Which vessels connect arteries to veins, allowing exchange with tissues?

• A. Vein
• B. Capillary
• C. Arteriole
• D. Lymph

Answer: (B)

Capillaries are the vessels that connect arteries to veins and are the site where exchange with tissues occurs. They form tiny networks between arterioles and venules, and their walls are only a single layer of endothelial cells with a thin basement membrane. This thinness allows oxygen and nutrients to diffuse from blood into surrounding tissues and carbon dioxide and other wastes to diffuse from tissues into the blood to be carried away. The vast surface area of capillary beds and their close proximity to cells maximize these exchanges. Veins and arteries are not the primary exchange sites—arteries carry blood away under pressure, veins return it to the heart, and lymph vessels carry lymph, which is separate from the blood’s exchange with tissues. Arterioles feed into capillaries but do not themselves facilitate the bulk exchange in tissues.