Capillary endothelium

The endothelium is the layer of cells that lines the inside of the entire vascular system, without a break, end to end, from the widest vessel to the finest. They are flat cells, so flat that histology calls them simple squamous epithelium, and they form the innermost part of the wall, the tunica intima. In a large artery that layer is only the lining. In a capillary it is almost everything: the endothelial cell is the wall.

And it is a wall that decides. Small molecules such as gases and fats cross the membrane of these cells directly, without permission and without help. Some large proteins in the plasma, the liquid part of the blood, cross a different way: they travel packaged in vesicles, tiny bubbles the cell forms to bring things in and out whole. It is the difference between air, which passes through a window screen on its own, and whatever will not fit through the mesh, which gets in only if someone puts it in a box.

For a long time the endothelium was thought to be nothing more than the border between the blood and the vessel. It is not. It helps regulate capillary exchange, changes blood flow and releases substances called endothelins, which are able to contract the smooth muscle of the vessel wall.