The endothelium is a thin membrane that lines the inside of the heart and blood vessels. Endothelial cells release substances that control vascular relaxation and contraction as well as enzymes that control blood clotting, immune function and platelet (a colorless substance in the blood) adhesion. Endothelial dysfunction has been shown to be of significance in predicting stroke and heart attacks due to the inability of the arteries to dilate fully. The dysfunction may be a result of high blood pressure, diabetes, high cholesterol and smoking. Studies have shown that endothelial dysfunction precedes the development of artherosclerosis, a chronic disease characterized by abnormal thickening and hardening of the arterial walls with resulting loss of elasticity. Artherosclerosis may cause a stroke or heart attack.