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Excretion in humans is carried out through kidneys, lungs and skin.

Excretion in humans is carried out through kidneys, lungs and skin.

Lungs do excrete metabolic waste products such as carbon dioxide and water.

Nitrogenous wastes are formed as a result of metabolic cellular activities. The most common metabolic wastes are ammonia, urea, and uric acid.

Proteins can’t be stored in the body. When protein is metabolized, nitrogenous wastes such as ammonia and urea are formed. These are then expelled from the body.

Organisms consume food for survival, growth and repair. Most of the food is digested and utilised by the body for the production of energy; the rest is expelled out from the body by the digestive system in the form of faeces. This process is NOT called excretion, though; it is called  egestion  (also known as defecation).

On the contrary,  excretion  is the process where  metabolic wastes  are eliminated from an organism. In humans, this function is performed through kidneys, lungs and skin. In animals, the  main excretory products  are:

There are a few significant modes of excretion observed in living organisms.

Based on the excretory product, five modes of excretion are known in animals. They are:

The process of eliminating ammonia from the body is known as  ammonotelism ,  and the organisms which exhibit this nature are called ammonotelic. Most fish, protozoans, echinoderms, poriferans and crustaceans fall into this category. Aquatic animals excrete ammonia directly into the environment; where the compound is quickly diluted. It is also very toxic to tissues.

In some mammals and amphibians, urea is excreted as the metabolic waste products. Such organisms are called  ureotelic . In these organisms, ammonia that is produced is converted to urea in the liver of animals and is released back into the blood. The kidneys filter the urea and are expelled outside the body. Some of the urea is retained in the matrix of the kidney to maintain a desired osmolarity in the organisms. Humans are ureotelic as we expel the urea through urine. Moreover, urea is comparatively less toxic than ammonia.

Uricotelic animals remove nitrogenous wastes as uric acid in the form of pellets or paste. Metabolically, this process is quite costly; however, the water loss is minimal, and it is the least toxic. Moreover, since uric acid is not readily soluble in water, the excrements form pasty white suspensions. Most reptiles, birds, and insects are classified as uricotelics.

Certain molluscs and echinoderms excrete excess amino acids. This feature is called aminotelism .

Spiders convert the ammonia into guanine before excretion. This characteristic is also found in some reptiles, birds and earthworms. It is also insoluble in water; hence no water is required for its excretion.

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