Animals cannot eliminate free nitrogen, but can
eliminate it in the form of nitrogenous end products. The metabolism of excess
amino acids, and the nitrogen bases of nucleic acids namely purines and pyrimidines produce ammonia from which
urea or uric acid is formed depending upon the need.
I.
Ammonotellsm: The elimination of ammonia as the chief nitrogenous
waste material is termed ammonotellsm. Ammonia is formed by the oxidative deamination of
amino acids. Deamlnation
chiefly
occurs in the liver of the vertebrate body. The amino group, separated from the
amino acid, combines with hydrogen and becomes ammonia (NH3).
Ammonia is highly toxic and readily soluble in water, hence it should be
eliminated from the body quickly and in a very dilute solution. Therefore
excretion of ammonia is most common in aquatic species. Many lower
invertebrates expel ammonia through the whole body surface by simple diffusion. In bony fishes, most of the
ammonia is lost as ammonium ions (NH^ across the epithelium of the gills, with
kidneys excreting only minor amounts of nitrogenous wastes. The animals which
excrete ammonia are called ammonotelic animals.
II.
Ureotelism: Terrestrial adaptation necessitated the production of
lesser toxic nitrogenous wastes such as urea and uric acid, for the
conservation of water. The elimination of urea as the principal nitrogenous
waste material is termed ureotelism It is produced in the vertebrate liver by a metabolic
cycle (ornithine cycle) that combines ammonia with carbon dioxide. The circulatory
system transports the urea to the kidneys for filtration and elimination. Urea
is 100,000 times less toxic than ammonia. This permits some animals to
transport and store urea safely at higher concentrations (Physiological uremia
as seen in cartilaginous fishies). Thus 'urea-excreting' animals require much
less water. Some amount of urea may be retained in the medullary fluid of
kidneys to maintain the desired osmolality. Earthworms, cartilaginous fishes,
most of the amphibians (which spend most of the time on land) and mammals
excrete urea as their chief nitrogenous waste and are called ureotelic animals.
Uricotelism: The elimination of uric acid as the chief nitrogenous
waste material is called uricotelism. Uric acid is mainly formed from ammonia
mostly in the liver of sauropsids and in the Malpighian tubules of tracheate
arthropods. Uric acid is less toxic than urea and being insoluble in water can
be excreted as semisolid paste or pellets with very little water loss. This become
a great advantage for animals with little access to water. Tracheate
arthropods, land snails, many reptiles and birds excrete uric acid as their
major nitrogenous waste, hence they are called uricotelic animals.
