The first specific, recognizable hemostatic mechanism is a process known as vasoconstriction. Vascoconstriction is initiated by chemical signals stemming from a damaged area of a blood vessel. Vasoconstriction, or vascular constriction, immediately reduces the quantity of blood flowing through the damaged area. Its action is the physical decrease in the size of the vessel and the redirection of blood flow around, and away from, the damaged area. Vasoconstriction is akin to putting a clamp on a pliable piece of plastic tubing. Vasoconstriction also allows for a closer interaction of the coagulation proteins and platelets with the damaged vessel wall. In addition, the epithelial cells lining the vessel also contract, which allows plasma to leak from the injured area. This is part of our normal inflammatory reponse and is the reason for swelling at the site of an injury.
Vasoconstriction is an exceedingly important hemostatic mechanism as it prepares the damaged vessel for subsequent repair activities. These activities will be discussed next.