Which forces influence filtration and reabsorption in capillary exchange?

• A. Capillary hydrostatic pressure and interstitial hydrostatic pressure drive filtration; capillary oncotic pressure and interstitial oncotic pressure drive reabsorption.
• B. Capillary hydrostatic pressure drives reabsorption; oncotic drives filtration.
• C. Interstitial hydrostatic pressure drives filtration; capillary oncotic drives reabsorption.
• D. Capillary oncotic pressure and interstitial oncotic pressure drive filtration; hydrostatic forces drive reabsorption.

Answer: (D)

Capillary exchange is controlled by Starling forces—the balance of hydrostatic and oncotic pressures across the capillary wall that push fluid in or pull it out.

• Capillary hydrostatic pressure (Pc) pushes fluid out of the capillary into the interstitial space.

• Interstitial hydrostatic pressure (Pi) pushes fluid into the capillary (opposes filtration).

• Capillary oncotic pressure (πc) pulls fluid back into the capillary from the interstitial space.

• Interstitial oncotic pressure (πi) pulls fluid out of the capillary into the interstitium.

The net filtration pressure can be summarized as NFP = Pc − Pi − πc + πi. Filtration occurs when outward forces dominate, and reabsorption occurs when inward forces dominate. In practical terms, filtration is promoted by higher capillary hydrostatic pressure and higher interstitial oncotic pressure, while reabsorption is promoted by higher capillary oncotic pressure and higher interstitial hydrostatic pressure.

This four-force balance is what governs where fluid moves during capillary exchange, and it helps explain why fluid filters out at the arterial end and is reabsorbed at the venous end.