Final answer:
Lower mean pulmonary blood pressure results in reduced capillary hydrostatic pressure, shifting the net filtration pressure towards reabsorption due to a higher blood colloidal osmotic pressure, thus leading to increased fluid return to the capillaries.
Step-by-step explanation:
When the mean pulmonary blood pressure is lower, the capillary hydrostatic pressure (CHP) is also reduced. Since the blood colloidal osmotic pressure (BCOP) remains higher than the lowered CHP, there will be an increased tendency for fluid to re-enter the capillaries from the interstitial space. The net filtration pressure, which is the balance of forces that determines the movement of fluid into and out of the capillary, will be shifted towards reabsorption. Therefore, a lower pulmonary blood pressure leads to a higher rate of fluid return to the capillaries, preventing excessive fluid loss from the vascular compartment into the tissues. This helps to maintain blood volume and pressure in the face of the low pulmonary blood pressure.