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 regulating the passage of substances between the blood and surrounding tissues, controlling blood pressure, and maintaining the overall health and functionality of the cardiovascular system. Research in this area focuses on understanding the function of the endothelium, its role in various diseases such as atherosclerosis and hypertension, and potential therapeutic approaches to improve endothelial function.