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Renal Perfusion Pressure and Vascular Resistance
Renal blood flow, like blood flow through any organ, is defined by the pressure difference between the arterial and venous ends of the vascular bed, and by the vascular resistance of that bed. The equation relating pressure difference (DP), resistance (R), and flow (Q) is given in the figure and is the same for any organ.
The distinguishing feature of the renal circulation is the presence of two arteriolar resistance sites in series across the glomerular capillary bed. The mean perfusion pressure in the renal artery is about 100 mm Hg. Perfusion pressure across the entire kidney drops from 100 mm Hg in the renal artery to about 8 mm Hg in the renal vein. The major contributions to this decrease are a 40 mm Hg decrease across the afferent arteriole and a 42 mm Hg decrease across the efferent arteriole as the blood enters and leaves the renal glomerulus.
The unique serial arrangement of afferent and efferent arterioles permits the maintenance of a high, 60 mm Hg, hydraulic filtration pressure across the glomerular capillary membranes. The maintenance of the glomerular capillary pressure depends on the relative values of the afferent and efferent arteriolar resistances. Changes in afferent and/or efferent arteriolar resistance may affect both renal blood flow and glomerular filtration pressure. A further decrease in perfusion pressure, to about 18 mm Hg, occurs across the efferent arteriole and pressure decreases to about 10 mm Hg as blood flows along the peritubular capillaries. As in other capillary beds the transcapillary fluid movement is determined by the difference in capillary hydraulic and oncotic pressures. Because protein is not filtered its concentration increases along the glomerular capillary. The oncotic pressure of the plasma entering the efferent arteriole is greater than the systemic arterial oncotic pressure and the peritubular capillary hydraulic-oncotic pressure gradient favors fluid reabsorption and facilitates reabsorption of large volumes of the filtrate from the tubules.
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