Excretion in animals

 Excretion may be defined as the separation and elimination of the metabolic wastes from the body usually in an aqueous solu tion. The waste materials are of several kinds and vary not only from animal to animal, but also in the same animal from time to time. However, the term excretion is generally used with reference to the elimination of the nitrogenus waste products. from the animal body.

During the oxidative metabolism of carbohydrates, carbon dioxide and water are formed as excretory products. The bile pigments bilirubin and biliverdin that are formed in the liver by the break down of haemoglobin are excretory substances which pass out through the intestine. The breakdown of proteins results in amino acids, the excess of which are excreted. While some aquatic animals excrete their extra nitrogen in the form of amino acids as such, most of the animals degrade their excess amino acids to ammonia, uric acid or urea.

Deamination
The first stage in the breakdown of amino acids is the removal of their nitrogenous groups as ammonia. Removal of the amino groups (-NH2) which converts the amino acid into a keto acid is called deamination. Such deamination results from the action of a variety of enzymes, which are either oxidative or hydrolytic.

Nature of Excretory Products
In vertebrates, deamination takes place particularly in liver cells, where any amino acid excess supplied by eaten food is deaminated. Ammonia produced in this way is highly toxic and must be disposed off. Elimination often occurs in the form of unchanged ammonia or in the form of a more complex nitrogenous substance which is either uric acid or urea. Based on the nature of the nitrogenous excretory end-product, animals are classified into 3 main categories :

(i) ammonotelic 

(ii) uricotelic
(iii) urcotelic.

Ammonotelic Animals
Animals which excrete their nitrogenous wastes as ammonia are said to be ammonotelic. Excretion of ammonia which is pre sumed to be the primitive excretory material, is especially suited to an aquatic habitat. Ammonia is highly soluble in water with which it forms ammonium hydroxide (NH4OH). The latter passes readily across water. the cell membranes and is Ammonia is found as the chinersed into the surrounding excretory product in the majority of primitively aquatic animals, certain protozoa, po chaete annelids, crustaceans, some molluscs and teleost fishes. It is also excreted by secondarily aquatic animals such as the larvae of the insects Aeschna, and aquatic tortoises and turtles. Anuran (amphibian) tadpoles excrete mainly ammonia, whereas the adults produce urea.

Uricotelic Animals
Animals which excrete their nitrogen predominantly in the form of uric acid are called uricotelic. All genuinely terrestrial animals, including insects, terrestrial gastropods, terrestrial reptiles land snails excrete solid pellets of uric acid, whereas reptiles and birds void a thick paste of uric acid crystals.

Ureotelic Animals
Animals which excrete urea as their major excretory product are said to be ureotelic. Urea is a useful product for a semi terrestrial animal that is not exposed to a scarcity of water, but is not necessarily surrounded by large quantities of it. It is, therefore, the main product of some earthworms, shore gastropods, adult amphibians and the semi-terrestrial tortoises and turtles. Elasmo branch fishes and mammals are special cases that also excrete urea. In humans, the blood contains about 04 g/100 ml of urea, whereas the level of urea in bladder is 2:0 g/100 ml of urine.

Although ammonia, uric acid and,urea are the major excretory substances, there are a few more types of nitrogenous excretory products:

Trimethylamine oxide

Marine teleost fishes excrete large proportion of their nitrogen-as trimethylamine oxide, a soluble non-toxic substance. It resembles in its properties with urea.

Guanine

Spiders excrete almost exclusively a substance luble than uric acid and, therefore, requires no water for its guanine which resembles with uric acid. elimination. Guanine is even less

Ornithinic Acid 

It is excreted in small amounts by birdsand is formed by a combination of benzoic acid in food with the amino acid ornithine.

Creatine

Mammals excrete creatine which is synthesized in the liver from three amino acids: arginine, glycine and methionine.

Creatinine 

Mammals contain a small quantity of creatinine in their blood (1 mg/100 ml), which is a derivative of creatine. The excess is eliminated along with urine.

It should be remembered, however, that no animal excretes one type of product exclusively.

Reference

1.Animal physiology and biochemistry by Verma Agarwal/ Excretion/pg: 348, 349, 350

Published by Chand publishers

2.Hoar's book of animal physiology