The metabolic wastes which are excreted in the animals are of several kinds and vary from animal to animal and in the same animal from time to time.
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The difference in the nature of waste products in animals is correlated with the metabolic processes taking place in the body of the animals.
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For convenience the metabolic wastes which are excreted by the animals may be grouped under the following heads:
1. Respiratory waste products:
Carbon dioxide and water are the by-products of catabolism of all classes of food stuffs. In small animals carbon dioxide is eliminated directly into the environment through the body surface but in higher animals it is eliminated almost exclusively with the expired air through the lungs. Excess of water is eliminated in the form of urine and sweat.
2. Nitrogen containing waste products:
The nitrogen containing waste products are derived partly from the deamination of the excess amino-acids taken in with the diet (exogenous source) and partly from the break-down of the animals own proteins and nucleic acids and also miscellaneous compounds (endogenous source).
The amount of waste nitrogen is determined by the utilization of protein for energy and by the rate of break down and turnover of body cell constituents.
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Some important specific nitrogen containing waste products of the animals are as follows :
(i) Ammonia:
It is one of the important nitrogenous compounds which is very toxic in the body. It is very soluble and can only be excreted in very dilute solution.
It is the major nitrogen excretory substance of many aquatic animals such as crustaceans. Ammonia is formed as a result of oxidative or hydrolytic deamination of amino acids.
(ii) Urea:
It is a common excretory product and is less toxic in nature in comparison to other nitrogenous compounds.
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It is generally found in aquatic animals such as fishes and those terrestrial forms such as mammals which are not well adapted to water conservation.
(iii) Uric acid:
Uric acid and its salts, the urates, are relatively insoluble and much less toxic than ammonia and urea; therefore, they can be stored in the body.
It is the major nitrogenous product of those animals which conserve water as one of the ways of survival on land, such as, birds, terrestrial reptiles, some snails and insects, etc.
It is the only nitrogenous excretory product which can be removed from the body in solid form, it, thus, permits nitrogen excretion without loss of water.
(iv) Amino acid:
In certain animals like molluscs (Limnaea, Paludina) and echinoderms (Pentacentrolus) the excess of amino acids is removed as such without undergoing any further change.
Amino acids are formed as a result of hvdrolytic proteins in the alimentary canal.
(v) Other nitrogenous compound:
Other nitrogenous excretory products include allantoin and allantoic acid. These are insoluble and are used during embryonic development by amniotes with shelled eggs.
There are also endogenously derived nitrogenous waste products such as guanine and adenine from nucleic acid break down and creatine from the creatine of muscles, which are excreted with other nitrogenous products in the urine.
3. Mineral ions as waste products:
The excess mineral ions taken in with the diet are also excreted by one means or another. In the vertebrates ionic composition of urine is controlled by hormones secreted by the adrenal glands.
Sodium, potassium, calcium, magnesium, chloride and ammonia are the essential mineral ions of the animals.
The ionic concentration of them may differ in different animals corresponding to the different environments in which they are found.
4. Water as an excretory product:
Most animals, whether they live on land or in ponds or in the sea, take in large quantities of water with their food and many mammals and birds drink it as well.
Some water is also formed during the metabolism of the food stuffs in the body. No doubt it is very essential for the body but its excess in the body may result in a serious condition of edema, therefore, its excess must be removed from the body by any means.
In freshwater animals the excess of water is eliminated in the form of dilute urine while in terrestrial animals it is removed in the form of urine as well as in the form of sweat.