Because fluid filtration from glomerular capillaries into the renal tubule system is the necessary first step in urine formation, the rate of this process is an important determinant of the rates of subsequent transport processes and ultimately of urine excretion. Glomerular filtration rate (GFR) depends on both the pressure and rate at which blood is delivered to the kidneys via the renal arteries (220). Changes in arterial pressure and renal blood flow (RBF) may result in changes in GFR; however, this is not always the case. Conversely, in some situations GFR may change even though arterial pressure and RBF do not. In such situations intrarenal regulatory mechanisms clearly are responding to other factors.

The immediate determinants of GFR are the net driving pressure across the filtration barrier and the permeability and area of the barrier. Particularly since the advent of appropriate micropuncture techniques, the magnitudes of these quantities have been determined in a wide variety of conditions. Direct micropuncture measurements of the net hydraulic pressure favoring filtration and of the single-nephron filtration fraction in rat kidneys (39, 40)