HANS-ULRICH OSTERWALDER / SCIENCE PHOTO LIBRARY HANS-ULRICH OSTERWALDER / SCIENCE PHOTO LIBRARY
Vein and artery. Artwork of a section through a vein (blue, at left) and an artery (red, at right). Veins carry deoxygenated (blue) blood to the heart, arteries take oxygenated (red) blood from the heart. Low pressure blood is helped up the thin-walled vein by the contraction of the surrounding muscles (pink). Valves in the vein lining ensure that blood can only flow one way. The artery carries high pressure blood and has thick, muscular walls. The layers of the artery wall are unfurled at top. A tough outer coat surrounds the muscle (striped), an elastic layer (holey) and a single celled inner layer.
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