What is the term for the smallest blood vessels where exchange with tissues occurs?

What is the term for the smallest blood vessels where exchange with tissues occurs?

• A. Capillaries
• B. Veins
• C. Arteries
• D. Cardiovascular system

Answer: (A)

Exchange between blood and surrounding tissues happens across the walls of capillaries—the smallest blood vessels with very thin, often one-cell-thick walls. This thin barrier, combined with their extensive network and slow blood flow, allows oxygen and nutrients to diffuse from the blood into tissues and carbon dioxide and wastes to move from tissues into the blood. Arteries and veins are larger vessels that transport blood to and from tissues, but they are not the primary sites of exchange. The cardiovascular system includes all these vessels, but the actual exchange occurs in capillaries.