What is the primary function of capillaries?

What is the primary function of capillaries?

• A. Pump blood through the body
• B. Transport blood away from the heart
• C. Store blood
• D. Site for exchange of substances

Answer: (D)

Capillaries are the site where substances are exchanged between blood and tissues. Their walls are extremely thin—essentially a single layer of endothelial cells—so oxygen and nutrients can diffuse out of the blood into the surrounding tissue, while carbon dioxide and wastes diffuse from the tissue into the blood. The capillary network provides a huge surface area and slows the blood just enough to allow this diffusion to occur efficiently, making it the primary place where exchange happens in the circulation. By contrast, pumping blood through the body is the heart’s job, arteries transport blood away from the heart, and veins mainly handle return and storage; those roles don’t describe the main function of capillaries.

Capillaries are the site where substances are exchanged between blood and tissues. Their walls are extremely thin—essentially a single layer of endothelial cells—so oxygen and nutrients can diffuse out of the blood into the surrounding tissue, while carbon dioxide and wastes diffuse from the tissue into the blood. The capillary network provides a huge surface area and slows the blood just enough to allow this diffusion to occur efficiently, making it the primary place where exchange happens in the circulation. By contrast, pumping blood through the body is the heart’s job, arteries transport blood away from the heart, and veins mainly handle return and storage; those roles don’t describe the main function of capillaries.