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At the arterial end of a capillary the CHP is 35 mm Hg and the BCOP is 25 mm Hg. What is the net filtration pressure?
A10 mm Hg, driving fluid out
B10 mm Hg, drawing fluid in
C60 mm Hg, driving fluid out
D25 mm Hg, drawing fluid in
Answer & Solution
Correct answer: A. 10 mm Hg, driving fluid out
1. Net filtration pressure is capillary hydrostatic pressure minus blood colloidal osmotic pressure.
2. Here that is 35 mm Hg minus 25 mm Hg, which equals 10 mm Hg.
3. A positive value means filtration, so fluid moves out of the capillary into the tissue.
4. Adding the two pressures instead of subtracting them would give 60 mm Hg, which is the trap in the wrong arithmetic option.
5. Interstitial hydrostatic and colloidal pressures are negligible in health, so they drop out of the calculation.
_Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 20 "The Cardiovascular System: Blood Vessels and Circulation", section 20.3 Capillary Exchange_
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