Figure 430

Formation and excretion of urea, the major nitrogenous waste product of protein catabolism.

which carries it to the liver (Figure 4-30). The liver contains enzymes that can link two molecules of ammonia with carbon dioxide to form urea. Thus, urea, which is relatively nontoxic, is the major nitrogenous waste product of protein catabolism. It enters the blood from the liver and is excreted by the kidneys into the urine. Two of the 20 amino acids also contain atoms of sulfur, which can be converted to sulfate, SO42~, and excreted in the urine.

Thus far, we have discussed mainly amino acid ca-tabolism; now we turn to amino acid synthesis. The keto acids pyruvic acid and a-ketoglutaric acid can be derived from the breakdown of glucose; they can then be

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

Amino Acids For Arteries

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