Abstract
In birds, the kidney does not function as the sole organ of osmoregulation, as it does in mammals. The urine of birds, at a relatively low osmotic potential, enters the terminal portion of the gastrointestinal tract and is moved by a reverse peristalsis into the colon. In the colon, the urine comes in contact with epithelial tissues that modify its composition. If the osmotic potential of the urine exceeds the plasma osmolality by more than 200 mOsm/Kg H2O, refluxing of urine into the colon is reduced, which prevents fluid from moving in a serosal-to-mucosal direction. Uric acid, the principal mode of nitrogen excretion by birds, has a very low aqueous solubility, which allows water to be conserved. However, to keep the uric acid in a colloidal suspension in the urine, a significant amount of protein is filtered and secreted by the kidney. This imparts a significant cost for excreting nitrogen as uric acid. J. Exp. Zool. 283:702–707, 1999. © 1999 Wiley-Liss, Inc.
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