Hemostasis

Overview
Hemostasis refers to the physiologic process whereby bleeding is halted in most animals with a closed circulatory system. Stopped bleeding is commonly referred to, however, as coagulation, but coagulation is only one type of hemostatic process.

Hemostasis in physiology
When a blood vessel is wounded, several steps occur to staunch the flow of blood, namely:


 * Vasoconstriction constricts the blood vessel, minimizing vessel diameter and slowing bleeding.
 * Primary hemostasis occurs, wherein platelets, one of the formed elements of the blood, bind to collagen in the exposed walls of the blood vessel to form a hemostatic plug within seconds after an injury.
 * Secondary hemostasis or coagulation occurs. This involves a complex cascade of coagulation factors, ultimately resulting in the transformation of fibrinogen, a blood protein, into polymerized fibrin, making a clot. This process takes several minutes.
 * The clot attracts and stimulates the growth of fibroblasts and smooth muscle cells within the vessel wall, and begins the repair process which ultimately results in the dissolution of the clot (fibrinolysis).

Disorders of hemostasis can be roughly divided into platelet disorders, such as idiopathic thrombocytopenic purpura, and disorders of coagulation, such as hemophilia.

Hemostasis may also refer to the complex interaction between vessels, platelets, coagulation factors, coagulation inhibitors and fibrinolytic proteins to maintain the blood within the vascular compartment in a fluid state. The objective of the hemostatic system is to preserve intravascular integrity by achieving a balance between hemorrhage and thrombosis.

Hemostasis can be induced by adenosine diphosphate (ADP) at the site of a mosquito bite to recruit platelets and oppose bloodfeeding; however mosquitoes have developed salivary apyrase to degrade ADP to counter this defense (Hurd, 2003).

Hemostasis by hemostatic clamps
Hemostasis may refer to the process of manually clamping a blood vessel, usually with hemostatic clamps, in surgery or dissection, to prevent bleeding from that vessel. This also may be done when an abnormal blood vessel forms, as these vessels may have thin walls and be prone to rupturing.