Permitting the exchange of nutrients and gases between the blood and tissue cells is the primary function of which vessels?

• A. Arteries
• B. Capillaries
• C. Veins
• D. Lymphatic vessels

Answer: (B)

Capillaries are the sites where nutrients and gases are exchanged between blood and tissue cells. Their walls are only one cell thick, creating the shortest diffusion distance, and they form vast networks with a huge surface area that maximize contact with every tissue cell. This setup lets oxygen and nutrients move from blood into interstitial fluid and cells, while carbon dioxide and wastes move from cells back into the blood to be carried away. Arteries and veins transport blood to and from tissues but have thicker, more muscular walls designed for pressure and bulk flow rather than exchange. Lymphatic vessels deal with interstitial fluid and immune functions, not direct blood–tissue exchange.