The pressure-flow relationship of different nephron populations in the rat.

The mechanisms behind the autoregulation of the total renal blood flow and the glomerular filtration rate are unclear. In this investigation a modified microsphere technique was applied to measure the blood flow at different depths in the renal cortex during normotensive and hypotensive conditions. No autoregulation was found in the outer cortex while it was well pronounced in the inner one. During similar conditions, glomerular capillary pressure, welling point pressure and intratubular pressure were recorded. By combining these results with the blood flow data, the preglomerular and postglomerular resistances were calculated. It was then found that the preglomerular resistance decreased and the postglomerular resistance increased when the blood pressure was lowered. The results indicate a redistribution of blood flow from the outer parts to the inner parts of the cortex when the blood pressure is decreased. The redistribution of the blood flow might explain the well known linear relationship between the arterial pressure and the urine flow. The single nephron filtration rate of the outermost glomeruli could be calculated and the results seem to indicate a non-equilibrium at the end of the glomerular capillaries.