CNRI / SCIENCE PHOTO LIBRARY CNRI / SCIENCE PHOTO LIBRARY
Vein cross-section, light micrograph. This is a cross-section through a small vein, one of the blood vessels bringing blood back to the heart from the body tissues. The section shows some of the layers that make up the walls of a blood vessel. This wall surrounds the central space (lumen) which here contains red blood cells (red). The layers, from inner to outer, are the tunica intima (endothelial), the tunica media (smooth muscle), and the tunica externa (connective tissue). The walls of veins are not as thick as arteries, as the blood is at a lower pressure.
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