Renal Blood Supply
Blood flow to the two kidneys is normally about 22 per cent of the cardiac output, or 1100 ml/min. The renal artery enters the kidney through the hilum and then branches progressively to form the interlobar arteries,arcuate arteries, interlobular arteries (also called radialarteries) and afferent arterioles, which lead to the glomerular capillaries, where large amounts of fluidand solutes (except the plasma proteins) are filtered to begin urine formation (Figure 26–3). The distal ends of the capillaries of each glomerulus coalesce to form the efferent arteriole, which leads to a second capillarynetwork, the peritubular capillaries, that surrounds the renal tubules.
The renal circulation is unique in that it has two cap-illary beds, the glomerular and peritubular capillaries, which are arranged in series and separated by the efferent arterioles, which help regulate the hydrostatic pressure in both sets of capillaries. High hydro-static pressure in the glomerular capillaries (about 60 mm Hg) causes rapid fluid filtration, whereas a much lower hydrostatic pressure in the peritubular capillaries (about 13 mm Hg) permits rapid fluid reab-sorption. By adjusting the resistance of the afferent and efferent arterioles, the kidneys can regulate the hydrostatic pressure in both the glomerular and the peritubular capillaries, thereby changing the rate of glomerular filtration, tubular reabsorption, or both in response to body homeostatic demands.
The peritubular capillaries empty into the vessels of the venous system, which run parallel to the arteriolar vessels and progressively form the interlobular vein,arcuate vein, interlobar vein, and renal vein, whichleaves the kidney beside the renal artery and ureter.