Final answer:
Reduced GFR results in increased peritubular capillary oncotic pressure due to higher concentration of proteins in the capillary blood, which enhances reabsorption of fluid back into the bloodstream.
Step-by-step explanation:
A reduced glomerular filtration rate (GFR) would result in increased peritubular capillary oncotic pressure. This is because with a lower GFR, fewer plasma proteins get filtered out into the Bowman's capsule; therefore, they remain in the capillary blood, increasing the blood colloid osmotic pressure (BCOP).
Osmotic pressure tends to draw water into the capillaries, and a higher concentration of proteins in the capillaries enhances this effect. In summary, low GFR leads to higher oncotic pressure in the peritubular capillaries, which then promotes reabsorption of fluid back into the bloodstream from the interstitial spaces.