| Veins and arteries | |
| Veins have thinner walls than arteries, but thicker than capillaries. |
| Arteries, Capillaries, and Veins | |
| The blood vessels can be thought of as highways your blood uses to move through your body. The three kinds of blood vessels are arteries, capillaries, and veins. Each kind has structures that help it do its job. |
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| Arteries are blood vessels that carry blood away from your heart to other parts of your body. Arteries have thick, muscular walls that stretch when the heart pushes blood into it. Arteries branch many times into smaller and smaller tubes. Almost every artery carries blood with lots of oxygen. |
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| Your smallest arteries branch to become your smallest blood vessels. The smallest kind of blood vessel is called a capillary. Side-by-side, ten of these tiny blood vessels would be barely as thick as one of your hairs. Some capillaries are so narrow that red blood cells must flow through them in a single-file line. |
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| The walls of capillaries are only one cell thick. Gases can pass through these thin walls. Oxygen moves from the blood in your capillaries to your cells. Carbon dioxide and other wastes go in the other direction. They move from your cells to the blood in your capillaries. | |
| Capillaries join together to form your tiniest veins. Veins are blood vessels that take blood from cells back to the heart. These tiny veins join many times to form larger and larger veins. | |
| Veins have valves. Valves are flaps that act like doors to keep blood flowing in one direction. Valves open to allow blood to flow to the heart. Valves close if the blood begins to flow away from the heart. Arteries and capillaries do not have valves. The pumping of the heart keeps the blood flowing in the right direction through the arteries and capillaries. |
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| Capillaries | |
| This capillary is as wide as only a few red blood cells. |