The primary role of capillaries in the circulatory system is to facilitate the exchange of nutrients, gases, and waste products between the blood and surrounding tissues. These tiny blood vessels connect arterioles and venules, providing a vast network that enables efficient transfer of substances at the cellular level. Oxygen and nutrients diffuse from the blood through capillary walls into tissues, while waste products, including carbon dioxide, move from the tissues into the blood. This exchange is vital for maintaining homeostasis and supporting cellular functions throughout the body.