Abstract
A model of the renal medulla is formulated, taking into account the transport properties of each medullary structure. Using known data on inflow to the vascular and tubular systems and on urine output in normal man in hydropenia, and assuming medullary osmotic profiles similar to published data on hydropenic dogs, transport coefficients are calculated for each medullary structure. The calculated coefficients compare reasonably well with measurements made in the rat kidney. A normal hydration case is calculated by reducing the collecting duct permeability until a urine output and concentration of 1.0ml/min and 700 mosm/l is obtained. The coefficients are then kept constant, and medullary osmotic profiles and urine output corresponding to various system imputs are calculated. These studies show that medullary osmolarities decrease and urine flow increases with increasing medullary blood flow and that an optimal rate of proximal reabsorption exists for producing a maximally concentrated urine. Such results are consistent with observations on renal function reported both experimentally and clinically.
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