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Blood Vessels: Your Internal Superhighway

Driving to work every day, you may hear the radio traffic person talking about another clogged artery in the far-flung road system. But no mere highway grid can rival your body's own transport plan—the cardiovascular system—for sheer flexibility, complexity, and efficiency.

Every minute of every day, millions of blood cells trek through about 60,000 miles of blood vessels—enough to stretch from New York City to San Francisco 23 times—delivering oxygen and nutrients to every tissue. Your cardiovascular system includes your heart and two basic kinds of blood vessels: arteries and veins.

Arteries carry blood and nutrients from your heart to all parts of the body. They have muscular walls that contract to help the blood circulate. The biggest artery in your body is the aorta. It carries oxygenated blood out of your heart and into the ever-narrower network of arteries that reaches throughout your body. The pulmonary artery takes oxygen-poor blood from the heart to the lungs to exchange carbon dioxide for oxygen.

Like all your arteries, the aorta is elastic and acts like a smart garden hose: It can expand to carry extra blood when needed—during exercise, for example. Blood flows through arteries by force of gravity and your heart's powerful pumping action. You can feel blood moving as a pulse by pressing against the arteries close to the skin at your wrist, neck, and a few other spots on your body.

The arteries divide into smaller vessels called arterioles which connect to capillaries. The capillaries are spread throughout all the tissues of the body. The capillaries connect to small veins. The veins return the blood to the heart. The small veins connect to larger veins and the blood eventually reaches the two largest veins, the superior and inferior vena cava that empty the blood into the heart. Veins aren't as muscular as arteries. They don’t contract to help pump blood, but they do contain small valves that prevent the blood from flowing backward into the capillaries.

Blood zips through your blood vessels at about three feet per second, or 20 miles an hour. But pressure drops and your blood slows significantly—and may even stop—in the brain and other tissues while blood cells trade oxygen and nutrients for carbon dioxide and other wastes.