The end result of the transport processes described above is the passage of essentially isotonic fluid from the tubule lumen, into the lateral intercellular spaces (LIS). This occurs mainly through tubular cell reabsorption and secretion, but some paracellular movement also occurs into the LIS, but mainly in the late proximal tubule. From here, the NaCl-NaHCO3 solution can move in two possible directions; either into the peritubular capillaries, or back into the tubular lumen.
A proportion of the Na+ always leaks back and, hence, the sodium re-absorption process can be called a 'pump-leak' system (it is also known as a gradient time-limited transport process). The rate of proximal tubular sodium re-absorption can be varied by changes in the rate of backflux into the tubule, as well as by changes in the rate of active sodium extrusion from the cells. The rate of backflux is affected by the rate of uptake from the lateral intercellular spaces into the capillaries. Re-absorption from the tubule is finally completed by the uptake of water and solutes into the peritubular capillaries. The faster the capillary uptake the lower the rate of backflux.
The forces governing the movement of fluid across the walls of the peritubular capillaries are Starling forces, i.e. colloid osmotic pressure (COP) and hydrostatic pressure gradients.
The forces favouring capillary uptake are:
(capillary COP) + (LIS hydrostatic pressure)
The forces opposing capillary uptake are: (capillary hydrostatic pressure) + (COP in LIS) or
Therefore, rate of capillary uptake is related to: (forces favouring uptake) - (forces opposing uptake)
nL and nPC = COPs in the LIS and peritubular capillary respectively
PL and PPC = hydrostatic pressures in LIS and peritubular capillary respectively.
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