Why the peritubular beds are low in pressure ?

Resorption of water and sodium is governed by the starling forces surrounding the peritubular capillaries. Several factors can modify the peritubular starling forces and in so doing can modulate net fluid resorption by the kidneys. Because blood in the capillaries has passed through the glomerulus and a proportion of the fluid has been filtered into the nephron, peritubular capillary blood has a high oncotic pressure and a low hydrostatic pressure. In contrast, the interstitial fluid oncotic pressure and hydrostatic pressure are relatively low. The oncotic pressure of the peritubular capillaries is actually relatively high because a significant percentage of the vascular fluid is filtered out in the glomerulus and into the tubule, leaving a higher concentration of plasma proteins in the blood entering the peritubular capillaries. It is the hydrostatic pressure of the peritubular capillaries is relatively low because much of the intravascular pressure is lost after blood passes through the afferent and efferent arterioles. Overall, the balance of forces is toward movement of sodium and water into the peritubular capillaries.