A large number of investigators have measured the end-products of nitrogen metabolism in the blood and their excretion in the urine in health and in disease as an aid in evaluating the functional capacity of the kidneys. Ambard and Weill1 were the first to relate the concentration of urea in the blood to its concentration in the urine and to the urinary volume as expressed by "Ambard's coefficient." Addis and his collaborators2 showed that more constant results could be obtained if, under standard conditions of fluid intake, the rate of urea excretion was measured as urea in 1 hour's urine urea in 100 cc. of blood. They introduced the assumption, and supported it by animal experiments, that the weight of the kidney is proportional to surface area rather than body weight, which was used by Ambard. Under Addis' standard conditions the volume of urine in normal persons was
SCHOENTHAL L, LURIE D, KELLY M. UREA CLEARANCE IN NORMAL AND IN DEHYDRATED INFANTSRENAL FUNCTION IN INTESTINAL INTOXICATION. Am J Dis Child. 1933;45(1):41–53. doi:10.1001/archpedi.1933.01950140051005
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