Excretion is the removal of waste products from the body of an organism. Based on the type of product, there are three types of animals. The kidney is the primary excretory organ in humans. Lung, skin, liver and intestine are accessory excretory organs.
Physiology of Excretion
The process of excretion has two parts
1. Formation of urea in the liver by urea cycle or ornithine cycle.
2. Formation of urine.
Different Modes of Excretion
Depending upon excretory products, all total there are five modes of excretion in animals. They are:
Ammonotelism: Excretory product is ammonia.
Ureotelism: Excretory product is uric acid.
Aminotelism: Excretory product is amino acids.
Uricotelism: Excretory product is uric acid.
Guanotelism: Excretory product is guanine.
The process in which ammonia is eliminated from the body is called ammonotelism. And the organism which exhibits this as an excreted material known as ammonotelic. Examples of organisms are fish, protozoans, echinoderms, poriferans and crustaceans. Such excretion is mainly shown in aquatic animals, as they excrete ammonia directly into the environment, where dilution of compounds occurs very quickly as this compound is very toxic to tissues.
In organisms like mammals and amphibians, waste material is in the form of urea and organisms which excrete urea are known as ureotelic. As ammonia is converted into urea in the liver of animals and released back into the blood. Further kidney filters the urea and excretes it outside the body. Some of the urea remain inside the matrix of the kidney so that they can maintain the required osmoregulation in the organism. Humans always excrete urea and because of this, they are ureotelic in nature. However, in comparison to ammonia, urea is less toxic.
Organisms that are uricotelic in nature remove their nitrogenous wastes as uric acid in the form of pellets or paste. In these organisms loss of water is very less and is least toxic also. As uric acid is insoluble in water because of this reason, the excreta is in the paste form. Examples of uricotelic organisms are birds and insects.
In this type of excretion, mode excreta is in the form of excess amino acids. This process is known as aminotelism. Example: Mollusca.
In this mode of excretion, organisms convert ammonia into guanine before excretion. Some which show such type of excretion are reptiles, birds, earthworms. As guanine is not soluble in water so no water is required.