Lung blood vessels. Coloured angiogram of the heart and pulmonary arteries. The heart is at lower right. The left and right pulmonary arteries branch from the top of the heart. The pulmonary arteries carry deoxygenated blood from the heart to the lungs. The arteries branch many times into smaller and smaller vessels. The smallest vessels are wrapped around tiny air sacs (alveoli). Alveoli are the site of gaseous exchange; oxygen enters the blood and carbon dioxide leaves. The oxygenated blood then travels through the pulmonary veins (not seen) back to the heart where it is pumped around the body. Angiography uses a contrast medium to show blood vessels on an X-ray.
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