Where is the exchange site of gases between the blood and tissues?

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

Answer: (A)

Gas exchange with body tissues occurs primarily across capillaries because their walls are extremely thin and form an extensive network right next to every cell. This small distance and large surface area allow oxygen to diffuse from blood (where it’s in higher concentration) into tissues (lower concentration) and carbon dioxide to diffuse from tissues (higher) into blood (lower). The capillary endothelium is just one cell thick, and red blood cells pass slowly enough to maximize contact with tissue cells, making diffusion efficient for both gases. Arteries transport blood away from the heart but aren’t sites of exchange with tissues, and veins return blood to the heart. Alveoli are where gas exchange occurs between air and blood in the lungs, not between blood and tissue cells.