The heart is the center of your body's circulatory system, and it performs two important functions: supplying oxygenated blood to your body and helping rid your body of waste products, such as carbon dioxide. With every beat your heart intakes, it pumps blood to and from your lungs, which in turn clear and re-oxygenate your blood.
Your whole circulatory system is made up of numerous arteries, arterioles, veins, venules, and capillaries, all of which carry blood, at varying amounts and pressures, throughout your body. Arteries carry blood away from your heart to the rest of your body. Your arteries need to be strong and flexible to maintain blood pressure between heartbeats. The smaller arteries and arterioles have muscular walls, so they can adjust their diameter to either increase or decrease the blood flow to various parts of your body, as required.
The systemic circuit carries blood from the left side of your heart to your organs. Once that blood has perfused the organs it comes back to the right side of your heart, through your veins, where it is sent through the pulmonary circuit and lungs to release carbon dioxide and become re-oxygenated.
Your veins carry blood back to your heart. Veins are generally wider than arteries, yet veins carry their large volumes of blood at lower speeds and pressure than do arteries. Veins have much less muscle and elastic tissue in their walls and can stretch considerably to become virtual blood pools.
Your capillaries are extremely small, thin-walled endothelial tubes that function as connectors between your arteries and your veins. Capillaries diffuse nutrients and gases into and from your tissues, in response to simple diffusion and osmotic pressures. Blood from the gastrointestinal tract passes through a second capillary network in your liver, before returning to your heart.