The endothelium is the layer of tissue which lines the inside of the arteries. It regulates the flow of blood through the arteries, by producing (or not producing) a substance called nitric oxide. Nitric oxide causes the arteries to expand to facilitate blood flow. The more nitric oxide produced by the endothelium, the more easily blood can flow through the arteries and the less likely an individual is to experience cardiovascular problems such as stroke and heart attack.
For more information, see Endothelium.