Which statement best describes the forces that drive capillary filtration and reabsorption?

• A. Capillary hydrostatic pressure pushes fluid out; plasma oncotic pressure pulls fluid in; the net movement depends on their balance.
• B. Capillary hydrostatic pressure pulls fluid in; plasma oncotic pressure pushes fluid out; filtration occurs when oncotic pressure exceeds hydrostatic pressure.
• C. Both hydrostatic and oncotic pressures push fluid out of the capillary.
• D. Capillary hydrostatic pressure remains constant along the capillary, so there is no net filtration.

Answer: (B)

Capillary filtration and reabsorption are driven by Starling forces: capillary hydrostatic pressure pushing fluid out of the capillary and plasma oncotic pressure pulling fluid back in. The capillary hydrostatic pressure tends to push fluid into the interstitium, while the plasma oncotic pressure—largely from albumin—pulls fluid into the capillary. Net movement follows their balance: when hydrostatic pressure is higher than oncotic pressure, filtration occurs; when oncotic pressure is higher, reabsorption occurs. This explains why fluid exits at the arterial end where Pc is higher and is reabsorbed toward the venous end as Pc declines. The statement that hydrostatic pressure pulls fluid in is incorrect, and the idea that both pressures push out or that hydrostatic pressure remains constant along the capillary does not fit the physiology.