The single cell layer that lines the inner surface of blood vessels where they are in contact with the blood that flows through the circulatory system. Healthy endothelial function promotes relaxation or opening of a blood vessel (vasodilation) and inhibits blood clot formation (platelet aggregation).

In cardiovascular disease, arteries develop atherosclerotic plaque. Atherosclerosis impairs normal endothelial function, increasing the risk of narrowing blood vessels (vasoconstriction) and clot formation, which may lead to heart attack (myocardial infarction) or brain infarct (stroke).