BIOLOGY Volume 3 - A Guide to General Biology - 2004

20. EXCRETION AND OSMOREGULATION

20.4. Urea Production in Humans

Urea is the end product of Nitrogen METABOLISM in humans and other mammals. The advantages of urea as such an end product are as follows:

1) non-toxicity, and therefore The ability to be transported harmlessly in the Blood from the site of production (the Liver) to the excretory Organs (the Kidneys);

2) high solubility, making it easy to transport in solution without requiring large amounts of Water;

3) small molecular size, which ensures easy filtration in the kidneys.

The body cannot store excess Amino Acids obtained from food. Those that are not immediately used for the synthesis of Proteins or other nitrogenous compounds must be eliminated. This takes place in the liver through two processes:

1) deamination — the removal of the amino group to form ammonia;

2) detoxification — The conversion of toxic ammonia into a harmless compound, urea, which is transported to the kidneys.

Deamination

Amino acids are oxidized in the presence of oxygen. The amino group — (—NH₂) is then removed, yielding a keto acid. Through subsequent transformations, this keto acid can enter the Krebs cycle and be used as an energy source for cellular Respiration. The amino group is released as ammonia

(NH₃).

Detoxification

In the liver, ammonia is converted into urea According to the following scheme:

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This occurs via a cyclic sequence of reactions known as the Ornithine cycle, which is simplified in Fig. 20.10. Starting from ornithine, it can be seen that overall, two molecules of ammonia and one molecule of carbon dioxide are converted into one molecule of urea and one molecule of water (or more precisely, two molecules of the latter are formed at different stages while one is consumed), and ornithine is regenerated to participate in the next cycle.

Via Blood Plasma, urea is transported from the liver to the kidneys.

Fig. 20.10. Simplified scheme of the ornithine cycle in the liver. Ornithine and citrulline are amino acids, but they are not incorporated into proteins and are not obtained from diet.

20.1. List in sequence the Blood Vessels and organs through which urea passes on its way from the liver to the kidneys.