Lesson 1 of 13 · 7 min
Nitrogenous wastes
NCERT §16 (opening text)
Meher's muscles broke down protein all through training. The nitrogen in it has to go somewhere, and her body cannot just breathe it out like CO₂. So where does it go?
The story this chapter follows: Meher's hot Sports Day
The lesson in notes
In short
Animals build up ammonia, urea, uric acid, CO₂, water and ions such as Na⁺, K⁺, Cl⁻, phosphate and sulphate, some from metabolism and some from other routes such as eating too much of something. These must be removed, wholly or partly.
The chief nitrogenous wastes are ammonia, urea and uric acid. Ammonia is the most toxic and needs a lot of water to be flushed out; uric acid is the least toxic and can be removed while losing very little water.
Ammonotelism is excretion of ammonia. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic.
Ammonia dissolves readily, so it usually leaves by diffusion as ammonium ions across the body surface, or across the gill surface in fish. The kidneys play no significant part in removing it.
Life on land made water precious, so land animals switched to less toxic wastes, urea and uric acid, which save water.
Ureotelic animals excrete mainly urea: mammals, many terrestrial amphibians and marine fishes. Their liver turns metabolic ammonia into urea, which enters the blood and is filtered out and excreted by the kidneys.
Some ureotelic animals keep a little urea in the kidney matrix to hold the osmolarity they need there.
Uricotelic animals excrete uric acid as a pellet or paste, losing hardly any water: reptiles, birds, land snails and insects.