Serum urea concentrations.

From a historical viewpoint, urea is the first organic compound to have been synthesized by man in vitro; physiologically, by contrast, urea is the "last" one to be formed by him in vivo after a protein meal. Thus in the mammal, urea is the chief end product of protein metabolism, comprising some 80% of the non-protein nitrogen normally excreted in the urine per 24 hours. 1 Urea is generated in the liver as the end product of the activity of the first enzymatic cycle to have been described by Krebs, the ornithine-arginine-urea cycle. 2 Amino acids, derived from dietary protein or from peripheral tissues, consequent to the interchange of serum (dietary) and tissue amino acids, serve as the precursors fed into the enzymatic "degradation" process. The rate of urea formation is determined by the quantity of amino acids presented to the liver per unit of time. Accordingly, urea synthesis is