Blood vessels
The ventricles of the heart pump blood into thick-walled blood vessels known as arteries. Arteries are strong, elastic vessels that are able to withstand the high pressures generated within them by the forceful action of the heart, the elasticity allowing the artery to expand as a pressure wave of blood passes through its interior. Blood destined for the lungs leaves the right ventricle through the pulmonary artery. Blood that supplies the rest of the body leaves the left ventricle via the body's largest blood vessel, the aorta. All arteries divide into progressively smaller vessels, eventually becoming very fine branches known as arterioles. As the arteries divide, the pressure within gradually diminishes. The very finest arterioles become capillaries, the smallest blood vessels in the body, having walls that are a single cell in thickness. The exchange of oxygen, nutrients and waste products, such as carbon dioxide, takes place between blood and tissues through the walls of these microscopic capillaries. There are around one billion capillaries in the human body and, if these were laid end-to-end, they would cover a distance of approximately 100,000 km (62,000 miles). Blood passes through the capillaries and drains into small veins, known as venules. These merge to form veins, which return blood to the heart through two large vessels called the superior and inferior vena cava, which drain into the right atrium. Blood pressure in the veins is significantly lower than in arteries, and veins have thin walls to let them expand and hold large volumes of blood. Many veins run through muscles, and when the muscles contract this encourage blood flow through the veins back to the heart. Veins contain one-way valves to stop blood flowing in the wrong direction. |
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