The peritubular capillaries are branches of the efferent arterioles, which in turn arise from the glomerular capillaries. Consequently, the Starling forces in the peritubular capillaries can be modified by the glomerular filtration process. The peritubular capillary COP (nPC) depends on the capillary plasma protein concentration. As plasma proteins are concentrated in the glomerular capillaries by the filtration process, COP in the peritubular capillaries increases with filtration fraction. The peritubular capillary COP is normally high compared with that in the glomerular capillaries, resulting in fluid re-absorption by the peritubular capillaries.
This dependence of nPC on the filtration fraction is a mechanism that automatically adjusts proximal tubular reabsorption to compensate for changes in glomerular filtration. When GFR increases, the forces available to reabsorb the increased volume of filtrate also increase and vice versa. This glomerulo tubular mechanism normally ensures re-absorption of a fixed proportion of the gomerular filtrate by the proximal tubule, i.e. there is constant proximal tubular fractional re-absorption.
However, proximal tubular fractional re-absorption can be altered by other factors, e.g. by alterations in the effective circulating volume. Such alterations are considered later.
The peritubular capillary hydrostatic pressure (PPC) is determined mainly by venous pressure. However, changes in tone of the afferent and efferent glomerular arterioles may also affect PPC by varying the amount of arterial pressure transmitted to the peritubular capillaries.
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