Animals consume a lot of food. Some of it is digested and utilized by the body for the production of energy; rest is excreted out from the body. Apart from food, other waste products that are produced are urea, uric acid, ammonia, carbon dioxide, water and ions like sodium, potassium, chloride, phosphates etc. Ammonia, urea and uric acids are the major sources of nitrogenous wastes. The most toxic substance, ammonia requires more amount of water for its elimination whereas uric acid, which is less toxic, requires less water.
The process of eliminating ammonia from the body is known as ammonotelism and the organisms which exhibit this nature are called ammonotelic. Mostly aquatic organisms like bony fishes, insects etc. fall in this category. Ammonia is readily soluble, so it is excreted in the form of ammonium ions either by simple diffusion across body surface or by gills.
In some mammals and amphibians, urea is excreted. They are called ureotelic. In these organisms, ammonia that is produced is converted to urea in the liver of animals and is released in blood which is filtered and released out by the kidneys. Some of the urea is retained in the matrix of the kidney to maintain a desired osmolarity in the organisms.
In case of uricotelic animals, nitrogenous wastes are removed as uric acid in the form of pellets or pastes with a minimum loss of water. Reptiles, birds, snails etc. fall in this category.
Apart from these structures, kidneys are present in Vertebrates, flame cells in Platyhelminthes, earthworms have Nephridia, which help them in the removal of nitrogenous wastes and maintain a fluid, ionic balance. Cockroaches have malpighian tubules which also help in osmoregulation. Similarly, prawns have antennal glands or green glands for the similar purpose.
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