through the lungs and the body. It consists of a network of vessels containing blood that circulates because of pressure differences generated by the heart.

The heart contains two pumps that move blood through the pulmonary and systemic circulations. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The pumping of the heart is a function of cardiomyocytes, distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle. The signal for contraction begins in the wall of the right atrium. The electrochemical signal causes the two atria to contract in unison then the signal causes the ventricles to contract. The blood from the heart is carried through the body by a complex network of blood vessels arteries take blood away from the heart, and veins bring blood back to the heart.

Watch the video: Human Respiratory System. Respiratory System Anatomy. Biology. Letstute (May 2022).


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