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The Excretory System

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The role of the liver is to filter toxins and remove them from the body in the form of nitrogenous waste. Nitrogenous waste is ammonia, urea and uric acid. In the human body only ammonia and urea are produced and the liver quickly changes the harmful ammonia into the tolerable urea for excretion from the body.

Ammonia is toxic to humans in high concentrations (see Table 1). Ammonia is the by-product of protein being broken down. Ammonia is formed by the breakdown of amino acids taken from excess proteins. The liver converts the ammonia into urea, which is then removed from the body in the form of urine (BBC Science and Nature – Human Body and Mind – Organ Layer – visited 14 April 2014).

This conversion is done through the urea cycle (see Figure 1) in the liver. The urea cycle is the conversion of carbon dioxide and ammonia into urea and water. After the ammonia is changed into urea, it is transferred into the bloodstream and on to the kidneys. Then the kidneys filter the urea out of the blood. The urea is mixed with water and the urine is expelled from the body via the bladder whilst keeping the water balance isotonic. Urea is the by-product of the urea cycle, which converts the highly toxic ammonia into the tolerable urea. The liver is the only place in the body where the complete urea cycle is observable (Neonate/Infant/Child with Hyperammonemia – 2013).

Water balance is maintained throughout the body but the kidneys play a major role. The kidneys normalize the quantity of water in the body and balance the absorption of mineral ions in the blood (Homeostasis – Kidneys and water balance – ABPI – resources for schools – visited 14th April). They achieve a state of homeostasis that, according to ABPI resources for schoo...

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