A. The afferent and efferent arterioles are the major sites of control of renal blood flow.Figure 2-1 illustrates the changes that occur in the hydrostatic pressure of blood as it flows from the renal artery through the arterioles and capillary beds into the renal vein. The largest falls in pressure occur in the afferent and efferent arterioles. These are the sites of greatest resistance to flow, and therefore, the major sites of control of blood flow.

   
Fig. 2-1.Pressure gradients and control points in the renal circulation. H.P. = hydrostatic pressure. _b = colloid osmotic pressure.

B. The afferent and efferent arterioles maintain a high pressure in the glomerular capillary bed and control the rate of blood flow through the bed.The unique positioning of the glomerular capillary bed between the two major resistance sites, the afferent and efferent arterioles, permits the maintenance of a relatively high hydrostatic pressure in that bed and also provides a mechanism for close control of the pressure and flow.

C. The afferent and efferent arterioles maintain a low pressure in the peritubular capillary bed. Hydrostatic pressure is low in this capillary bed because of the high resistance of the afferent and efferent arterioles upstream.

QUESTIONS:  
1.What are the two major sites of resistance to blood flow in the kidney?

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