endothelium

The endothelium is the delicate, single-cell-thick lining on the inside of every blood vessel—the smooth ‘wallpaper’ that blood is always in contact with. Far from being a passive coating, it is an active organ in its own right, sensing the blood it carries and constantly adjusting the vessel’s behavior.

Endothelial cells release signaling molecules that fine-tune vascular tone—most famously nitric oxide, which relaxes the underlying smooth muscle and widens the vessel. They also keep the blood flowing smoothly by presenting a non-stick surface that discourages clots, and they regulate what passes between blood and tissue. In this way the endothelium helps govern blood pressure, local blood flow, clotting and inflammation.

When the endothelium is damaged or dysfunctional—by high blood pressure, high cholesterol, diabetes, smoking or aging—it becomes less able to dilate vessels and more prone to inflammation and clotting. This endothelial dysfunction is widely regarded as one of the earliest steps in atherosclerosis, which makes the health of this thin lining central to cardiovascular disease long before any blockage appears.