Endothelial cells are specialized cells that line the interior surface of blood vessels, forming a barrier between the vessel's contents and surrounding tissues. They play an important role in regulating blood flow by releasing substances such as nitric oxide, which helps to relax and widen blood vessels, allowing for better circulation. Endothelial cells also help prevent clot formation within blood vessels by producing substances that prevent platelet aggregation. Overall, these cells play a crucial role in maintaining the health of blood vessels and promoting proper cardiovascular function.