Which type of blood vessel is primarily responsible for gas exchange with tissues?

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

Answer: (B)

Gas exchange with tissues happens mainly across capillary walls. Capillaries are the smallest vessels and have very thin walls made of a single layer of endothelial cells, so the distance over which gases must diffuse is minimal. Their expansive networks create a large surface area and bring blood into close proximity to every cell, allowing oxygen to diffuse into tissues and carbon dioxide to diffuse into the blood as blood flows slowly through them, giving cells time to absorb and release gases.

Arteries carry blood away from the heart under high pressure and have thick, muscular walls built to withstand that pressure, not to exchange gases. Veins return blood to the heart and have thinner walls and valves, but they’re not optimized for gas diffusion and often keep blood farther from tissue cells. Arterioles regulate flow into capillary beds but are not the primary site of gas exchange; the actual exchange occurs mainly in capillaries.

So, the best answer is that capillaries are responsible for gas exchange with tissues.