The metabolic wastes which are excreted by animals may be grouped as follows:
Respiratory Waste Products
Carbon dioxide and water are the by-products of catabolism of all varieties of foodstuffs. In lower animals carbon dioxide is eliminated directly into the environment through the body surface. In higher animals it is excreted out along with the exhaled air through the lungs. Excess water is excreted in the form of urine and sweat.
Nitrogenous Waste Products
Nitrogen containing waste products are derived from the deamination of the excess amino-acids consumed in the food and also from the breakdown of the proteins and nucleic acids. The following are the nitrogenous waste products produced by animals:
Ammonia
This is the main nitrogenous compound produced due to oxidative or deamination of amino acids during protein metabolism. Ammonia is highly toxic and it is important that it is expelled out of the body as and when it is formed as it can cause severe harm to the organism.
Urea
In higher animals ammonia combines with carbon dioxide in the liver and forms a less toxic substance called urea which is periodically flushed out of the system of the animal. Man eliminates nitrogenous waste in the form of urea.
Uric Acid
This is non-toxic and almost insoluble in water. Thus excretion of uric acid takes place with minimum loss of water. In man small quantities of uric acid is excreted in urine. In birds and reptiles, uric acid is excreted as a thick white paste or in the form of pellets. This enables these animals to conserve water.
Amino Acids
Excess amino acids or proteins in higher animals are broken down into ammonia which are converted into less toxic substances like urea and uric acid in the liver. But in lower animals like molluscs and echinoderms the excess amino acid gets removed without undergoing any change.
