Blood Flow Velocities And Pressures In The Renal Vasculature

The glomerulus is a unique structure designed to produce rapid ultrafiltration of fluid from the plasma across the glomerular capillary endothelium and into

FIGURE 1 Scanning electron micrograph showing a vascular cast of a glomerular capillary tuft and its associated afferent arteriole (AA) and efferent arteriole (EA). Within the glomerular tuft smaller capillaries are seen interconnecting larger loops. (Electron micrograph courtesy of Dr. Andrew P. Evan, Indiana University Medical center, Indianapolis, IN.)

Bowman's space. As shown by the scanning electron micrograph in Fig. 1, the glomerular capillaries are supplied by the afferent arteriole. However, the blood that leaves the glomerulus does not flow out through a venule as in most capillary systems, but through a second resistance vessel, the efferent arteriole. Because of this arrangement, most of the total drop in pressure from the arterial to the venous side of the circulation is divided between the two arterioles, resulting in a higher blood pressure in the glomerular capillary network and a lower blood pressure in the peritubular capillary network. The distribution of the fall in hydrostatic pressure is given in Table 1.

As can be seen in Table 1, the largest drop in pressure from the mean systemic arterial pressure occurs in the afferent and efferent arterioles, which have roughly equal resistances. Because of the higher transmural pressure in the afferent arteriole, thick rings of vascular smooth muscle that provide the contractile force to maintain and alter the resistance surround it. The afferent arteriole provides about 35% of the total resistance to flow within the kidney, causing a fall in blood pressure from 90 mm Hg in the medium-sized arteries to —60 mm Hg in the glomerular capillaries. Because of the lower transmural pressure, the smooth muscle of the efferent arteriolar is thinner and distributed more irregularly, but it drops the pressure by a further 33-35 mm Hg.

Glomerular Capillaries

The relative resistance of these two arterioles determines the average pressure within the glomerular capillaries, and it is estimated to be about 60 mm Hg in humans. This pressure is considerably higher than in the capillary beds of other organs. Because of the large cross-sectional surface area provided by the numerous capillary loops (Fig. 1), the blood pressure falls by at most 2 mm Hg along the glomerular capillaries and the flow velocity is relatively slow.

The tone of the smooth muscle in the arterioles is the primary determinant of their resistance. However, it is conjectured that their resistance might also be altered by mesangial cells, which are interstitial cells located between the afferent and efferent arterioles within the glomerulus and just external to it. Mesangial cells contract or relax in response to various hormones. contraction of the mesangial cells could augment the resistance of the arterioles and possibly change the number of open capillary loops in the glomerular tuft, but there is presently no definitive information about the role of these cells in regulating arteriolar resistances.

on leaving the glomerulus, the efferent arteriole branches to form the peritubular capillary network. The densest region of the peritubular capillary network is in the cortex. Each efferent arteriole supplies capillaries near the nephron of its glomerular origin, and about 90% of the postglomerular blood flows only through the cortex. The remaining 10% of the blood flow supplies the medulla via the elongated capillary loops formed by the ascending and descending vasa rectae. Vasa rectae

TABLE 1 Pressures and Resistances in the Renal Vasculature
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