Which vessels are the smallest blood vessels where oxygen exchange occurs with body tissues?

Which vessels are the smallest blood vessels where oxygen exchange occurs with body tissues?

• A. Arteries
• B. Veins
• C. Capillaries
• D. Arterioles

Answer: (C)

Gas exchange with body tissues happens across capillary walls. Capillaries are the smallest blood vessels, just wide enough for a red blood cell to pass, and their walls are a single layer of cells. That very thin barrier, plus a vast network that blankets tissues, creates a large surface area and slow enough flow for oxygen to diffuse out of the blood into surrounding tissue and for carbon dioxide to diffuse from tissue back into the blood. Arteries have thick, muscular walls built to push blood under high pressure from the heart, so they’re not where exchange primarily occurs. Veins carry blood back to the heart and have thinner walls and valves, but they’re not optimized for diffusion either. Arterioles are tiny branches of arteries that regulate how much blood reaches the capillaries; they adjust flow, but the actual exchange happens mainly in the capillaries.

Gas exchange with body tissues happens across capillary walls. Capillaries are the smallest blood vessels, just wide enough for a red blood cell to pass, and their walls are a single layer of cells. That very thin barrier, plus a vast network that blankets tissues, creates a large surface area and slow enough flow for oxygen to diffuse out of the blood into surrounding tissue and for carbon dioxide to diffuse from tissue back into the blood. Arteries have thick, muscular walls built to push blood under high pressure from the heart, so they’re not where exchange primarily occurs. Veins carry blood back to the heart and have thinner walls and valves, but they’re not optimized for diffusion either. Arterioles are tiny branches of arteries that regulate how much blood reaches the capillaries; they adjust flow, but the actual exchange happens mainly in the capillaries.