Hemostasis is a process that attempts to stop the bleeding and keep blood within a damaged blood vessel.
Hemostasis involves three steps
- Temporary blockage of a break by a platelet plug
- Blood coagulation, or formation of a fibrin clot that seals the hole until tissue repair.
Blood coagulation and platelet-mediated hemostasis are the two important defense mechanisms against bleeding.
The coagulation cascade is triggered as soon as blood contacts the injured endothelial lining. The responses of the coagulation cascade are ideally coordinated with the formation of the platelet plug that initially occludes a vascular lesion.
Mechanism of Hemostasis
Vasoconstriction [contraction of the injured vessel] is the blood vessel’s first response to injury and is done by smooth muscles in the vessel.
On injury, there is an immediate reflex, initiated by local sympathetic pain receptors, which helps promote vasoconstriction which reduces the amount of blood flow to the site and reduces blood loss.
Collagen, which is exposed at the site of injury promotes platelets adherence to the injury site. Platelets release cytoplasmic granules which contain serotonin, ADP and thromboxane A2 which contribute to vasoconstriction.
Platelets are involved in following sequence of events during the hemostatic process
- Shape change
- Secretion and activation of circulating platelets
- Binding/aggregation of additional platelets.
During the first few minutes after endothelial cell disruption, an initial unstable platelet plug forms (adherence).
On contact with the injured endothelium cells, platelets change shape, release granules and ultimately become sticky. Activated platelets express glycoprotein receptors that interact with other platelets, producing aggregation and adhesion.
Platelets release cytoplasmic granules such as adenosine diphosphate (ADP), serotonin and thromboxane A2.
Adenosine diphosphate (ADP) attracts more platelets to the affected area, serotonin is a vasoconstrictor and thromboxane A2 assists in platelet aggregation, vasoconstriction and degranulation.
These, along with catecholamines, cations, clotting factor, and platelet-derived growth factor, activate platelets in the area, leading to the additional aggregation.
As more platelets stick and release their chemicals, a platelet plug is created. This is referred to as primary hemostasis.
Once the platelet plug has been formed by the platelets, clotting cascade is initiated by the clotting factors leading to formation of fibrin clot.
This step of coagulation is called as secondary hemostasis.
Thus platelets take part in hemostasis at three different levels
- Stick to endovascular collagen as well as to each other to form a physical barrier to blood loss.
- Platelet phospholipid surface provides a surface for activation of factors V and X, facilitating coagulation mechanism
- Vasoconstrictive effect
The classic coagulation cascade is composed of two basic parts
- Intrinsic pathway
- Extrinsic pathway
The extrinsic pathway is initiated by injury to the avascular wall or nonvascular tissue. Nonvascular tissue cells contain tissue factor which is an integral membrane protein. Damage to the blood vessel wall exposes plasma to tissue factor.
Factor VII [ a circulating protein] binds to tissue factor, creating a complex. In doing so, factor VII is activated to factor VIIa.
This complex, activates factors IX and X to factors IXa and Xa [in the presence of Ca++ and phospholipids].
Factors IXa and Xa may
remain associated with the tissue factor bearing cell
may diffuse into the blood and bind to the surface of nearby activated platelets of the primary platelet plug.
Factor Xa and its cofactor Va form a phospholipid bound complex called the prothrombinase complex.
This complex is highly activated on the surface of platelets and, in the presence of ionated calcium, cleaves prothrombin (factor II) to thrombin (factor IIa).
Thrombin in turn cleaves fibrinogen (factor I) to fibrin (factor Ia), which is covalently cross-linked by factor XIIIa into fibrin strands.
Factor VIII enhances the activation of factor X. Factor VIII circulates bound to vWF [von willebrand factor], which is an adhesive protein important for generation of the initial platelet plug.
After activation, factor VIIIa dissociates from vWF and forms a complex on the platelet surface, which also activates factor X to Xa.
Mechanism of activation of the intrinsic pathway is less well understood, so is its physiology.
This pathway is thought to begin with trauma to the blood vessel or exposure of blood to collagen in a damaged vascular wall.
AS a response, two events occur. First, factor XII (Hageman factor) is converted to its active form (factor XIIa). Second, platelets are activated.
Factor XIIa enzymatically activates factor XI to factor Xia. This reaction requires the presence of high —molecular weight kininogen and prekallekrin.
Factor XIa is a protease that converts factor IX to factor IXa, which in turn converts factor X to factor Xa. Once factor Xa is generated, the remainder of the pathway is similar to the extrinsic pathway.
Cofactors of Coagulation
Various substances are required for the proper functioning of the coagulation cascade:
Calcium and phospholipid
Calcium and phospholipid are required for the tenase [cleaver of factor X] and prothrombinase complexes to function
Calcium is also required at other points in the coagulation cascade.
Vitamin K is an essential factor to a hepatic gamma-glutamyl carboxylase that adds a carboxyl group to glutamic acid residues on factors II, VII, IX and X, as well as Protein S, Protein C and Protein Z.
Deficiency of this vitamin leads to inhibition of clotting factors.
Regulatory Mechanisms of Coagulation Cascade
The regulatory mechanisms work to ensure the optimally required amount of coagulation and prevent prevent widespread thrombosis.
Tissue Factor Pathway Inhibitor
As noted, coagulation is normally initiated when vessel or tissue injury exposes circulating factor VIIa to tissue factor, which leads to activation of factors IX and X, eventually resulting in limited quantities of thrombin. Tissue factor pathway inhibitor mediates the feedback inhibition of the tissue factor–factor VIIa complex, thus leading to decreased activation of both factor IX and X.
Antithrombin III is a protein synthesized by liver and endothelial cells which binds and directly inactivates thrombin and the other serine proteases (factors IXa, Xa, and XIa).
Activated Protein C and Protein S
Proteins C and S are both vitamin K–dependent and together they inactivate factors Va and VIIIa.
Thrombomodulin is an endothelial cell receptor that binds thrombin but when the complex is formed, the conformation of the thrombin molecule is changed. This altered thrombin molecule then readily activates protein C and loses its platelet-activating and protease activities.
The fibrinolytic system is present to keep clot formation in check by actually degrading the fibrin strands.
Antithrombin is a serine protease inhibitor that degrades the serine proteases: thrombin, FIXa, FXa, FXIa, and FXIIa. It is constantly active, but its adhesion to these factors is increased by the presence of heparan sulfate (a glycosaminoglycan) or the administration of heparins.