In theory, therefore, GFR is highly dependent on hydrostatic pressure. For the kidney, flow (J) is positive, favoring filtration, meaning that plasma flows from higher hydrostatic pressure in the capillary to lower hydrostatic pressure in the tubular space, despite the unfavorable oncotic gradient (there is higher protein concentration in the capillary). Where Kf is the filtration coefficient, and σ is the reflection coefficient, both inherent and fixed values of the epithelium. The reabsorption of 178.5 L requires a sophisticated tubular network. Average urine output, on the other hand, averages only about 1.5 L daily. GFR is approximately 120 ml per min (180 L per day).
Within the plasma, organic and inorganic solutes are freely filtered- meaning that they can be found in the ultrafiltrate (the fluid in Bowman’s space) and plasma at the same concentrations. The RPF is approximately 600 to 720 ml per minute. Thus, the renal plasma flow (RPF) is a more accurate expression and is calculated as follows: RBF*(1-Hct) Of this renal blood flow (RBF), only the plasma can cross the structures comprising the glomerulus. The kidneys receive 20% to 25% of the cardiac output (about 1.0 to 1.1 liters per minute) with the blood entering individual glomerular tufts via the afferent arteriole and exiting through the efferent arteriole. Glomerular filtration rate (GFR) represents the flow of plasma from the glomerulus into Bowman’s space over a specified period and is the chief measure of kidney function.
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