HEMOSTASIS AND BLOOD COAGULATIONALLAN SIGANGAMBCHB III

Normal hemostasis comprises a series of regulated processes that maintain blood in a fluid , clot free state in normal vessels while rapidly forming a localized hemostatic plug at the site of vascular injury.

Achieved through the following mechanisms:

1. Transient arteriolar vasoconstriction

2. Platelet adherence, activation and aggregation

3. Activation of thrombin leading to subsequent formation of a blood clot

4. Fibrinolysis

primary hemostasis - It takes place when there are injuries to small vessels during which the affected vessels contract to seal off the wound and platelets are mobilized, aggregate, and adhere to components of the subendothelium of the vasculature. Platelet adhesion requires the presence of various factors such as von Willebrand factor (vWF) and platelet receptors (IIb/IIIa and Ib/IX). Additional platelets are attracted to the site of injury by the release of platelet granular contents, such as adenosine diphosphate (ADP)

Secondary hemostasis - involves the response of the coagulation system to vessel injury. It is required to control bleeding from large wounds and is a continuation of the primary hemostatic mechanisms.

Whereas the outcome of primary hemostasis is the formation of the platelet plug, the outcome of secondary hemostasis is the formation of a thrombus.

ANTITHROMBOTIC PROPERTIES OF NORMAL EPITHELIUM Platelets - intact endothelium prevents platelets from engaging the

highly thrombogenic subendothelial ECM. Prostacyclins ie PGI2 and nitric oxide produced by endothelium

impedes their adhesion, potent vasodilators , inhibit aggregation Endothelial cells also produce Adenosine diphosphatase which

degrades ADP further inhibiting platelet aggregation. Coagulation factors – the inhibitory effects are mediated by factors

expressed on endothelial surfaces particularly heparin like molecules, thrombomodulin, and tissue factor pathway inhibitor.

Heparin like molecules act indirectly by enhancing the inactivation of thrombin by antithrombin III.

Thrombomodulin binds to thrombin thereby modifying the substrate specificity of thrombin so that it cleaves and activates protein C instead of fibrinogen.

TFPI directly inhibits tissue factor –factor VIIa complex and factor Xa. Fibrinolysis - endothelial cells synthesize tissue type plasminogen

factor that cleaves plasminogen to plasmin which in turn cleaves fibrin to degrade thrombi.

PROTHROMBOTIC PROPERTIES OF INJURED/ACTIVATED ENDOTHELIUM Activation of platelets - endothelial injury brings

platelets into contact with Subendothelial ECM which includes among its constituents von Willebrand factor . vWF is held fast to the ECM through interactions with collagen as well as Gp1b found on the surface of platelets allowing it to as glue that binds platelets tightly to denuded vessel walls.

Activation of clotting factors – in response to TNF / IL-1 or endotoxin endothelial cells produce tissue factor the major in vivo activator of coagulation and downregulate thrombomodulin. Also bind coagulation factors IXa and Xa augmenting the catalytic activities.

Antifibrinolytic effects – activated endothelial cells secrete plasminogen activator inhibitors which limit fibrinolysis.

PLATELETS Platelet function depends on several integrin family

glycoprotein receptors, a contractile cytoskeleton and 2 types of cytoplasmic granules:

Alpha granules – express the adhesion molecule P-selectin on their membranes and contain fibrinogen, fibronectin, factors V & VIII , platelet factor 4 , platelet derived growth factor , transforming growth factor beta.

Dense bodies (delta granules) - ADP, ATP , ionized calcium , histamine, serotonin , epinephrine.

PLATELET ADHESION

Depends on vWF and Gp1b. Under stress( i.e in flowing blood) vWF undergoes a conformational change allowing it to bind to collagen in the ECM and to platelet Gp1b.

PLATELET ACTIVATION Platelet adhesion leas to an irreversible shape change and

secretion of both granule types ( platelet activation) Calcium and ADP released are important as Calcium is

required by several coagulation factors and ADP is a potent activator of resting platelets.

Activated platelets synthesize thromboxane A2 (TxA2) a PG that activates additional nearby platelets as well play an important role in platelet aggregation.

Irreversible shape change includes change from smooth discs to spheres with numerous long, spiky membrane extensions as well as subtle changes in their plasma membranes.

Enhances subsequent aggregation and increase surface area available for interaction with coagulation factors .

Subtle membrane changes include an increase in the surface expression of negatively charged phospholipds providing binding sites for both calcium and coagulation factors and a conformation change in platelet GpIIb /IIIa that permits it to bind to fibrinogen.

PLATELET AGGREGATION Prompted by bridging interactions between fibrinogen and

GpIIb/IIIa receptors on adjacent platelets. [ Glanzmann thrombasthenia]

Concurrent activation of the coagulation cascade generates thrombin which stabilizs the platelet plug through two mechanisms:

1. Thrombin activates a platelet surface receptor (protease activated receptor[PAR]) which in concert with ADP and TxA2 further enhances platelet aggregation. Platelet contraction follows creating the secondary hemostatic plug.

2. Thrombin activates fibrinogen to fibrin

COAGULATION CASCADE

Critical labs assess the cascade using two standard assays:

1. Prothrombin time – screens for factors VII,X,II,V and fibrinogen. (extrinsic pathway) .

2. Partial thromboplastin time – screens for the activity of factors XII,XI, IX, VIII,X,V,II and fibrinogen

once activated coagulation cascade must be tightly restricted to the site of injury.

Besides restricting factor activation to sites of exposed phospholipids , clotting is controlled by three general categories of anticoagulants:

1. Antithrombin III – inhibit the activity of thrombin and factors Ixa,Xa,XIa and XIIa . Activated by binding to heparin like molecules ie the adminsitration of heparin to limit thrombosis.

2. Protein c & S - vitamin k dependent proteins that act in a complex to proteolytically inactivate cofactors Va, VIIIa

3. tissue factor pathway inhibitor – inactivates factor Xa and tissue factor-factor VIIa complexes.