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Excretory Products and their Elimination 

Removal of waste product from the body is called excretion.

Waste products are synthesized in the cells due to metabolic activity. These products come into the ECF from the cells. Its total amount is 15 litre.

Extracellular fluid –

Plasma

CSF

Peritoneal fluid

Tissue fluid

Claudy Bernard called ECF Meleium interior. It forms "infernal environment of body.

Maintenance of chemical composition of ECF is called homeostasis.

Homeostasis term was given by Walter Cannon.

On the basis of main excretory products, animals can be divided' into 3 groups.

Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by the animals. Ammonia is the most toxic form and requires large amount of water for its elimination, whereas uric acid, being the least toxic, can be removed with a minimum loss of water.

 

 

The process of excreting ammonia is Ammonotelism.

Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature. Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions. Kidneys do not play any significant role in its removal.

Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic animals. Ammonia produced by metabolism is converted into urea in the liver of these animals and released into the blood which is filtered and excreted out by the kidneys.

Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid in the form of pellet or paste with a minimum loss of water and are called uricotelic animals.

  • EXCRETORY STRUCTURES

Protonephridia or flame cells are the excretory structures in Platyhelminthes (Flatworms, e.g., Planaria), rotifers, some annelids and the cephalochordate - Amphioxus.

Protonephridia are primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation.

Nephridia are the tubular excretory structures, of earthworms and other annelids. Nephridia help to remove nitrogenous wastes and maintain a fluid-and ionic balance.

Malpighian tubules are the excretory structures of most of the insects including cockroaches. Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation.

Antennal glands or green glands perform the excretory function in crustaceans like prawns.

  • EXCRETORY PRODUCTS

Amino acids: These are end products of protein digestion absorbed into the blood from small intestine. Certain invertebrates, like some molluscs (eg llnio, Limnae, etc.) and some echinoderms (eg Asterias) excrete excess amino acids as such. This is called aminotelic excretion or aminotelism.

Ammonia (NH4+ or NH3): In most animals, excess amino acids are deaminated, i.e, degraded into their keto and ammonia groups. The keto groups are used in catabolism for producing ATP, whereas ammonia is excreted as such or in other forms. Ammonia is highly toxic and highly soluble in water. Its excretion as such, therefore, requires a large amount of water. That is why, most of the aquatic arthropods, bony and freshwater fishes, amphibian tadpoles, turtles, etc excrete ammonia. This type of excretion is called ammonotelic excretion or ammonotelism.

Urea CO(NH2H)2: This is less toxic and less soluble in water than ammonia. Hence, it can stay for some time in the body. Many land vertebrates (adult amphibians, mammals) and such aquatic animals which cannot afford to lose much water (e.g. elasmo branch fishes) marine bony fish, adult frog, earthworms, nematodes, turn their ammonia into urea for excretion. This type of excretion is called ureotic excretion or ureotelism.

Uric acid: Animals living in dry (arid) conditions, such as land gastropods, most insects, land reptiles (snakes and lizards), birds and kangroo rat (mammal) etc have to conserve water in their bodies. These, therefore, systhesize crystals of uric acid from their ammonia for the formation of uric acid xanthine oxidase enzyme is necessary. Uric acid crystals are non-toxic and almost insoluble in water. Hence, these can be retained in the body for a considerable time before being discharged from the body. Uric acid is the main nitrogenous excretory product discharged in solid form. This excretion is called uricotelic excretion or uricotelism.

Trimethylamine oxide: Certain marine molluscs, crustaceans and teleost fishes first form trimethylamine from their ammonia by a process known as methylation. Then, the trimethylamine is oxidised to trimethylamine oxide for excretion. This oxide is soluble in water, but non-toxic.

Guanine: Spiders typically excrete their ammonia in the form of guanine. Some guanine is also formed in unio, penguin, birds and it is insoluble in water. Hence, no water is required for its excretion.

Hippuric and ornithuric adds: Sometimes food of rabbit and other mammals may contain traces of benzoic acid, or this acid may be formed in small amounts during fat metabolism. It is highly toxic. As it is absorbed in blood, it is combined with glycine and changed into less toxic hippuric acid for excretion. In birds, benzoic acid is combined with ornithine and changed into ornithuric acid for excretion.

Creatine and creatinine: Muscle cells contain molecules of creatine phosphate, which are high energy molecules and serve for storage of bio-energy like ATP. It is synthesised by 3 amino acids (G.A.M.) (Glycine, Arginine and Methionine). Excess amount of this phosphate is, however, excreted out as such, or after being changed into creatinine. 

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