How does capillary hydrostatic pressure change along the capillary bed, and what is the implication for filtration and reabsorption?

• A. Higher capillary hydrostatic pressure at the arterial end promotes filtration; lower pressure toward the venous end reduces filtration and favors reabsorption, aided by a constant plasma oncotic pressure.
• B. Capillary hydrostatic pressure is constant along the capillary bed; filtration remains constant.
• C. Higher capillary hydrostatic pressure at the venous end promotes filtration.
• D. Oncotic pressure drives filtration throughout the capillary bed.

Answer: (C)

Capillary exchange is governed by Starling forces: hydrostatic pressure pushes fluid out of the capillaries, while plasma oncotic pressure (mostly from albumin) pulls fluid back in. Hydrostatic pressure is highest at the arterial end of the capillary and gradually falls as blood moves toward the venous end. Because the plasma oncotic pressure stays relatively constant along the length, the higher hydrostatic pressure at the arterial end favors filtration (fluid leaving the capillary). As hydrostatic pressure declines toward the venous end, the tendency for filtration decreases and reabsorption becomes more likely, with water drawn back into the capillaries by the oncotic pull. The net effect is filtration predominating near the arterial end and reabsorption toward the venous end, with the excess interstitial fluid returned to the circulation via the lymphatic system.