The Integrative Aspects of Fluid and Electrolyte Balance in Birds

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9515450 Dantzler In mammals the kidney is the sole organ responsible for regulating the water and salt content of the body and mammals have a urinary bladder in which to store the output of the kidneys (urine) until the bladder can be emptied at a convenient and safe time. In birds on the other hand, the kidneys function together with the lower gastrointestinal (GI) tract to regulate the salt and water content of the body. This is because birds do not have a urinary bladder in which to store urine. Instead, the urine enters the terminal portion of the GI tract, the cloaca. The urine does not remain here to be excreted, but is moved into the GI tract ( the colon) by muscular contractions of the cloaca and colon. In the colon, the composition of the urine can be modified by the cells that make up the lining of the colon. Thus, the kidney must work together with the lower GI tract to maintain the fluid and salt balance. How this integration of function is controlled is one of the major emphasis of the proposed research. A better understanding of how birds regulate water and salt balance is of major importance to growers of birds for commercial purposes, especially the poultry industry where quite frequently disturbances of water and salt balance in flocks can have significant economic impact. Another emphasis of the proposed work deals with how birds excrete the nitrogen that is formed as a by product of metabolism. Mammals excrete nitrogen as urea which is quite soluble in water and seldom causes problems unless there has been major kidney damage. Birds on the other hand excrete their excess nitrogen as uric acid, a compound that is virtually insoluble in water and tends to precipitate from solution and form large plate-like crystals. However, when bird urine is examined no crystals of uric acid are found despite of very high concentrations of uric acid. Instead of crystals the urine contains small spheres that are sufficiently small to pass along the kidney tubules without pl ugging the tubules. We have already determined that these small spheres are up of uric acid and protein. Very little is known by what mechanism and where in the kidney these spheres of uric acid form. Birds do not often form kidney stones, and it is not understood why stones are not more prevalent with the large amount of uric acid in the urine. However, out breaks of kidney stones do occur in commercial flocks with disastrous effects. Understanding the basic process of how bird kidneys handle uric acid will help people to deal more efficiently with out breaks of kidney stones in commercial flocks and in the trade that deals with exotic birds. In summary, the results of the studies should lead to a better understanding of kidney function in birds and how the kidneys function together with the lower GI tract to maintain whole animal water and salt balance.