Endothelium

Endothelium is a layer of cells that line the walls of the circulatory system which carries blood around the body.[1] The cells form a membrane so that certain substances, such as nutrients, can move through the walls.  In an adult human there is about 1 kg of endothelium, which would cover an area of about 7 m2.[1] The endothelium plays an important part in controlling the flow of blood around the body.