CGlomerulotubular balance in the proximal tubule

- maintains reabsorption at a constant fraction (2/3 or 67%) of the filtered Na+ and H20.

(1) For example, if GFR spontaneously increases, the filtered load of Na+ also increases. Without a change in reabsorption, this situation would lead to increased Na+ excretion. However, glomerulotubular balance functions such that Na+ reabsorption also will increase, ensuring that a constant fraction is reabsorbed.

(2) The mechanism of glomerulotubular balance is based on Starling forces in the peritubular capillaries, which alter the reabsorption of Na+ and H20 in the proximal tubule (Figure 5-9).

Lumen

Cells of the proximal tubule

Peritubular capillary blood

Cells of the proximal tubule

Peritubular capillary blood

Figure 5-9. Mechanism of isosmotic reabsorption in the proximal tubule. The dashed arrow shows the pathway. Increases in ttc and decreases in Pc cause increased rates of isosmotic reabsorption.

- The route of isosmotic fluid reabsorption is from the lumen, to the proximal tubule cell, to the lateral intercellular space, and then to the peritubular capillary blood.

- Starling forces in the peritubular capillary blood govern how much of this isosmotic fluid will be reabsorbed.

- Fluid reabsorption is increased by increases in uc of the peritubular capillary blood and decreased by decreases in ttc.

- Increases in GFR and filtration fraction cause the protein concentration and ttc of peritubular capillary blood to increase. This increase, in turn, produces an increase in fluid reabsorption. Thus, there is matching of filtration and reabsorption, or glomerulotubu-lar balance.

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