Hemostasis
Encyclopedia
Hemostasis or haemostasis (from the haimóstasis "styptic (drug)") is a process which causes bleeding to stop, meaning to keep blood within a damaged blood vessel (the opposite of hemostasis is hemorrhage
Bleeding
Bleeding, technically known as hemorrhaging or haemorrhaging is the loss of blood or blood escape from the circulatory system...

). Most of the time this includes blood changing from a liquid to a solid state. Intact blood vessels are central to moderating blood's tendency to clot. The endothelial
Endothelium
The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...

 cells of intact vessels prevent blood clotting with a heparin-like molecule and thrombomodulin
Thrombomodulin
Thrombomodulin, CD141 or BDCA-3 is an integral membrane protein expressed on the surface of endothelial cells. In humans, thrombomodulin is encoded by the THBD gene...

 and prevent platelet aggregation with nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...

 and prostacyclin
Prostacyclin
Prostacyclin is a member of the family of lipid molecules known as eicosanoids.As a drug, it is also known as "epoprostenol". The terms are sometimes used interchangeably.-History:...

. When endothelial injury occurs, the endothelial cells stop secretion of coagulation and aggregation inhibitors and instead secrete von Willebrand factor
Von Willebrand factor
von Willebrand factor is a blood glycoprotein involved in hemostasis. It is deficient or defective in von Willebrand disease and is involved in a large number of other diseases, including thrombotic thrombocytopenic purpura, Heyde's syndrome, and possibly hemolytic-uremic syndrome...

 and tissue thromboplastin
Thromboplastin
Thromboplastin is a plasma protein aiding blood coagulation through conversion of prothrombin to thrombin.Although sometimes used as a synonym for tissue factor, this is a misconception. Historically, thromboplastin was a lab reagent, usually derived from placental sources, used to assay...

 which initiate the maintenance of hemostasis after injury. Hemostasis has three major steps: 1) vasoconstriction, 2) temporary blockage of a break by a platelet plug, and 3) blood coagulation, or formation of a clot that seals the hole until tissues are repaired.

Overview

Hemostasis is maintained in the body via three mechanisms:
  • Vascular spasm - Damaged blood vessels constrict
    Vasoconstriction
    Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in...

    .
  • Platelet plug formation - Platelet
    Platelet
    Platelets, or thrombocytes , are small,irregularly shaped clear cell fragments , 2–3 µm in diameter, which are derived from fragmentation of precursor megakaryocytes.  The average lifespan of a platelet is normally just 5 to 9 days...

    s adhere to damaged endothelium to form platelet plug (primary hemostasis) and then degranulate.
  • Blood coagulation
    Coagulation
    Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...

     - Clots form upon the conversion of fibrinogen
    Fibrinogen
    Fibrinogen is a soluble plasma glycoprotein, synthesised by the liver, that is converted by thrombin into fibrin during blood coagulation. This is achieved through processes in the coagulation cascade that activate the zymogen prothrombin to the serine protease thrombin, which is responsible for...

     to fibrin
    Fibrin
    Fibrin is a fibrous, non-globular protein involved in the clotting of blood. It is a fibrillar protein that is polymerised to form a "mesh" that forms a hemostatic plug or clot over a wound site....

    , and its addition to the platelet plug (secondary hemostasis).

Steps

  • The first step is immediate constriction of damaged vessels caused by vasoconstrictive paracrine released by the endothelium. Vasoconstriction temporarily decreases blood flow and pressure within the vessel. When you put pressure on a bleeding wound, you also decrease flow within the damaged vessel.
  • Vasoconstriction is rapidly followed by the second step,mechanical blockage of the hole by a platelet plug. The plug forms as platelets stick to the exposed collagen(platelet adhesion) and become activated, releasing cytokines into the area around the injury. Platelet factors reinforce local vasoconstriction and activate more platelets which stick to one another( platelet aggregation) to form a loose platelet plug.
  • Simultaneously, exposed collagen and tissue factor (a protein-phospholipid mixture) initiate the third step, a series of reactions known as the coagulation cascade
    Coagulation
    Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...

    . The cascade is a series of enzymatic reactions that ends in the formation of a fibrin
    Fibrin
    Fibrin is a fibrous, non-globular protein involved in the clotting of blood. It is a fibrillar protein that is polymerised to form a "mesh" that forms a hemostatic plug or clot over a wound site....

    protein fiber mesh that stabilizes the platelet plug. The reinforced platelet plug is called a clot
    Thrombus
    A thrombus , or blood clot, is the final product of the blood coagulation step in hemostasis. It is achieved via the aggregation of platelets that form a platelet plug, and the activation of the humoral coagulation system...

    . Some chemical factors involved in the coagulation cascade also promote platelet adhesion and aggregation in the damaged region.

Eventually, as the damaged vessel repairs itself, the clot retracts and is slowly dissolved by the enzyme plasmin
Plasmin
Plasmin is an important enzyme present in blood that degrades many blood plasma proteins, most notably, fibrin clots. The degradation of fibrin is termed fibrinolysis. In humans, the plasmin protein is encoded by the PLG gene.- Function :...

.

Hemostasis in emergency medicine

In medicine, hemostasis is achieved through multiple techniques. Hemostasis prevents shock.
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