Anatomy: Animals Inside

Excretory System

What type of nitrogenous waste do animals excrete?

Ammonia, urea, and uric acid are nitrogenous waste products—the result of the breakdown of various molecules, including nucleic acids and amino acids. Since it is highly toxic, excretion of pure ammonia is possible only for aquatic animals because ammonia is very soluble in water. Urea and uric acid are excreted by terrestrial animals. Urea is approximately 100,000 times less toxic than ammonia, so it may be stored in the body and eliminated with relatively little water loss. In addition, uric acid requires very little water for disposal and is often excreted as a paste or dry powder—for example, guano from seabirds and bats is excreted as solid white droppings.


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