PHYSIOLOGY OF HEMOSTASIS

Maintenance of intravascular blood in a flowing state is a complex balance between pro-coagulant activity to prevent excessive bleeding after trauma and anticoagulant activity to arrest disproportionate clotting response and prevent generalized coagulation.

After vascular trauma, hemostasis is achieved by a series of four sequential, interrelated and appropriately limited responses, including—(a) vascular response, i.e. reflex vasoconstriction to restrict bleeding, (b) platelet response to form a temporary plug at the site of bleeding, (c) coagulation response to convert this plug into a stable clot, and (d) fibrinolytic response to re-establish vascular potency. Impairment of first three responses leads to hemorrhagic tendency, while abnormalities in fibrinolytic mechanisms produce hypercoagulable states with thromboembolic complications.

• Vascular response is characterized by immediate vasoconstriction at the site of injury to control bleeding, mediated by sympathetic adrenergic activity. Although this response is potentiated further by release of serotonins and other factors from platelets, it rarely lasts for more than few minutes.

• Platelet response: Post-injury, exposure of flowing blood to sub-endothelial tissues initiates a sequence of platelet responses, i.e.

± Adherence of platelets to von Willebrand factor (vWf)-a protein serving as ligand for platelets at exposed/injury site

± Activation of adhered platelets to release various factors, e.g. ADP and thromboxane A2, etc. from their storage granules, which promote further platelet aggregation and activate clotting mechanisms.

± Aggregation of platelets, leading to formation of a temporary platelet plug which acts as nidus for clot formation by coagulation mechanisms.

• Coagulation response is a complex and sequential activation of various plasma proteins, i.e.

*No longer considered as clotting factors (as not independent proteins). Factors' numbers represent order of discovery, not the order of activation.

- Activation of _fX by intrinsic or extrinsic pathway. Intrinsic pathway begins with activation of _fXII by two plasma proteins, i.e. prekallikrein and kininogen that leads to sequential activation of _fXI and _fIX. _fIX in presence of _fVIII, calcium and phospholipids activates _fX. _fX and _fIX activation is also stimulated by an Extrinsic pathway involving tissue thromboplastin, _fVII, and Calcium.

- Activated _fXa activates _fV, which along with plate­let phospholipid and calcium converts inactive prothrombin into active thrombin (fIIa).

- Thrombin facilitates conversion of plasma fibrino­gen to fibrin, which is further cross-linked by _fXIII to form a stable fibrin-platelet plug.

• Anticoagulant activity: Excess thrombin generation is limited by many anticoagulants, which include- inhibitors of thrombin generation, e.g. protein C, its cofactor protein-S and tissue factor pathway inhibitor (TFPI); and inhibitors of thrombin activity, e.g. anti­thrombin III (AT-III), #945;₂ macroglobulins (#945;₂M) and heparin cofactor.

It is stimulated by binding of thrombin to thrombo­modulin on exposed endothelial surface, which activates protein C. Activated protein C inactivates fV and f_VIII, while AT-III regulates fXa. TFPI-the final inhibitor, stops activation of _fX by _fVII and tissue factor.

• Fibrinolytic system aims to lyses the clot to re­establish vascular integrity (Fig. 19.9). Critical enzyme in this process is the Plasmin, which forms by activation of plasminogen by tissue plasminogen activator (tPA) and urokinase (UK), and cleaves the fibrin to form fibrin-split products (FSP) including a specific fragment, i.e. D-dimer. Excess plasmin activity

Fig. 19.9: Fibrinolytic system.

‘Assayed in D dimer assay

#945;₂AP: #945;₂ antiplasmin; tPA: Tissue plasminogen activator; PAI₁: Plasminogen activation inhibitors

is primarily inhibited by #945;₂antiplasmin (#945;2AP), while many other proteins inhibit its generation by blocking plasminogen activators-most important in childhood being PAI1.

19.10