Hemostasis is the process of preventing blood loss from intact vessels and of stopping bleeding from a severed vessel. The pre-vention of blood loss from intact vessels requires adequate num-bers of functional platelets. Platelets nurture the endothelium and thereby maintain the structural integrity of the vessel wall. Two processes are involved in arresting bleeding: primary and secondary hemostasis.
In primary hemostasis, the severed blood vessel constricts. Cir-culating platelets aggregate at the site and adhere to the vessel and to one another. An unstable hemostatic plug is formed. For the coagulation process to be correctly activated, circulating inactive coagulation factors must be converted to active forms. This process occurs on the surface of the aggregated platelets at the site of vessel injury. The end result is the formation of fibrin, which reinforces the platelet plug and anchors it to the injury site. This process is termed secondary hemostasis (Fig. 33-4). The process of blood coagulation is highly complex. It can be activated by the intrinsic or the extrinsic pathway. Both pathways are needed for maintenance of normal hemostasis.
Many factors are involved in the reaction cascade that forms fibrin. When tissue is injured, the extrinsic pathway is activated by the release from the tissue of a substance called thromboplastin. As the result of a series of reactions, prothrombin is converted to thrombin, which in turn catalyzes the conversion of fibrinogen tofibrin. Clotting by the intrinsic pathway is activated when the col-lagen that lines blood vessels is exposed. Clotting factors are acti-vated sequentially until, as with the extrinsic pathway, fibrin is ultimately formed. Although the intrinsic pathway is slower, this sequence is probably most often responsible for clotting in vivo.
As the injured vessel is repaired and again covered with en-dothelial cells, the fibrin clot is no longer needed. The fibrin is digested via two systems: the plasma fibrinolytic system and the cel-lular fibrinolytic system. The substance plasminogen is required to lyse (break down) the fibrin. Plasminogen, which is present in all body fluids, circulates with fibrinogen and is therefore incorpo-rated into the fibrin clot as it forms. When the clot is no longer needed (eg, after an injured blood vessel has healed), the plas-minogen is activated to form plasmin. Plasmin actually digests the fibrinogen, and the breakdown particles of the clot (fibrin degra-dation products) are released into the circulation. Through this sys-tem, clots are dissolved as tissue is repaired, and the vascular system returns to its normal baseline state.