The endothelium is the layer of thin, flat cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary. In small blood vessels and capillaries, endothelial cells are often the only cell-type present. Endothelial cells are involved in many aspects of vascular biology, including:
- Vasoconstriction & vasodilation, and hence the control of blood pressure
- Blood clotting (thrombosis & fibrinolysis)
- Formation of new blood vessels (angiogenesis)
- Inflammation and swelling (edema)
Endothelial cells also control the passage of materials and the transit of white blood cells into and out of the bloodstream. In some organs, there are highly differentiated endothelial cells to perform specialized 'filtering' functions. Examples of such unique endothelial structures include the renal glomerulus and the blood-brain barrier. Endothelial tissue is a specialized type of epithelium tissue (one of the four types of biological tissue in animals.
Endothelial dysfunction, or the loss of proper endothelial function, is a hallmark for vascular diseases, and often leads to atherosclerosis. This is very common in patients with diabetes mellitus, hypertension or other chronic pathophysiological conditions. One of the main mechanisms of endothelial dysfunction is the diminishing of nitric oxide, often due to high levels of asymmetric dimethylarginine, which interferes with the normal L-arginine-stimulated nitric oxide synthesis.