Starling forces regulate reabsorption of NaCl and water across the proximal tubule. As previously described, Na+ , Cl−, HCO3 −, amino acids, glucose, and water are transported into the intercellular space of the proximal tubule. Starling forces between this space and the peritubular capillaries facilitate movement of the reabsorbed fluid into the capillaries. Starling forces across the wall of peritubular capillaries •Fig. 34.11 Starling forces modify proximal tubule solute and water reabsorption. (1) Solute and water are reabsorbed across the apical membrane. This solute and water then cross the lateral cell membrane. Some solute and water reenters the tubule fluid (3), and the remainder enters the interstitial space and then flows into the capillary (2). The width of the arrows is directly proportional to the amount of solute and water moving by pathways 1 to 3. Starling forces across the capillary wall determine the amount of fluid flowing through pathway 2 versus pathway 3. Transport mechanisms in the apical cell membranes determine the amount of solute and water entering the cell (pathway 1). Pi, interstitial hydrostatic pressure; Ppc, peritubular capillary hydrostatic pressure; πi, interstitial fluid oncotic pressure; πpc, peritubular capillary oncotic pressure. Thin arrows across the capillary wall indicate the direction of water movement in response to each force.
consist of hydrostatic pressure in the peritubular capillary (Ppc) and lateral intercellular space (Pi) and oncotic pressure in the peritubular capillary (πpc) and lateral intercellular space (πi). Thus reabsorption of water as a result of transport of Na+ from tubular fluid into the lateral intercellular space is modified by the Starling forces. Accordingly:
Equation 34.2 where J is flow (positive numbers indicate flow from the intercellular space into blood). Starling forces that favor movement from the interstitium into the peritubular capillaries are πpc and Pi (Fig. 34.11 ). The opposing Starling forces are πi and Ppc. Normally the sum of the Starling forces favors movement of solute and water from the interstitial space into the capillary. However, some of the solutes and fluid that enter the lateral intercellular space leak back into the proximal tubular fluid. Starling forces do not affect transport by the loop of Henle, distal tubule, and collecting duct because these segments are less permeable to water than the proximal tubule is.
A number of factors can alter the Starling forces across the peritubular capillaries surrounding the proximal tubule. For example, dilation of the efferent arteriole increases Ppc, whereas constriction of the efferent arteriole decreases it.
An increase in Ppc inhibits solute and water reabsorption by increasing back-leak of NaCl and water across the tight junction, whereas a decrease stimulates reabsorption by decreasing back-leak across the tight junction.
Peritubular capillary oncotic pressure (πpc) is partially determined by the rate of formation of the glomerular ultrafiltrate. For example, if one assumes a constant plasma flow in the afferent arteriole, the plasma proteins become less concentrated in the plasma that enters the efferent arteriole and peritubular capillary as less ultrafiltrate is formed (i.e., as GFR decreases). Hence, πpc decreases. Thus πpc is directly related to the filtration fraction (FF = GFR/renal plasma flow [RPF]). A fall in the FF resulting from a decrease in GFR, at constant RPF, decreases πpc. This in turn increases the backflow of NaCl and water from the lateral intercellular space into tubular fluid and thereby decreases net reabsorption of solute and water across the proximal tubule. An increase in FF has the opposite effect.