Regulation of renal and lower gastrointestinal function: role in fluid and electrolyte balance

For the majority of vertebrates, the kidneys are not the sole organs that function to maintain homeostasis of body fluid and electrolytes. Mammals are unusual in this respect, as the kidneys are the organs that fill this role. For non-mammalian vertebrates, other organs such as gills, skin, salt glands, urinary bladders and the gastrointestinal (GI) system function in concert with the kidneys in the control of fluid and ion balance. Birds are of particular interest and unique as they do not possess a urinary bladder and the renal output enters the lower GI tract. The physiology of the interaction of avian kidneys and lower GI tract is an excellent example of integrative physiology and several aspects of it have been examined, for example, the role of the avian antidiuretic hormone (arginine vasotocin, AVT) in controlling renal output. AVT produces both a tubular and glomerular antidiuresis. The glomerular antidiuresis is important, as the fluid from the kidneys that enters the GI should not be highly concentrated. Another hormone, aldosterone, has been shown to play an important role in regulating the transport of sodium by the GI epithelium. In addition, the lower GI tract plays a significant role in recycling a portion of the nitrogen that leaves the kidneys as uric acid. Furthermore, the output of avian kidneys contains large amount of protein that is conserved by the lower GI tract.