The process of excreting nitrogenous waste in the form of ammonia is termed ammonotelic. The phenomenon is recognized as Ammonotelism. Birds, amphibians reptiles, most of the aquatic animals including fishes, some terrestrial invertebrates, larvae, and mammals including humans excrete urea acid as waste.
Ammonia, which is highly soluble in water and which forms ammonium hydroxide (NH4 OH) that injures cells directly by the alkaline caustic action. Therefore excretion of ammonia requires large amounts of uric acid. Part of uric acid is oxidized further to form allantoin and allantoic acid. Teleost fish excrete allantoate or hydration product of allantoin. In most fishes and amphibians, allantoate is hydrolyzed into urea and glyoxylate. For some marine invertebrates, it goes a step further by hydrolyzing urea to ammonia and carbon dioxide. Creatine is formed in the liver from amino acids. The resultant creatinine is produced from creatine.
Almost all animals possess a certain mechanism of getting rid of the waste substances formed in their body during metabolic activities. These waste substances include carbon dioxide, water, urea, uric acid, ammonia, etc. such substances could be harmful if retained in the body. Besides metabolic wastes, excess salt (eg. Sodium from food), water and even certain excess vitamins need to be eliminated. Certain medicines including antibiotics are removed from the blood into the urine. Removal of all harmful, unwanted products (especially nitrogenous wastes) from the body is called excretion. The excretory system is primarily associated with removal of nitrogenous wastes. Urea is the main nitrogenous waste produced in our body. It is formed by the breakdown of surplus amino acids and nucleic acids in the liver. Blood transports urea to the kidneys for filtration and removal in the form of urine.
Importance of excretion
- Excretion is necessary for the elimination of nitrogenous wastes formed during the metabolism of proteins (amino acids) and nucleic acids.
- For elimination of excess salts like Sodium chloride (NaCl), vitamins, bile pigments (from the breakdown of old RBCs)
- The removal of excess water or its retention in case of shortage. This helps to maintain the required quantity of water – osmoregulation in the body.
|Category||Product formed||Solubility in water||Examples|
|Ammonotelic||Ammonia (highly toxic)||Highlysoluble, therefore needs plenty of water for its excretion||Freshwater aquatic animals e.g. bony fish, Amoeba, etc.|
|Ureotelic||Urea (less toxic)||Less soluble, thus needs less water for excretion||Mammals including man, dog etc, marine fishes and amphibians like frog and toad|
|Uricotelic||Uric acid (least toxic)||Insoluble solids or semi-solid. Needs very little water just to flush out the uric acid||Birds, reptiles, and insects.|
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