OPTIMAL TRANSPORT PARAMETERS OF THE INNER MEDULLARY COLLECTING DUCT IN INTERACTION BETWEEN URINE CONCENTRATING AND UREA EXCRETING MECHANISMS - A COMPUTER-SIMULATION STUDY

Although the accumulation of urea in the renal medulla is essential fo r the formation of concentrated urine, it is also necessary for the ki dney to excrete considerable amounts of urea into the urine as a waste product of protein degradation. Thus, the urine concentrating capacit y is attained by the interaction with the efficiency of urea excretion . To seek the best condition for this phenomenon, we developed an obje ctive function for evaluating urea excreting capacity relative to urin e concentrating capacity by using a mathematical model consisting of c omponents of the countercurrent multiplication system: the ascending t hin limb, capillary network, and inner medullary collecting duct. The values of the objective functions were calculated as three-dimensional functions of transport parameters for the inner medullary collecting duct, including hydraulic conductivity, urea permeability, and reflect ion coefficient for urea, The results of the computer analysis reveale d that the maximum value of the objective function was attained when v alues for transport parameters of the inner medullary collecting duct corresponded to those experimentally obtained values reported previous ly. We conclude that the maximum urine concentrating capacity is limit ed by the efficiency of urea excreting capacity of the kidney, and vic e versa.