Which statement is NOT an accurate description of the benefits of slow, low-pressure blood in the capillary beds?

Which statement is NOT an accurate description of the benefits of slow, low-pressure blood in the capillary beds?

• A. Lower pressure in the capillary bed helps to increase pressure in the venous circulation.
• B. Slow flow allows more time for nutrient and gas exchange.
• C. Capillaries provide a large surface area for diffusion.
• D. They are lined with phagocytes that can catch prey in capillaries.

Answer: (A)

The key idea being tested is how slow, low-pressure flow in capillary beds facilitates exchange. When blood moves slowly through capillaries, there’s more time for nutrients, gases, and wastes to move across the thin capillary walls, and the enormous network of capillaries provides a large surface area for diffusion. This is why the statements describing extended contact time and a large diffusion surface are correct benefits of capillary flow. The statement that lower pressure in the capillary bed helps to increase pressure in the venous circulation isn’t accurate. Venous pressure is determined by the overall return of blood to the heart and downstream resistance, not by capillary pressure pushing on the veins. Capillary hydrostatic pressure is already part of a gradient that drops as blood moves toward the venous end, and it does not function to boost venous pressure. In short, reducing capillary pressure would not be a mechanism to raise venous pressure. As for the idea of phagocytes lining capillaries catching prey, that is not the typical benefit described for capillary function. The main advantages of capillary flow are the diffusion-driven exchange enabled by slow flow and large surface area, not reliance on phagocytic activity within the capillary beds.

The key idea being tested is how slow, low-pressure flow in capillary beds facilitates exchange. When blood moves slowly through capillaries, there’s more time for nutrients, gases, and wastes to move across the thin capillary walls, and the enormous network of capillaries provides a large surface area for diffusion. This is why the statements describing extended contact time and a large diffusion surface are correct benefits of capillary flow.

The statement that lower pressure in the capillary bed helps to increase pressure in the venous circulation isn’t accurate. Venous pressure is determined by the overall return of blood to the heart and downstream resistance, not by capillary pressure pushing on the veins. Capillary hydrostatic pressure is already part of a gradient that drops as blood moves toward the venous end, and it does not function to boost venous pressure. In short, reducing capillary pressure would not be a mechanism to raise venous pressure.

As for the idea of phagocytes lining capillaries catching prey, that is not the typical benefit described for capillary function. The main advantages of capillary flow are the diffusion-driven exchange enabled by slow flow and large surface area, not reliance on phagocytic activity within the capillary beds.