The endothelium is a thin layer of cells that lines the interior surface of blood vessels, including arteries, veins, and capillaries. It plays a crucial role in maintaining vascular health by regulating blood flow, preventing blood clotting, and controlling the exchange of substances between the bloodstream and surrounding tissues. In addition to its protective functions, the endothelium produces various substances that influence blood pressure and inflammation. Damage to the endothelium can lead to cardiovascular diseases, such as atherosclerosis and hypertension, highlighting its importance in overall health and disease prevention.