The urinary concentrating mechanism: a model of integrative physiology.

One of the most efficient water conservation mechanisms in mammals is the ability of the kidneys to produce concentrated urine for the elimination of waste products while losing a minimum of water. Depending on the food supply of differing animals, components of the glomerular ultrafiltrate are concentrated in the final urine to different extents. For example, carnivorous animals preferentially eliminate urea, the waste product of protein metabolism, whereas herbivores concentrate potassium but conserve urea as nitrogen source of protein synthesis, and marine mammals concentrate salt ingested from sea water. Thus, although there are differences in the underlying renal mechanisms, the general organization of these systems is similar in all mammals, including humans.