The pressure in the glomerular capillaries of the rat kidney was determined by micropuncture of individual nephrons. The proximal tubule was blocked by injection of viscous oil. The intratubular hydrostatic pressure increased until it reached a steady state, the intratubular stop-flow pressure. Since the glomerular capillary wall is an ultrafiltration membrane, impermeable to proteins, the sum of intratubular step-flow pressure plus the plasma colloid osmotic pressure should equal the pressure in the glomerular capillaries. This pressure increased as the arterial pressure was raised from 60 to 90 mm Hg. Thereafter it remained constant at 88 Β± 4 mm Hg despite further elevation of the arterial pressure up to 160 mm Hg. Determination of the stop-flow pressure in Bowman's capsule and in the proximal tubules of denervated kidneys gave the same results as above.
The hydrostatic pressure drop in the afferent arterioles was determined by subtracting the glomerular capillary pressure from the arterial pressure. This pressure drop was negligible at arterial pressures below 90 mm Hg and increased linearly as the arterial pressure was elevated from 90 to 160 mm Hg.
Both the above findings are compatible with and confirm the principle of autoregulation of renal blood flow and glomerular filtration rate.