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Why do organisms excrete nitrogenous wastes?

Why do organisms excrete nitrogenous wastes?


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Organisms excrete nitrogenous wastes in the form of urea, uric acid or ammonia. But isn't that a bit of a waste? There is a shortage of biologically available nitrogen in the ecosystem, and plants convert nitrogen in ammonia and nitrates into amino acids. Why do organisms then deaminate these amino acids and then excrete them?

It seems to me that it would make much more sense to store the nitrogen for further use. I understand that ammonia is toxic, but surely it could be stored in some specialized structure.


Some amino acids are essential, that is, they cannot be synthesized. For all the others, however, you need building blocks. Nitrogen, and specifically ammonia, is required for non-essential amino acid synthesis, the transfer of which is accomplished through a process called transamination. Amino acids are also broken down for energy as a metabolic process, producing glutamate (in addition to energy-containing molecules such as pyruvate). The body has thus produced some amino acids and metabolized others for energy, but glutamate must be deaminated to remove the nitrogen, which by this point is now in excess.

To evolve a specialized storage unit for highly toxic substances could be useful, I suppose, but it would be unlikely and would pose a huge risk compared to just getting rid of 'em. I'd rather urinate a few times a day than save a lot of toxic material for later and run the risk of something malfunctioning and killing me; besides, eventually that storage unit would get full. You could ask the same question for any substance we might eventually want.


Preamble: The problem with questions on evolution

Obviously, evolutionary biology is a reputable discipline, but unfortunately there are two types of questions on evolution that are posted on this site that I believe are often of dubious value and can lead to dangerously unscientific habits of mind.

The first is questions of the type “Why did such and such evolve this way?”. The problem with this type of question is that there is generally no way of proving a hypothesis because it makes no predictions - the end product of the evolution is known. There is sometimes an anatomical or biochemical paper-trail to support arguments, and I have presented one such in an answer to another question. However many answers to this type of question are pie-in-the-sky or circular reasoning.

The second type - typified by this one - asks “Why has such and such not evolved, it would obviously be advantageous?” The posters of such questions tend not to ask themselves why, because if they did they would realize that the answer is likely to be either:

“It would be too difficult, costly or would have disadvantageous consequences”

or

“The assumption that it would convey a selective advantage (and that is what evolution is about) is incorrect.”

The accepted answer to this question addresses the former explanations. Although this is an old question at the time of my answering it, I am doing so partly because a similar question came up recently, but also because I feel it would be generally useful exercise to consider the second explanation - to emphasize the need to question the assumptions of those who think that they know better than Nature.

A story my mother told me

The story is apocryphal, but it is true my mother related it to me. It concerns two youths who had applied for a general office job. (This was not quite Dickensian, but not so much later.) In order to chose between them they were each given a parcel to unwrap. The first youth carefully untied the string, took off the paper wrapping, neatly folded it, and placed the parcel, the string and the wrapping paper on the table. The second youth ripped off the wrapping and stuffed it into the waste paper basket and placed the parcel on the table. Who do you think got the job? In the era in which the question was posed, most people (including myself) assumed that the youth who had saved the string and paper for reuse would get the job, rather than the youth who wasted them. However (in the story) it was the latter who got the job - the manager was interested in speed and efficiency, not in economy in office stationary. And the moral is that if you make the wrong assumptions (that avoiding waste was paramount) you will come to the wrong conclusion. So it is with “wasting nitrogen”.

Is the assumption in the question valid for human metabolism?

Although the original question had plants in mind, like @Amory I shall consider it from the perspective of a single species, Homo sapiens. The argument in the question is, in effect:

“There is a shortage of biologically available nitrogen in the ecosystem, so why do organisms deaminate amino acids and excrete them rather than store the nitrogen for future use.”

The first logical flaw is equating the shortage (limitation?) of nitrogen in the ecosystem with a shortage in any individual species. Let us, however, consider a modified argument:

“Nitrogen is a necessary component of proteins and nucleic acids, the synthesis of which is necessary for life. If this were stored in good times it could help survival when dietary protein became unavailable.”

This seems more reasonable. But one first needs to ask whether there is actually no storage of nitrogen. In effect there is in the form of structural protein, which is broken down in starvation. This is relevant, as considering this phenomenon allows one to address the real question:

“Would (further) storage of nitrogen in some more dedicated molecular form provide a selective advantage by enhancing survival during periods of starvation?”

This is the office boy question, but whereas we had no way of knowing what was most important in the office - speed or frugality - here we have information from which to draw a conclusion. In prolonged starvation protein is broken down to amino acids. Why? To utilize the carbon backbone of glucogenic amino acids to provide glucose for the brain when all other substrates for gluconeogenesis have been exhausted. This is key to survival. What happens to the nitrogen? It is partly used as ammonia to buffer the acid produced from some of the ketone bodies produced from ketogenic amino acids and the rest is excreted as urea. It is certainly not used to make protein or nucleic acid, which would entail using precious carbon constituents. Thus, it is clear that energy requirements, and particularly the generation of glucose are the limiting factors in survival during starvation - not nitrogen. The conclusion, therefore, is that there is no point in storing nitrogen in times of plenty: what needs to be stored is carbohydrate/fat.

One might raise the existence of Kwashiorkor, a disease of protein malnutrition. However this is a relatively rate disease, and it affects only children: it is not the normal situation in starvation. Clearly the demands for protein and nucleic acid synthesis are much greater in growth than adulthood.

Coda: Another counter-intuitive evolutionary development in nitrogen metabolism

The fallacy of “wouldn't it be better to evolve… ” is evident if one considers another aspect of nitrogen metabolism - our lack of the apparently useful facility to make all the amino acids, resulting in a dependency of man on certain so-called 'essential' amino acids. Here the situation is not one of failing to evolve a facility, but actually loosing it - simpler organisms from which we have evolved had it. The inescapable conclusion is that such a facility can never have been sufficiently advantageous to result in an evolutionary disadvantage for those who had lost it. Presumably we have evolved in a way that allows us (or a sufficient number of us) to survive in our particular ecological niche, not to be supermen.


Excretion in Animals: Definition, Modes and Excretory Wastes

It is the elimination of metabolic waste products from the animal body to regulate the composition of the body fluids and tissues. The terms excretion and defecation should not be confused. Defecation is the removal of wastes and undigested food, collec­tively called faeces, through the anus.

It is a process that regulates the body’s salt and water concentration.

Modes of Excretion:

Depending upon the excretory product, animals show five types of nitrogenous excre­tion in which ammonotelism, ureotelism and uricotelism are major types and aminotelism and guanotelism are minor types. Nitrogenous waste substances such as ammonia, urea or uric acid are produced during protein metabolism according to the species.

Small amount of nitrogenous waste substances are also produced during the metabolism of nucleic acids. Ammonia is the most toxic, followed by urea and uric acid. The latter is the least toxic.

Why Deaminate Amino Acids?

Animal body can store fats and carbohydrates but they are unable to store proteins or amino acids. Therefore, the amino acids that the body cannot utilize immediately are deaminated, that is, their amino groups (— NH2) are removed. The remaining organic acid may be used as energy source or converted into carbohydrate or fat.

Many aquatic animals like protozoans, (e.g., Amoeba, Parame­cium), sponges (e.g., Sycon), cnidarians or coelenterates (e.g., Hydra), liver fluke, tape worms, Ascaris, Nereis, Earthworm, Leech, most aquatic arthropods (e.g., Prawn ), most aquatic molluscs (e.g., Pila) bony fishes (e.g., Labeo), Amphibian tadpole (e.g., tadpole of frog), tailed amphibians (e.g., Salamanders), and crocodiles excrete ammonia.

Animals which excrete ammonia are called ammonotelic and excretion of ammonia is termed as the ammonotelism.

Excretion of urea is known as ureotelism and the animals which excrete urea are called ureotelic. Ureotelic animals include Ascaris, earthworm (both are ammonotelic and ureotelic), cartilaginous fishes like sharks and sting rays, semi-aquatic amphibians such as frogs and toads, aquatic or semi aquatic reptiles like turtles, terrapins and alligators, and man and all other mammals.

Urea is less toxic and less soluble in water than ammonia. Hence, it can stay for some time in the body. Sharks retain large quantity of urea in their blood, therefore, blood osmotic pressure approaches that of sea water, which minimizes water loss from their body.

How Urea is produced?

Liver converts toxic ammonia (NH3) into much less toxic urea which is excreted in urine. Urea is the end product of protein metabolism (amino acid metabolism).

Urea is synthe­sized in liver and transported to kidneys for excretion in urine. Urea is produced through urea cycle which was discovered by Hans Krebs and Kurt Henseleit (1932), hence it is known as Krebs-Henseleit cycle. The individual reactions, however, were described in more detail later on by Ratner and Cohen.

Urea has two amino (-NH2) groups, one derived from NH3 and the other from aspartate. Carbon atom is supplied by CO2. Urea cycle includes five steps involving five distinct enzymes. The first two enzymes are present in mitochondria while the rest are localized in cytosol (the cytoplasm minus the mitochondria and endoplasmic reticulum).

(i) Synthesis of Carbamoyl Phos­phate:

Carbamoyl phosphate synthase 1 (CPS 1) of mitochondria catalyses the condensation of NH4 + ions with CO2 to form carbamoyl phosphate. This step con­sumes two ATPs.

(ii) Formation of Citrulline:

Citrul- line is synthesized from carbamoyl phosphate and ornithine by ornithine transcarbamoylase. Ornithine is regenerated and used in urea cycle. Ornithine and citrulline are basic amino acids.

(iii) Synthesis of arginosuccinate:

Arginosuccinate synthase condenses citrulline with aspartate to produce arginosuccinate. This step requires ATP.

(iv) Cleavage of arginosuccinate:

Arginosuccinate cleaves arginosuccinate to give argi­nine and fumarate. Fumarate liberated here provides a connecting link with Krebs cycle, gluconeogenesis, etc.

Arginase is the fifth and final enzyme that cleaves arginine to form urea and ornithine. This ornithine enters mitochondria for its reuse in the urea cycle. The urea cycle (also called ornithine cycle) is irreversible.

Excretion of uric acid is known as uricotelism and the animals which excrete uric acid are called uricotelic. Animals which live in dry conditions have to conserve water in their bodies. Therefore, they synthesize crystals of uric acid from their ammonia. Uric acid crystals are non-toxic and almost insoluble in water.

Hence, these can be retained in the body for a considerable time. Uricotelic animals include most insects, (e.g., cockroach) some land crustaceans (e.g., Oniscus commonly known as “wood louse”), land snails (e.g., Helix commonly known as “land snail”), land reptiles (e.g., lizards and snakes) and birds.

The concentration of uric acid is so high in guano (waste matter dropped by sea birds, used as fertilizer) that uric acid is commercially extracted from guano which is collected from uninhabited marine or littoral (part of country which is along the coast) islands. Primates including man also excrete some uric acid which is formed in their body by the breakdown of nucleic acids.

Certain invertebrates like some molluscs (Unio, Limnaea, etc.) and some echinoderms (e.g., Asterias) excrete excess amino acids as such. These animals are called aminotelic and their mode of excretion is called aminotelism.

Spiders excrete guanine and are said to be guanotelic and their mode of excretion is called guanotelism.


Some animals perform two modes of excretion. That is called dual excretion. Some important examples of dual excretion are mentioned here. Earthworms excrete ammonia when sufficient water is available while they excrete urea instead of ammonia in drier surroundings.

When lung fishes and Xenopus (African toad) live in water they are normally ammonotelic but they become ureotelic when they lie immobile in moist air or mud during summer. Amphibian tadpoles (larvae) are aquatic and ammonotelic but they become ureotelic during their metamorphosis. Crocodiles spend most of their time in water and creases.

Although man is ureotelic yet he excretes a small amount of uric acid in his urine. But it is too little amount as compared to total urinary nitrogen. However, in some patients the concentration of uric acid is raised in their body fluids and subsequently uric acid is deposited in joints, cartilages and kidneys causing gout and kidney failure. In gouty arthritis, crystals of uric acid are deposited in the joints causing a severe pain.

Other Nitrogenous Wastes:

1. Creatine and Creatinine:

Muscle cells contain molecules of creatine phosphate, which are highly energy molecules and serve for storage of bioenergy like ATP. Excess amount of this phosphate is however, excreted out as such, or after being changed into creatinine. The latter is passed out through urine.

2. Trim ethylamine oxide (TMO):

Marine teleost fishes excrete a large proportion of their nitrogen as trim ethylamine oxide (TMO). Large amounts of this compound is also stored in their body for osmoregulation, (i.e., to minimize loss of water and entry of salts).

It is excreted in small amount by birds and is formed by a com­bination of benzoic acid (formed during fat metabolism) with the amino acid ornithine.

It is formed when benzoic acid is combined with glycine. It is less toxic.

5. Bilirubin and Biliverdin:

These are the bile pigments which are formed in the liver due to breakdown of haemoglobin of worn out RBCs. These are excreted through bile. In jaundice, level of bilirubin is high in the blood resulting yellow skin, white eyes, etc.

It is formed from uric acid as a result of an oxidation reaction catalyzed by the enzyme uricase. Higher primates including man do not have enzyme uricase. Allantoin is an excretory product of embryos of amniotes. In a very young embryo, the excretory matter is stored in allantois.

Other Excretory Wastes:

1. Bile Salts:

Bile salts are the sodium and po­tassium salts of bile acids, which are conjugated with glycine or taurine. The bile acids are derived from cholesterol. Glycine is an amino acid and taurine is derivative of an amino acid. The conjugated bile acids namely, glycocholic acid and taurocholic acid form bile salts in combination with sodium or potassium salts.

Bile salts always keep the cholesterol and lecithin in solution. So, in the absence of bile salts, cholesterol precipitates along with lecithin and forms gallstone. 95% of bile salts are absorbed into blood from small intestine.

Most of the bile salts are converted into salts of deoxycholic acid and lithocholic acid. Salts of deoxycholic acids are absorbed completely. Only 1% of lithocholate (salts of lithocholic acid) is absorbed. Major portion of this is excreted with faeces. The bile salts absorbed from intestine are transported by hepatic portal vein back to the liver via the enterohepatic circulation. From liver, the bile salts are re-excreted through bile.

2. Excretion of Drugs, Hormones and Other Substances:

The liver is well known for its ability to detoxify or excrete into bile many drugs, including sulfonamides, penicillin, ampicillin and erythromycin. Several hormones secreted by the endocrine glands are either chemically altered or excreted by the liver, including thyroxine and essentially all the steroid hormones such as oestrogen, cortisol and aldosterone.

One of the major routes for excreting calcium from the body is secretion by the liver into the bile, which passes into the gut and lost in the faeces. The liver also excretes heavy metals like lead, arsenic and bismuth.

The other substances excreted in bile are heavy metals such as copper and iron, some toxins, some bacteria like typhoid bacteria, cholesterol, lecithin and alkaline phosphatase. Heavy metals and drugs are also excreted in the saliva.

3. Carbon Dioxide:

It is mainly expelled out by lungs. Some carbon dioxide is also excreted through sweat and defecation.

4. Water:

Excess of water is a waste product and is eliminated in urine, faeces, sweat and expired air.

5. Vitamins:

The excess of water soluble vitamins like vitamin В complex and vitamin С is removed from the body in urine.

6. Spices:

Onions, garlic and some other spices have volatile components which leave the body through lungs, the rest are removed by the kidneys.

Sebaceous glands (oil glands) secrete an oily secretion called sebum that contains some lipids such as sterols, other hydrocarbons and fatty acids.

Sudoriferous glands (= sweat glands) in the skin and gastrointestinal tract also expel heat which is the result of various metabolic processes.


What Is the Importance of Excretion?

Excretion is an essential process in which waste products are removed from the body. Without excretion, waste products build up in the body and cause serious health issues.

Urea is a mixture of nitrogenous wastes that damage the body if not removed by the excretory system. Blood is important to the excretory system. It carries waste from cells through the bloodstream to the excretory organs for removal from the body.

The excretory system consists of many parts and organs that work as a whole. It rids the body of metabolic waste, which contains salts, carbon dioxide and urea. The lungs remove carbon dioxide, absorbing and removing waste while providing oxygen to the body. The skin is part of the excretory system and helps rid the body of contaminants through perspiration. Perspiration removes urea from the body in a water-based liquid excreted from sweat glands located in the skin.

The urinary system is comprised of the kidneys, the urethra, the ureters and the bladder. The kidneys filter contaminants from the bloodstream and remove them from the body as urine. The kidneys also serve to diffuse any useful substances that pass through the urinary system, sending them back into the bloodstream to be utilized.


Excretory System Structure

The excretory system is necessary for preventing the toxic build up of nitrogenous wastes, such as ammonia or urea. However, the excretory system of animals has evolved in many different ways since the dawn of life on Earth.

In fish and aquatic animals, the excretory system is fairly simple. The gills are a major site of excretion, and some waste products are simply added to the blood to be excreted in the gills. These animals also rely on their skin and glands to excrete excess salts and other waste products. In fact, freshwater and saltwater fish have drastically different kidney functions, based on the concentration of salt in the surrounding water.

In terrestrial animals, such as humans, the excretory system is structured to retain as much water as possible. Birds and reptiles have even developed uric acid, which is a more concentrated and safer form of urea. As a whole system, every part and organ of the excretory system can be functioning at the same time to remove wastes from the body. However, if the structure of the excretory system gets damaged by disease, many bad consequences can ensue.


Nitrogen excretion: three end products, many physiological roles

There are diverse physiological functions of nitrogen end products in different animal groups, including excretion, acid-base regulation, osmoregulation and buoyancy. Animals excrete a variety of nitrogen waste products, but ammonia, urea and uric acid predominate. A major factor in determining the mode of nitrogen excretion is the availability of water in the environment. Generally, aquatic animals excrete mostly ammonia, whereas terrestrial animals excrete either urea or uric acid. Ammonia, urea and uric acid are transported across cell membranes by different mechanisms corresponding to their different chemical properties in solution. Ammonia metabolism and excretion are linked to acid-base regulation in the kidney, but the role of urea and uric acid is less clear. Both invertebrates and vertebrates use nitrogen-containing organic compounds as intracellular osmolytes. In some marine invertebrates, NH4+ is sequestered in specific compartments to increase buoyancy.


30 Easy Questions to Study the Excretory System

The main nitrogenous wastes excreted by living organisms are ammonia, uric acid and urea. Living organisms that secrete ammonia are called ammonotelic. Organisms that secrete uric acid are called uricotelic. Organisms that secrete urea are called ureotelic. 

4. Why are most ammonotelic organisms aquatic animals?

Aquatic animals, such as crustaceans, bony fish and amphibian larvae, are generally ammonotelic because ammonia diffuses more easily through membranesਊnd is more water-soluble than the other types of nitrogenous waste. Ammonia is still the nitrogenous waste most economical to synthesize in terms of energy.

5. Why did ammonotelic excretion cease to be used after animals left aquatic habitats and started to live in terrestrial habitats?

Ammonia is a highly toxic molecule if not diluted and quickly excreted from the body. For this reason, ammonotelic਎xcretion was abandoned in terrestrial habitats because the availability of water for dilution was reduced in this medium, making it so that waste could not be excreted to the exterior so quickly.

6. With regards to toxicity and the need for dilution in water, how different are ureotelic and uricotelic excretions? What are some examples of animals that use these types of excretion?

Urea is more water-soluble than uric acid (an almost insoluble substance). Urea is also more toxic. However, both are less toxic than ammonia.

Some invertebrates, chondrichthyan fish, adult amphibians and mammals are ureotelic. Reptiles, birds and most arthropods are uricotelic.

7. What is the nitrogenous waste produced by amphibian larvae and by the adult animal?

Since amphibian larvae are aquatic, they excrete ammonia. The terrestrial adult excretes urea.

8. Why is uricotelic excretion essential for avian and reptile embryos?

In reptiles and birds, the excretory system is uricotelic since uric acid is insoluble, less toxic and suitable to be stored within the eggs where their embryos develop.

9. How do the embryos of placental mammals excrete nitrogenous wastes?

Placental animals, including embryos, excrete urea. In the adult placental mammal urea is excreted through the urine. In embryos, the molecule passes to the mother’s blood through the placenta and is excreted in the mother’s urine.

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The Human Excretory System

10. What is the main nitrogenous waste produced by humans?

Human beings excrete mainly urea, which is eliminated in urine. 

11. How is urea formed in the human body?

Urea is a product of the degradation of amino acids. During this process, amino acids lose their amine group, which is then transformed into ammonia. In the liver, ammonia reacts with carbon dioxide to form urea and water, through a process called ureogenesis.

During the intermediary reactions of ureogenesis, a molecule of ornithine is consumed and another is produced. For this reason, ureogenesis is also known as the ornithine cycle.

The Excretory System Review - Image Diversity: ureogenesis

12. What organs make up the excretory system?

The excretory system is formed of the kidneys (two), the ureters (two), the bladder and the urethra. 

The Kidneys and Their Functions

13. What vessels carry blood to the kidneys? Is this blood arterial or venous?

The arterial vessels that carry blood to be filtered by the kidneys are the renal arteries. The renal arteries are branches of the aorta therefore, blood filtered by the kidneys is arterial (oxygen-rich) blood. 

14. Which vessels drain filtered blood from the kidneys?

The venous vessels that collect blood filtered by the kidneys are the renal veins. The renal veins carry blood that has been reabsorbed by the nephron tubules.

Nephrons

15. What is the functional unit of the kidneys?

The functional (filtering) unit of the kidneys is the nephron. A nephron is made of the afferent arteriole, the efferent arteriole, the glomerulus, the Bowman's capsule, the proximal tubule, the loop of Henle, the distal tubule and the collecting duct.

Each kidney contains around one million nephrons.

16. What are the three main renal processes that produce urine when combined?

Urine is made by these occurrence of three processes in the nephron: glomerular filtration, tubular reabsorption and tubular secretion.

In the nephron, blood carried by the afferent arteriole enters the glomerular capillary network where it is filtered. The filtration results in part of the blood returning to circulation through the efferent arteriole while the other part, known as glomerular filtrate, enters the proximal tubule of the nephron. In the nephron tubules (also known as convoluted tubules), substances of the glomerular filtrate, such as water, ions and small organic molecules, are reabsorbed by the cells of the tubule wall and enter into circulation. These cells also secrete other substances inside the tubules. Urine is formed of filtered substances that are reabsorbed and of secreted (by the tubules) substances. Urine is drained by the collecting ducts to the ureter of each kidney. It then enters the bladder and is later਍ischarged through the urethra.

The nephron tubules are surrounded by an extensive capillary network that collects reabsorbed substances and supplies others to be secreted.

17. What is the main transformation presented in glomerular filtrate compared to blood?

Glomerular filtrate is the name given to plasma after it has passed the glomerulus and entered the Bowman’s capsule. Glomerular filtrate has a different composition compared to urine, since the fluid has not yet undergone tubular reabsorption and secretion.

The main difference between blood and glomerular filtrate is that the latter contains a minimum amount of proteins as well as no cells or blood platelets.

18. What is proteinuria? Why is proteinuria a sign of glomerular renal injury?

Proteinuria means the passing of proteins in the urine. Under normal conditions, proteins are too large to be filtered by the glomerulus and are practically absent in urine (the few filtered proteins may also be reabsorbed by the nephron tubules). Proteinuria is an indication that a more than expected amount of proteins is passing through the glomerulus, and is an indicator of glomerular disease, such as diabetic nephropathy.

The glomerulus also blocks the passage of blood cells and platelets (hematuria is often a sign of urinary disease, although it does not specifically implicate the kidneys, since the blood may come from the lower parts of the excretory tract). 

19. Where does most of the water reabsorbed after glomerular filtration go? What other substances are reabsorbed by the nephron tubules?

Only 0.5 to 1% of glomerular filtrate is eliminated as urine. The remaining volume, containing mainly metabolic ions, glucose, amino acids and water, is reabsorbed by the nephron tubules (by means of active or passive transport) and regains blood circulation.

The convolute tubules of the nephron are responsible for the reabsorption of substances.

20. Why do the cells of the nephron tubules contain a large amount of mitochondria?

The cells of the tubule wall have a large number of mitochondria because many substances are reabsorbed or secreted through them by means of active transport (a process that spends energy). Therefore, many mitochondria are necessary to supply the energy for (ATP supply) this type of transport.

21. What is tubular secretion? What are some examples of substances secreted through the renal tubules?

Tubular secretion is the passage of substances from the blood capillaries that surround the nephron tubules to the tubular lumen so that these substances can be excreted with urine. Ammonia, uric acid, potassium, bicarbonate and hydrogen ions, metabolic acids and bases, various ingested drugs (medicines) and other substances are secreted by the nephron tubules.

22. In what part of the nephron does the regulation of the acidity and alkalinity of plasma take place?

The regulation of the acid-basic equilibrium of the body is carried out by the kidneys and depends on tubular reabsorption and secretion.

23. How are kidneys involved in the regulation of the acid-basic equilibrium of the body? How are alkalosis and acidosis corrected by the kidneys?

The kidneys can regulate the acidity or alkalinity of the plasma by varying the excretion of hydrogen and bicarbonate ions.

During alkalosis (an abnormally high level of plasma pH), the kidneys excrete more bicarbonate ions and the equilibrium of the formation of bicarbonate from water and carbon dioxide shifts towards the formation of more hydrogen ions and bicarbonate ions, thus lowering plasma pH. When the body undergoes acidosis (an abnormally low level of plasma pH), the kidneys excrete more hydrogen ions and retain more bicarbonate ions and, as a result, the equilibrium of the formation of bicarbonate from water and carbon dioxide shifts towards more hydrogen consumption, increasing the pH of plasma.

24. How are the kidneys involved in controlling blood volume? How is the volume of blood in the body related to arterial pressure?

The kidneys and the hormones that control them are the main physiological regulators of the total volume of blood in the body. As more water is reabsorbed by the nephron tubules, the volume of blood increases and, as more water is excreted in urine, the volume of blood lowers.

The volume of blood in turn has a direct relationship with blood pressure. Blood pressure increases when blood volume increases and it lowers when blood volume lowers. This is the reason why one of the main groups of antihypertensive drugs is diuretics. Doctors often prescribe diuretics to patients with high blood pressure so that they excrete more water and therefore lower their blood pressure.

Hormones and the Excretory System

25. Which three main hormones are involved in the regulation of the renal function?

Antidiuretic hormone (ADH, or vasopressin), aldosterone and atrial natriuretic factor (or ANF) are the mains hormones that are involved in the regulation of the excretory system.

26. What is the function of antidiuretic hormone? Where is it produced and what stimuli increase or decrease its secretion?

Antidiuretic hormone is secreted by the hypophysis (also known as the pituitary gland) and it has an effect on the nephron tubules, increasing their reabsorption of water. When the body needs to retain water, for example, in the case of blood loss and an abrupt decrease in blood pressure, or in the case of an abnormally high blood osmolarity, ADH secretion is stimulated.

When the body has an excess of water, such as in the event of excessive ingestion or abnormally low blood osmolarity, the secretion of ADH is blocked and diuresis increases. ADH is also known as vasopressin since it increases blood volume and therefore increases blood pressure.

27. Why does the ingestion of alcohol increase diuresis?

Alcohol inhibits the secretion of ADH (antidiuretic hormone) by the pituitary gland. That is why when people are drunk, they urinate in excess.

28. What is the effect of aldosterone and where is it produced?

Aldosterone is a hormone that has an effect on the nephron tubules, stimulating the reabsorption of sodium. Therefore, it contributes to increasing blood osmolarity, consequently increasing blood pressure.

Aldosterone is made by the adrenal glands, which are located above the upper portion of the kidneys.

29. What evolutionary hypothesis could explain the heart’s role in secreting a hormone that regulates renal function? What hormone is this? 

The renal regulator hormone secreted by the heart is atrial natriuretic factor (or ANF). ANF increases the excretion of sodium in the nephron tubules, causing less reabsorption of water and more urinary volume, thus lowering blood pressure. Atrial natriuretic factor is secreted when there is an increase in the length of heart muscle fibers in response to high blood pressure. ANF is a natural antihypertensive substance. Since the health of the heart depends largely on the stability of normal blood pressure, evolution likely preserved atrial natriuretic factor to allow information from the heart to work as an additional mechanism in the renal control of blood pressure.

Now that you have finished studying Excretory System, these are your options:


The body parts that the excretory system (of human beings) includes are &minus

Kidneys are located in the abdomen (see the image given below), one on either side of the backbone.

Urine that produced in the kidneys passes through the ureters into the urinary bladder where it gets stored until released through the urethra.

On the other hand, plants have completely different process for excretion than those of animals.

Oxygen (released in the day-time) itself can be considered as a waste product generated during photosynthesis.

Many plant waste products are stored in leaves that fall off.

Some other waste products, in plants, are stored as resins and gums, especially in old xylem.


Nonspecific mechanisms of waste disposal

A multitude of disposal mechanisms exist throughout the plant and animal kingdoms for the elimination of excess plant and animal material. Among plants, the shedding and dropping of bark, leaves, and twigs might, in a broad sense, be said to represent disposal mechanisms. Certain plants, in addition, secrete or exude resins, sap, and other substances that accumulate in excessive quantities within the plant.

Specialized, mobile, amoeba-like cells exist in the blood and tissues of animals and engulf particulate wastes resulting from the disintegration of dead cells or the intake of foreign particles into the bodies of animals. Waste matter thus stored inside these small cells is removed from contact with the organism or its metabolism and may be considered to be eliminated whether or not the material is ever actually eliminated from the body of the organism during its normal life cycle.

Toxic substances are produced by normal metabolic activities. Though some of these poisons are eliminated in their original chemical form, others, such as some nitrogenous compounds, are altered biochemically to less toxic compounds. In this manner, more of the original waste may be safely stored, or permitted to accumulate without harmful effects to the organism, until it can be eliminated. In addition, toxic chemicals that are inadvertently ingested or produced by bacterial action (infection) are frequently converted to nontoxic forms by enzymatic and antibody (immune) reactions. Such materials can then be eliminated safely with other wastes along normal pathways of excretion.

Heat is eliminated from the bodies of animals by conduction to the external surface of the organism. In animals possessing a circulatory system, heat travels in its fluid from the deeper portions of the body to the surface. At the body surface, heat is lost by physical processes of convection, radiation, conduction, and evaporation of sweat.


Flame Cells of Planaria and Nephridia of Worms

As multi-cellular systems evolved to have organ systems that divided the metabolic needs of the body, individual organs evolved to perform the excretory function. Planaria are flatworms that live in fresh water. Their excretory system consists of two tubules connected to a highly branched duct system. The cells in the tubules are called flame cells (or protonephridia ) because they have a cluster of cilia that looks like a flickering flame when viewed under the microscope, as illustrated in Figure (PageIndex<2>)a. The cilia propel waste matter down the tubules and out of the body through excretory pores that open on the body surface cilia also draw water from the interstitial fluid, allowing for filtration. Any valuable metabolites are recovered by reabsorption. Flame cells are found in flatworms, including parasitic tapeworms and free-living planaria. They also maintain the organism&rsquos osmotic balance.

Figure (PageIndex<2>): In the excretory system of the (a) planaria, cilia of flame cells propel waste through a tubule formed by a tube cell. Tubules are connected into branched structures that lead to pores located all along the sides of the body. The filtrate is secreted through these pores. In (b) annelids such as earthworms, nephridia filter fluid from the coelom, or body cavity. Beating cilia at the opening of the nephridium draw water from the coelom into a tubule. As the filtrate passes down the tubules, nutrients and other solutes are reabsorbed by capillaries. Filtered fluid containing nitrogenous and other wastes is stored in a bladder and then secreted through a pore in the side of the body.

Earthworms (annelids) have slightly more evolved excretory structures called nephridia , illustrated in Figure (PageIndex<2>)b. A pair of nephridia is present on each segment of the earthworm. They are similar to flame cells in that they have a tubule with cilia. Excretion occurs through a pore called the nephridiopore . They are more evolved than the flame cells in that they have a system for tubular reabsorption by a capillary network before excretion.


The Importance of Homeostasis

The failure of homeostatic regulation in just one body system will cause conditions to deteriorate and it may be fatal. For the health of an organism, all homeostatic regulation mechanisms must function properly. The information below describes how various body systems contribute to overall homeostasis.

Nervous System

The nervous system maintains homeostasis by controlling other parts of the body. It comprises the central nervous system and the peripheral nervous system. The peripheral nerves are those outside of the brain and spinal cord which go to the limbs and organs. The brain and spinal cord make up the central nervous system. The hypothalamus in the brain is particularly important for maintaining homeostasis because it controls the actions of the medulla oblongata (involuntary functions), the autonomic nervous system (smooth muscle and glands), and the pituitary gland (hormone excretion).

Endocrine System

This system comprises the glands that excrete hormones into the bloodstream. Hormones have a myriad of functions in the body that maintain homeostasis by targeting certain tissues. Besides regulating bone growth, muscle metabolism, and energy production, there are hormones that regulate fluid balance, the production of red blood cells, blood pressure, and inflammation.

Integumentary System

Skeletal System

The bones of the skeleton protect the brain, spinal cord, and internal organs and serve as a reservoir of calcium, phosphorous, and other minerals. Calcium, for example, is needed for muscle contraction. Red and white blood cells and other cells of the immune system are made and stored in the bone marrow. The skeleton also makes movement of the body possible which is important for homeostasis. An example of this is when an animal’s core temperature becomes too hot, it can move into the shade of a tree or into the water to cool itself.

Muscular System

Muscles not only work with the skeleton to move the body, but they make digestion and breathing possible. The layers of muscle also protect internal organs and generate heat when they contract (useful for shivering when the body is cold). Finally, the heart is made of cardiac muscle and its pumping of blood is necessary for many of the homeostatic control systems in the body.

Lymphatic System

This system is key to maintaining homeostasis by controlling blood volume and tissue fluids. The lymphatic system works with the capillaries in the cardiovascular system to remove excess fluid which can build up and cause edema and swelling. The lymphatics are also a critical part of the immune system and immune response. After B cells mature in the bone marrow, they migrate to the lymph nodes where they stand guard against foreign invaders in the body. Other parts of the lymphatic system that help maintain homeostasis are the lymph glands, tonsils, adenoids, spleen, and thymus gland.

Respiratory System

The respiratory system transports gases like oxygen and carbon dioxide in and out of the lungs. This is critical to maintaining the proper pH of the blood. If the blood is too acidic, the brain slows the breathing to increase the amount of bicarbonate ions (carbon dioxide) in the blood. Conversely, to adjust the blood chemistry when the pH is too low, respiration increases so that more carbon dioxide is expelled. The respiratory system also acts to dissipate heat when the body temperature gets too hot. This is done through open-mouth breathing or panting in animals that don’t have sweat glands.

Digestive System

The digestive system helps maintain homeostasis by eliminating toxins and waste and supplying nutrients to the body. It also serves the critical immune system function of destroying bacteria and viruses than enter the body through food and water intake. Also, the heat generated during the digestive process contributes to regulation of the core temperature.

Urinary System

The body eliminates nitrogenous waste through urine which is important for maintaining homeostasis in the body. The urinary system also helps control blood pressure by regulating the amount of fluid and ions in the body. Also, the kidneys produce the hormone erythropoietin which stimulates red blood cell production in the bone marrow.