Your circulatory system consists of your heart and blood vessels. Together, these provide a continuous flow of blood to your body, supplying the tissues with oxygen and nutrients. Your veins carry blood back toward the heart. Tiny vessels called capillaries in organs and tissues of the body deliver deoxygenated blood into small veins called venules, which join to form veins. Blood flows through the veins to the body's 2 main veins (called the vena cavae), which deliver the blood back into the heart.