endothelial cells

Endothelial cells line the inside of every blood vessel in the body. They form a one-cell-thick layer called the endothelium, which is also found on the inner walls of the heart chambers and lymphatic vessels, which carry excess blood plasma around the body. The endothelium is very extensive. If all the endothelium tissue in an adult body were to be spread out, it would take up the area of eight tennis courts!