A thin layer of cells lining the interior surface of blood vessels and lymphatic vessels is called endothelium. Endothelial cells perform many functions such as barrier function, blot clot formation, formation of new blood vessels and blood pressure regulation through vasoconstriction and vasodilation. The contraction of the muscular wall of the blood vessels results in narrowing of blood vessels which is called vasoconstriction. It plays a key role to stop internal bleeding and heavy loss of blood. The constriction of blood vessels restricts the blood flow and thus results in retention of body heat. The process of vasoconstriction is essential to maintain mean arterial pressure.