Clinical Laboratory · MU Plovdiv

30. Hemostasis as a complex functional system – phases and factors.

Inside: 7 sections · 1 labelled picture · 1 table · 20 flashcards · 8 min

Topic 30 of 48 in the MU Plovdiv syllabus for Clinical Laboratory (Medicine, Year 3). The free AceMed note covers it in 7 sections, about 1,001 words (8 min of reading), with 20 flashcards.

In this note

  1. Introduction
  2. Phases of haemostasis
  3. Vascular phase
  4. Platelets and primary hemostasis
  5. Plasma coagulation factors
  6. The coagulation cascade
  7. Clot retraction and laboratory assessment

Introduction

Haemostasis is the complex process that prevents blood loss following vascular injury and controls the flow of blood through blood vessels, keeping blood fluid elsewhere. The process of blood clotting and the subsequent dissolution of the clot, after repair of the injured tissue, is termed haemostasis. This topic covers the vascular reaction, the platelet plug (primary hemostasis), the plasma coagulation factors and the cascade that produces a stable fibrin clot (secondary hemostasis), clot retraction, and the basic laboratory tests. Fibrinolysis and the natural anticoagulants are dealt with in the next topic.

Components of haemostasis: platelets, blood vessels, clotting factors and factors of fibrinolysis.

Phases of haemostasis

The stages run as overlapping stages rather than strictly one after another: vascular spasm, platelet plug formation, blood coagulation (conversion of soluble fibrinogen to an insoluble fibrin network), then clot retraction, fibrinolysis and tissue repair.

  • Vascular phase - 2-5 s: vascular constriction limiting blood flow and release of von Willebrand factor and tissue thromboplastin after injury.
  • Platelet phase - 3-10 s: platelets become activated and aggregate at the site of injury, forming a loose platelet plug. Fibrinogen is primarily responsible for platelet clumping. The secreted von Willebrand factor allows platelets to adhere to collagen on the damaged endothelial lining of the vessels. Upon activation platelets release ADP and thromboxane A2.
  • Plasma phase - 30-120 s: clotting factors are activated from the intrinsic and extrinsic pathways.
  • Coagulation phase: activation of factor X and of thrombin leads to formation of fibrin; the loose plug is stabilised by a fibrin mesh that entraps it. Simultaneously exposed collagen and tissue factor initiate the coagulation cascade, a series of reactions involving 12 factors that ends in a fibrin protein fibre mesh stabilizing the platelet plug. The reinforced platelet plug is called the red thrombus.
  • Fibrinolysis phase - 6-48 h: dissolution of the fibrin formed during coagulation through the action of plasmin.
  • Vessel recovery - 10-60 days.

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