What is the primary purpose of capillaries?

• A. To carry oxygenated blood
• B. To allow nutrient and gas exchange between blood and tissues
• C. To transport blood back to the heart
• D. To maintain blood pressure

Answer: (B)

The main idea is that capillaries are the site where exchange happens between blood and tissues. Their walls are extremely thin—just a single layer of endothelial cells—creating a vast network with a huge surface area. This structure lets oxygen and nutrients diffuse from the blood into surrounding tissues, while carbon dioxide and other waste products diffuse from the tissues into the blood to be carried away. Capillaries form the microcirculation, linking small arteries to small veins, and the slow, filtered blood flow there provides the perfect environment for exchange to occur. By contrast, carrying oxygenated blood is more about arteries, returning blood to the heart is the job of the venous system, and maintaining blood pressure is mainly managed by arteries and their smooth muscle.