Figure 214

Cellular mechanisms of increased solute and water reabsorption by the proximal tubule in patients with "effective" arterial volume depletion. A, Normal effective arterial volume in normal persons. B, Low effective arterial volume in patients with both decreased glomerular filtration rates (GFR) and renal plasma flow (RPF). In contrast to normal persons, patients with low effective arterial volume have decreased GFR and RPF, yet the filtration fraction is increased because the RPF decreases more than does the GFR. The increased filtration fraction concentrates the plasma protein (indicated by the dots) in the peritubular capillaries leading to increased plasma oncotic pressure (wonc). Increased plasma oncotic pressure reduces the amount of backleak from the peritubular capillaries. Simultaneously, the increase in filtration fraction reduces volume delivery to the

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FIGURE 2-14 (continued)

peritubular capillary, decreasing its hydrostatic pressure, and thereby reducing the renal interstitial hydrostatic pressure (P(). Even though the proximal tubule hydrostatic pressure (Pt) may be reduced, owing to diminished GFR, the hydrostatic gradient from tubule to interstitium is increased, favoring increased volume reabsorption. A—afferent arteriole; E—efferent arteriole.

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