Which statement about capillary exchange is true?

Which statement about capillary exchange is true?

• A. Slow flow enhances exchange.
• B. Capillary pressure is highest in venules.
• C. Diffusion does not occur in capillaries.
• D. They are not involved in gas exchange.

Answer: (A)

Capillary exchange relies on time for substances to move by diffusion and on the flow allowing contact with the capillary walls. When flow through capillaries is slow, blood spends more time in contact with the endothelial surface and interstitial fluid, so oxygen and nutrients can diffuse from blood to tissue and waste products can diffuse back. This longer transit time makes exchange more complete, which is why slow flow enhances exchange. The other statements don’t fit what actually happens in capillaries. Capillary pressure is highest at the arterial end of the capillary, not in venules. Diffusion does occur across capillary walls, and capillaries are the sites of gas exchange—particularly in the lungs, where oxygen moves into blood and carbon dioxide moves out, as well as in tissues where gases and solutes are exchanged.

Capillary exchange relies on time for substances to move by diffusion and on the flow allowing contact with the capillary walls. When flow through capillaries is slow, blood spends more time in contact with the endothelial surface and interstitial fluid, so oxygen and nutrients can diffuse from blood to tissue and waste products can diffuse back. This longer transit time makes exchange more complete, which is why slow flow enhances exchange.

The other statements don’t fit what actually happens in capillaries. Capillary pressure is highest at the arterial end of the capillary, not in venules. Diffusion does occur across capillary walls, and capillaries are the sites of gas exchange—particularly in the lungs, where oxygen moves into blood and carbon dioxide moves out, as well as in tissues where gases and solutes are exchanged.