Integration of renal and gastrointestinal function.

In birds, the kidney does not independently regulate the composition of the extracellular fluid, as is the case for mammals. The urine of birds enters the cloaca and is moved by a reverse peristaltic action into the colon. In the colon, the urine comes in contact with an epithelium that modifies its composition. If the osmotic potential of the urine is significantly higher than that of the plasma, the urine will not be refluxed into the colon. The composition of the ureteral urine is sensed in the cloaca which in turn modulates the refluxing activity. It appears to be the large contraction waves of the colon that are modified by changes in the composition of the urine, although radiographic evidence indicates that the small contraction waves of the colon do the actual refluxing. It is necessary for the urine to be moved into the colon. This may be driven by the need to recover some, if not all, of the protein in the urine. This protein maintains uric acid in a colloidal suspension that prevents the formation of uric acid crystals which would block the renal tubules. Thus, the kidneys and lower gastrointestinal tract must function in concert in the regulation of the composition of the extracellular fluid.