Endothelial cells are specialized cells that line the interior surface of blood vessels, including arteries, veins, and capillaries. They form a barrier between the blood and surrounding tissues, playing a crucial role in regulating blood flow, inflammation, and the exchange of substances like oxygen and nutrients. These cells also produce various substances that help maintain vascular health, such as nitric oxide, which relaxes blood vessels, and von Willebrand factor, which is important for blood clotting. Endothelial cells are essential for maintaining the integrity of the cardiovascular system and contribute to overall health.