Role of Platelets in Hemostasis

Hemostasis is the process through which blood changes from liquid form into a solid state. When bleeding occurs, chemicals that cause vasoconstriction -- contraction of the damaged blood vessel -- are released and trigger a series of events. There are three main mechanisms that take place to achieve hemostasis.

Vasconstriction

• Vasoconstriction of the damaged blood vessel is initiated by the release of chemicals in the area of the injury. These chemicals cause the smooth muscle of the vessel to contract, reducing the flow of blood. Pain sensations trigger the release of the hormone epinephrine which causes vasoconstriction as well. This part of the process lasts for approximately 30 minutes.

Platelet Activation

• Platelets, also known as thrombocytes, are sticky components of blood that respond to injury and attempt to block blood flow. This response is triggered when the blood is exposed to air. The role of the platelets is to form a clot or plug, which occurs through several steps. First, the platelets gather at the site of the injury where a substance is released that activates the platelets, causing them to adhere to the damaged cells of the blood vessel wall.

Platelet Adhesion

• Activated platelets that have adhered to the blood vessel wall then release several substances. Adenosine diphosphate (ADP) is released and recruits other platelets to the area of the injury. These platelets then attach to the platelets already at the injury site. The hormone serotonin, which causes vasoconstriction, is also released by the platelets. The platelets begin to break apart and connect to fibrin to form a platelet plug.

Blood Coagulation

• After the platelet plug is formed, 13 different clotting factors are released. These factors initiate a series of reactions that cause fibers of fibrin to form a web that traps blood cells and platelets to form a clot. New cells form under the clot or scab, and as the wound heals the scab eventually falls off.