Which vessels are the smallest blood vessels where exchange occurs between blood and tissues?

Which vessels are the smallest blood vessels where exchange occurs between blood and tissues?

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

Answer: (A)

Exchange between blood and tissues happens across capillary walls, which are the smallest blood vessels. Their walls are extremely thin—often just a single layer of endothelial cells with a basement membrane—so substances like oxygen, carbon dioxide, glucose, and wastes can diffuse readily between blood and the surrounding tissues. The vast capillary network also provides a large surface area and slows flow enough to maximize this diffusion. Arteries are large, thick-walled vessels designed to carry blood away from the heart under high pressure, so they’re built more for transport than for exchange. Veins return blood to the heart and have thinner walls but are optimized for low-pressure flow, with valves in many cases. Venules are small veins that collect blood from capillaries, but the actual exchange mainly occurs in the capillaries still. So, the vessels where exchange occurs and which are the smallest are the capillaries.

Exchange between blood and tissues happens across capillary walls, which are the smallest blood vessels. Their walls are extremely thin—often just a single layer of endothelial cells with a basement membrane—so substances like oxygen, carbon dioxide, glucose, and wastes can diffuse readily between blood and the surrounding tissues. The vast capillary network also provides a large surface area and slows flow enough to maximize this diffusion.

Arteries are large, thick-walled vessels designed to carry blood away from the heart under high pressure, so they’re built more for transport than for exchange. Veins return blood to the heart and have thinner walls but are optimized for low-pressure flow, with valves in many cases. Venules are small veins that collect blood from capillaries, but the actual exchange mainly occurs in the capillaries still.

So, the vessels where exchange occurs and which are the smallest are the capillaries.