Braun E J
Department of Physiology, University of Arizona, Tucson 85724, USA.
J Exp Zool. 1999;283(4-5):495-9.
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.
与哺乳动物不同，鸟类的肾脏并不独立调节细胞外液的成分。鸟类的尿液进入泄殖腔，并通过反向蠕动作用进入结肠。在结肠中，尿液与改变其成分的上皮组织接触。如果尿液的渗透势明显高于血浆的渗透势，尿液就不会回流到结肠。输尿管尿液的成分在泄殖腔中被感知，进而调节回流活动。虽然放射学证据表明结肠的小收缩波进行实际的回流，但似乎是尿液成分的变化改变了结肠的大收缩波。尿液进入结肠是必要的。这可能是由回收尿液中部分（如果不是全部）蛋白质的需求驱动的。这种蛋白质使尿酸保持在胶体悬浮液中，防止形成会阻塞肾小管的尿酸晶体。因此，肾脏和下胃肠道必须协同作用来调节细胞外液的成分。
