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There are three main types of blood vessel in the body:

• Arteries take blood away from the heart to the organs and other body tissues. Arteries have a narrow internal diameter and thick muscular walls. This allows them to carry blood that is at a high pressure.
• Capillaries are tiny, thin walled vessels that form a network to take blood through the organs and other body tissues. The dense networks of capillaries present a large surface area, which allows materials to be exchanged between body cells and the blood rapidly. Oxygen and dissolved foods diffuse into body cells from the blood, and carbon dioxide and other waste products diffuse out of body cells into the blood.
• Veins carry blood under low pressure from the capillaries and return the blood to the heart. The vein walls have thinner muscular walls than arteries and have a wider internal diameter. Veins contain valves to prevent the backflow of low-pressure blood.