<qa>  What is the most important of these substances?Blood vessels are lined by a single layer of cells called the endothelium. These cells actively release substances that influence a variety of functions including blood flow in the arteries. Nitric oxide NO is among the most important of these substances. Decrease in NO reduces blood vessel size and increases blood pressure BP. Subjects with hypertension high blood pressure and those with a strong family history of hypertension but without high blood pressure also exhibit endothelial dysfunction and decreased NO activity. This abnormality predisposes individuals with high blood pressure to an increased long term risk of adverse cardiovascular events such as heart attack and stroke. 