What are the small vessels where exchange of oxygen, carbon dioxide, nutrients, and wastes occurs between blood and tissues?

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

Answer: (D)

Capillaries are the small vessels where exchange occurs. Their walls are extremely thin—often a single cell layer—so substances can diffuse between the blood and surrounding tissues. Oxygen and nutrients move from the blood into tissue cells, while carbon dioxide and wastes move from the cells into the blood to be removed. Veins and arteries transport blood to and from tissues but aren’t the sites of this diffusion themselves. Alveoli are air sacs in the lungs where gas exchange with air happens, not with tissue cells. So the exchange between blood and tissues takes place across capillaries.