In the cardiovascular system, the heart pumps blood through vessels that form a continuous circuit. Blood carries oxygen and nutrients to cells and wastes away from them. The cardiovascular system is closed, blood normally remains within the blood vessels. Arteries carry blood away from the heart and branch into smaller arterioles. Veins return blood to the heart and collect blood from smaller venules. Capillaries are thin-walled blood vessels that connect arterioles to venules. Materials are exchanged between the blood and the tissues through the walls of capillaries.
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