Renal cortical interstitium and fluid absorption by peritubular capillaries

Every minute, the cortical peritubular capillaries in a 1-g rat kidney take up more than 0.5 ml tubular reabsorbate. Studies of renal lymph and measurements of pressure in capillaries (Pc) and interstitium (Pi) indicate that normally the protein colloid osmotic pressure of peritubular capillary plasma (COPp) provides the necessary absorptive force, keeping Pi at 2-4 mmHg, i.e., 8-10 mmHg lower than Pc. At reduced COPp, continued delivery of fluid from the tubules automatically raises Pi to maintain capillary fluid uptake. The transient Pi response to sudden exposure of the kidney to subatmospheric pressure shows that such adjustment of forces may take place in only 5 s. Most remarkable, adjustment of forces may take place in only 5 s. Most remarkable, reabsorption continues during protein-free perfusion of the isolated rat kidney, apparently effected by a Pi exceeding Pc. A relative retardation of interstitial uptake of ferritin from plasma in this case suggests fluid reabsorption through both small and large pores in the capillary wall. Collapse of the capillaries is presumably prevented by tight tethering to the capillary wall, giving the narrow interstitium a very low compliance.