GLOMERULAR FILTRATION AND RENAL BLOOD FLOW

The kidneys receive an enormous blood flow: more than 1 L/min, or about
20% of the cardiac output. This blood flow is far in excess of the kidney’s
metabolic need and provides the kidneys with the flexibility to alter their
blood flow in response to physiological demand. All of this blood flows through
glomeruli in the cortex.1 The vast majority continues on (via efferent arterioles) to peritubular capillaries in the cortex and then into the renal venous system. A much smaller fraction, about 5–10%, flows from efferent arterioles down into the medulla. This medullary blood derives from juxtamedullary glomeruli that are situated near the corticomedullary border. Consider some typical numbers. A normal hematocrit is 0.45, ie, 45% of the blood volume is composed of red blood cells and the remaining 55% is almost entirely plasma. Typical renal blood flow (RBF) is 1.1 L/min. The renal plasma flow (RPF) 5 0.55 × 1.1 L/min 5 605 mL/min. As stated in Chapter 1, a typical glomerular filtration rate (GFR) is about 125 mL/min. Thus, of the 605 mL of plasma that enters the glomeruli via the afferent arterioles, 125 mL, or 20%, filters into Bowman’s space. The remaining 480 mL passes via the efferent arterioles into the peritubular capillaries. This ratio—GFR/RPF—is known as the filtration fraction. Because freely filtered substances are passing into Bowman’s space along with the water, about 20% of all freely filtered substances (eg, sodium) that enter the kidney also move into Bowman’s space.