Which vessel is the smallest blood vessel where exchange of gases occurs between blood and tissues?

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

Answer: (C)

Gas exchange happens where blood is in very close contact with body cells and the vessel wall is extremely thin. Capillaries form tiny networks throughout tissues and have walls that are only one cell thick, making it easy for oxygen to diffuse from blood into cells and carbon dioxide to diffuse from cells into blood. They also provide a large surface area and slow blood flow to maximize diffusion. Arteries and arterioles have thicker walls and mainly transport blood away from the heart, while veins return blood to the heart, so they aren’t the primary sites for gas exchange. Capillaries are the specialized vessels where this exchange occurs.