14.1.3—Urea production

Excess amino acids cannot be stored as amino acids. In the liver, deamination removes the amino group and produces ammonia; the remaining carbon skeleton can be used in metabolism, while the nitrogen-containing waste is processed into urea.

Urea pathway boundary:

1. Excess amino acids are broken down and their amino groups are removed by deamination.
2. The removed nitrogen forms ammonia, which is very soluble and toxic if it accumulates in the blood.
3. Liver cells combine ammonia with carbon dioxide to form urea, a less toxic nitrogenous waste.
4. Urea dissolves in the blood and is transported to the kidneys.
5. The kidneys remove urea from the blood and it leaves the body in urine; this card does not describe nephron structure or the detailed formation of urine.

Converting ammonia to urea reduces the immediate toxicity of nitrogen waste and makes its transport in body fluids manageable. Water is required to carry dissolved urea and to remove it, so nitrogen excretion has a water-handling cost; the kidney-level details belong to later cards.

Urea is formed in the liver, not made by the kidney from scratch. Deamination is not the same as complete respiration of the carbon skeleton, and this card does not cover ultrafiltration, selective reabsorption or osmoregulation.