Starling forces play a crucial role in regulating the reabsorption of NaCl and water in the proximal tubule. Within this tubule, substances such as Na+, Cl−, HCO3−, amino acids, glucose, and water are transported into the intercellular space. The movement of this reabsorbed fluid into the peritubular capillaries is facilitated by Starling forces operating between the intercellular space and the capillaries.

The Starling forces across the wall of the peritubular capillaries are comprised of hydrostatic pressure in the peritubular capillary (Ppc) and the lateral intercellular space (Pi), as well as oncotic pressure in the peritubular capillary (πpc) and the lateral intercellular space (πi). These forces determine the amount of fluid flowing through different pathways, specifically pathway 2 and pathway 3, in which solute and water either enter the interstitial space or reenter the tubule fluid.

The reabsorption of water, which occurs due to the transport of Na+ from the tubular fluid into the lateral intercellular space, is influenced by the Starling forces. The equation J = Ppc - Pi + πpc - πi represents the flow, with positive values indicating flow from the intercellular space into the blood. The Starling forces that promote movement from the interstitium into the peritubular capillaries are πpc and Pi. On the other hand, the opposing forces are πi and Ppc. Normally, the sum of the Starling forces favors the movement of solute and water from the interstitial space into the capillary. However, some solutes and fluid may leak back into the proximal tubular fluid from the lateral intercellular space.

It's important to note that the Starling forces do not impact transport in the loop of Henle, distal tubule, and collecting duct as these segments are less permeable to water compared to the proximal tubule.

Various factors can alter the Starling forces across the peritubular capillaries surrounding the proximal tubule. For instance, dilation of the efferent arteriole increases Ppc, while constriction decreases it. An increase in Ppc inhibits solute and water reabsorption by promoting back-leak across the tight junction, whereas a decrease stimulates reabsorption by reducing back-leak.

The peritubular capillary oncotic pressure (πpc) is influenced by the rate of formation of the glomerular ultrafiltrate. As the ultrafiltrate formation decreases (e.g., due to a decrease in GFR), the concentration of plasma proteins in the plasma that enters the efferent arteriole and peritubular capillary also decreases, resulting in a decrease in πpc. Thus, πpc is directly related to the filtration fraction (FF = GFR/RPF). A decrease in FF, with constant RPF, leads to a decrease in πpc, which in turn increases the backflow of NaCl and water from the lateral intercellular space into the tubular fluid. Consequently, the net reabsorption of solute and water across the proximal tubule decreases. Conversely, an increase in FF has the opposite effect.
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.