General Overview

Hemostasis is the process by which a barrier to blood loss is created at the site of blood vessel injury and is limited to that site. Hemostasis most commonly occurs in the smallest veins and arteries, the venules and arterioles, as well as in capillaries. Primary hemostasis is the first phase in the hemostatic process, during which the primary hemostatic plug is formed by the platelets at the site of vessel injury. During secondary hemostasis, the primary hemostatic plug is strengthened by the addition of fibrin.²

During primary hemostasis, platelets interact among themselves and with the injured blood vessel to create a clump of platelets at the site of vessel injury known as the primary hemostatic plug. This plug temporarily arrests bleeding. However, it is fragile and can be dislodged with ease from the vessel wall.

The Role of Vasculature in Primary Hemostasis
Hemostasis is initiated by damaged blood vessels. Their first response to injury is to constrict or narrow the lumen of the arterioles, thereby minimizing both the flow of blood to the wounded area and the loss of blood from the wound. This initial vasoconstriction occurs and is balanced by a complementary process of vasodilation. These conflicting processes balance one another, preventing either from becoming too powerful.²

Hemostasis inhibits clot formation in the absence of injury by maintaining a nonreactive environment for the components of the hemostatic system. Endothelial cells are responsible for modulating functions that both form and prevent blood clots. In the absence of vessel injury, the negatively charged surface of the endothelial cell repels negatively charged circulating proteins and platelets. However, when injury to the endothelium occurs, coagulation proteins in the plasma and platelets are exposed to subendothelial tissues, rich in collagen and von Willebrand Factor (vWF). In high shear flow conditions that occur at the site of vessel injury, platelets come in contact with subendothelial collagen and vWF, causing platelet adhesion and initiating the process of primary hemostasis. Interaction between plasma components and the vessel wall leads to formation of the hemostatic plug.^1,2

One of the primary thrombogenic functions of the endothelium is production and processing of vWF. During vessel injury, vWF is secreted into the subendothelial tissue and into the plasma from the luminal side of the endothelium. vWF plays an important role in the initial stage of clot formation by binding to collagen fibers in the extracellular matrix and supporting the binding platelets.²

Primary hemostasis is followed by secondary hemostasis.²
To learn more about secondary hemostasis, click here.

To learn more about the role of platelets in primary hemostasis, click here.