What is the primary function of capillaries?

The primary function of capillaries is to facilitate gas exchange between blood and tissues. Capillaries are the smallest blood vessels in the body and they connect arterioles (which carry blood away from the heart) with venules (which carry blood back to the heart). Their walls are extremely thin, allowing oxygen and carbon dioxide, as well as nutrients and waste products, to pass through easily.

This exchange process occurs via diffusion, where oxygen from the blood passes through the capillary walls into the surrounding tissues, which require it for cellular respiration. Simultaneously, carbon dioxide, a waste product of metabolism in tissues, diffuses from the tissue cells into the capillaries to be transported back to the lungs for exhalation.

While other options, such as transporting blood and regulating blood pressure, are functions related to the overall circulatory system, these processes involve different types of blood vessels—arteries and veins. Capillaries specifically specialize in the exchange of gases and metabolites, which is crucial for maintaining tissue health and functionality. Thus, the correct answer to the question highlights the role of capillaries in ensuring that tissues receive the necessary substances for metabolism while also removing waste products efficiently.