A simple method for estimating tubular and net glomerular capillary pressures in the isolated perfused kidney

To estimate glomerular filter resistance it is necessary to measure net glomerular capillary pressure, tubular pressure, and glomerular filtration rate. Net glomerular capillary pressure is defined as the difference between glomerular capillary hydrostatic and oncotic pressures. A method has been developed for estimating both tubular pressure and net glomerular capillary pressure for the whole kidney from the time course of the rise in ureteral pressure on ureteral occlusion. Pressure and flow relations which apply in the kidney during ureteral occlusion have been represented by a simple mathematical model. Estimates of tubular pressure and net glomerular capillary pressure are made by representing the first 60s of the rise in ureteral pressure as the linear combination of two exponential terms. These estimates compare favourably with corresponding results obtained from a more general numerical analysis. Direct measurements of tubular pressure and net glomerular capillary pressure made by micropuncture of early proximal tubules have been compared with analytical estimates from subsequent ureteral occlusion. There was no significant difference between results obtained using either method for a wide range of perfusion pressures.