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In this article we will discuss about:- 1. Definition of Excretion 2. Modes of Excretion 3. Other Excretory Wastes.
Definition of Excretion:
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.
Osmoregulation:
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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.
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1. Ammonotelism:
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.
2. Ureotelism:
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).
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(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:
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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:
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Arginosuccinate cleaves arginosuccinate to give arginine and fumarate. Fumarate liberated here provides a connecting link with Krebs cycle, gluconeogenesis, etc.
(v) Formation of Urea:
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.
3. Uricotelism:
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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.
4. Aminotelism:
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.
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5. Guanotelism:
Spiders excrete guanine and are said to be guanotelic and their mode of excretion is called guanotelism.
Dual Excretion:
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.
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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).
3. Ornithuric acid:
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.
4. Hippuric acid:
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.
6. Allantoin:
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.
