MCAT Biology Question 95: Answer and Explanation

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Question: 95

5. At the venous end of a capillary bed, the osmotic pressure:

  • A. is greater than the hydrostatic pressure.
  • B. results in a net outflow of fluid.
  • C. is significantly higher than the osmotic pressure at the arterial end.
  • D. causes proteins to enter the interstitium.

Correct Answer: A

Explanation:

The exchange of fluid is greatly influenced by the relative balance between the hydrostatic and osmotic pressures of blood and tissues. The osmotic (oncotic) pressure remains relatively constant; however, the hydrostatic pressure at the arterial end is greater than the hydrostatic pressure at the venous end. As a result, fluid moves out of the capillaries at the arterial end and back in at the venous end. Fluid is reabsorbed at the venous end because the osmotic pressure exceeds the hydrostatic pressure. Proteins should not cross the capillary wall under normal circumstances.

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