Which statement describes Starling forces governing capillary exchange?

• A. Net filtration is driven by capillary oncotic pressure; reabsorption by capillary hydrostatic pressure.
• B. Net filtration by capillary hydrostatic pressure; reabsorption by capillary oncotic pressure.
• C. Net filtration by interstitial hydrostatic pressure; reabsorption by interstitial oncotic pressure.
• D. Net filtration by interstitial hydrostatic pressure; reabsorption by capillary oncotic pressure.

Answer: (B)

Fluid exchange across capillary walls is governed by the balance of hydrostatic and oncotic forces. Capillary hydrostatic pressure pushes fluid out of the capillaries into the interstitial space, while capillary oncotic pressure, created by plasma proteins, pulls fluid back into the capillaries. In the typical description, net filtration occurs when capillary hydrostatic pressure dominates, and reabsorption occurs when capillary oncotic pressure dominates. Interstitial pressures also play a role, but the classic statement focuses on these two forces as the primary drivers. So the best description is that net filtration is driven by capillary hydrostatic pressure and reabsorption by capillary oncotic pressure.