Arteries transport blood away from the heart towards either the lungs or the rest of the body. Blood returns to the heart via the veins. Networks of capillaries throughout the body connect arteries and veins and provide sites for the exchange of substances because of their very thin walls.
Blood is continuously circulated from the heart to the lungs (for absorption of oxygen and excretion of carbon dioxide), back to the heart and around the entire body (for two-way transport of oxygen and other substances) before returning to the heart again. The circulation of blood to the lungs is called the pulmonary circulation whilst circulation around the rest of the body is called systemic or general circulation.
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