Zoology · Ch 8 — Excretion
Mechanism of urine formation in human
Mechanism of urine formation in human
Before urine can be formed, the body first has to generate the waste it is designed to remove. When cells break down amino acids for energy, the amino group is released as ammonia, which is far too toxic to circulate in the blood in any quantity. The liver neutralises this danger by running the ornithine cycle (also called the urea cycle): ammonia and carbon dioxide are combined into carbamoyl phosphate, which is fed into a cyclical series of reactions involving ornithine, citrulline and arginine, ending with the release of a molecule of urea and the regeneration of ornithine so the cycle can run again. This urea is the main nitrogenous waste that human urine is built to carry away. Once it is in the bloodstream, actually turning it - along with water, ions and other wastes - into urine is the job of the nephron, and that h …
What this figure shows. The ornithine (urea) cycle as it occurs across the mitochondrion and cytoplasm of a liver cell: ammonia (NH4+) and CO2 combine with the help of carbamoyl phosphate synthetase to form carbamoyl phosphate, which condenses with ornithine to form citrulline; citrulline moves to the cytoplasm and combines with aspartate to form argininosuccinate, which splits into fumarate and arginine; arginine is then hydrolysed to release urea and regenerate ornithi …