Carbon dioxide, ammonia, urea, uric acid, water and excess ions like phosphate, sulphate, sodium, potassium and chloride are some of the excretory products produced in our body. The process of elimination of metabolic waste products from the animal body is called excretion.
Ammonia, urea and uric acid are known as nitrogenous wastes. Ammonia-excreting animals are called ammonotelic animals and the process is called ammonotelism. Excretion in the form of urea is called ureotelism and such animals are called ureotelic animals. Animals excreting nitrogenous wastes in the form of uric acid are called uricotelic animals and the process is called uricotelism. In mammals, apart from the kidneys, which are the main excretory organs, the skin, liver and lungs also help in excretion.
Carbon dioxide, ammonia, urea, uric acid, water and excess ions like phosphate, sulphate, sodium, potassium and chloride are some of the excretory products produced in our body.
Primitive animals like invertebrates possessed simple organs like protonephridia or flame cells, which in course of time evolved into complex organs, such as kidneys in vertebrates.
Generally, bony fishes, aquatic amphibians and aquatic insects excrete ammonia in the form of readily soluble ammonium ions through their gills or body surface by the process of diffusion.
As excretion of ammonia requires a lot of water, terrestrial animals have evolved a mechanism to convert ammonia into less toxic forms of urea and uric acid, which help conserve water.
Mammals including humans and many marine fish and terrestrial amphibians excrete urea. Another excretory structure is the nephridia found in many annelids like the earthworm.