The microcirculation of the renal medulla.

TRUETA et al. proposed in 1947 that morphological differences between renal cortical and medullary structures reflect a '"fundamental difference in the function of the cortical and juxtamedullary vasculonephric units.' This proposition was reinforced when Wirz et al. first published evidence for the countercurrent mechanism as the basis for urinary concentration in 1951. The countercurrent hypothesis attributes an important role to the medullary microcirculation in the regulation of urinary concentration and water balance as a countercurrent exchanger [e.g., see the mathematical concentration model of Stephenson (1972)] and as the route for removal from the medulla of water reabsorbed from the collecting tubules. Changes in the distribution of blood flow between cortex and medulla have been considered to play a role in salt balance (Stein, 1973), and a reduction in medullary blood flow has been proposed as the basis for several pathophysiologjcal disorders (Thurau, 1964; Fisher et al., 1970; Stein et al., 1973; Karlberg et al., 1983).