Tabuk University

Faculty of Applied Medical SciencesDepartment Of Medical Lab.

Technology

3rd Year – Level 5 – AY 1434-1435

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HEMATOLOGY – 2, MLT 307

Platelets, Blood coagulation and Haemostasis

By/Dr. Walid ZAMMITI; Phd

M.Sc; MLT

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OBJECTIVES

An overview of Haemostatic Mechanism, Platelet structure and function, Define the term Haemostasis coagulation, fibrinolysis, and hemorrhage. List the major and minor systems involved in maintaining hemostasis. Describe the events that take place in primary hemostasis Explain the role of platelets in the haemostatic process. List steps in platelet plug formation Name essential elements for the process of platelet adhesion. Describe the process of platelet aggregation. Describe the events take place in secondary hemostasis. Name the product responsible for stabilization of the haemostatic plug. List characteristics for the contact coagulation proteins. List characteristics for the prothrombin proteins. Interaction of the Fibrinolytic, Coagulation Systems and Related Pathology. Name the component of the coagulation and fibrinolytic system.

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Concept of Normal Haemostasis

The normal haemostatic response to vascular damage depends on interaction between the blood vessel wall, circulating platelets and blood coagulation factors.

Haemostasis : the arrest of bleeding by the physiological properties of vasoconstriction, platelets and coagulation factors.

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Without this balance, the individual may experience either excessive bleeding (poor clot formation or excessive Fibrinolysis)

Vaso-occlusion: uncontrolled formation of thrombin in vascular system, occluding vessels and depriving organs of blood.

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Components of hemostasis1. Blood vessels:  --considered to be an anticoagulant surface, to prevent a clot from

forming on their surface.2. Platelets: --help maintain the integrity of the vessel lining, and they 'plug' any

rupture in the circulatory vessels.3. Plasma proteins (coagulation factors):  --soluble circulating blood proteins function in the proper maintenance of

hemostasis.

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Platelets It is formed by fragmentation of the cytoplasm of megakaryocytes. Disc-shaped, non nucleated cells with complex internal structure. 3 major types of intracellular granules: 1.lysosomes  2.a-granules contain: *coagulation factors (fibrinogen, von Willebrand Factor, and coagulation

factors V and VIII); and platelet-derived growth factor (PDGF).  3. dense granules contain: *ADP, ATP and serotonin. **a- and dense granules contents released by platelet activation

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Platelet Structure*Thrompopoeitin is the main regulator of platelet production *Life span 7-10 d, Normal count – 150- 400x109/l*1/3 of plts. are trapped in normal spleen (more than 90% in massive splenomegaly)*Functions:

Hemostatic plug formationSome coagulation factors - release, synthesisSurface glycoproteins are important in platelet adhesion (vessel wall) and aggregation (platelet-platelet) ------(GpIa, GPIb,GPIIbIIIa)

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Process-primary haemostasis Normal coagulation is initiated within 20 seconds after

the injury which damage the endothelial cells. In order for hemostasis to occur, platelets must adhere

to exposed collagen, release the contents of their granules, and aggregate. The adhesion of platelets to the collagen exposed on endothelial cell surfaces is mediated by von Willebrand factor (vWF).

The function of vWF is to act as a bridge between a specific glycoprotein complex on the surface of platelets (GPIb-GPIX-GPV) and collagen fibrils

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When Platelets adhere, they become activated and change in shape to become spheres with pseudopodia .

Then they release their granules contents which induce aggregation (platelet to platelet), and further promote vasoconstriction.

This is called primary haemostasis.

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Secondary haemostasis

Secondary haemostasis then follows—plasma components called coagulation factors respond (in a complex cascade) to form fibrin strands which strengthen the platelet plug.

Contrary to popular belief, coagulation from a cut on the skin is not initiated by air or drying out, but by platelets adhering to and activated by collagen in the blood vessel endothelium.

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Coagulation cascadeFig.

The coagulation cascade of secondary hemostasis has two pathways, the Contact Activation pathway (formerly known as the Intrinsic Pathway)

And the Tissue Factor pathway (formerly known as the Extrinsic pathway) that lead to fibrin formation.

The pathways are a series of reactions, in which enzymes of serine protease (except factor XIII) are activated to become active components that then catalyze the next reaction in the cascade.

The two pathways lead to the formation of a fibrin clot. Although they are initiated by distinct mechanisms, the two converge on a common pathway that leads to clot formation.

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Coagulation:Intrinsic 12,11,9,8

(APTT)Extrinsic-7

(PT)

Prothrombin Thrombin

Fibrinogen Fibrin

Common Path (TT)FX FXa

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The intrinsic cascade (which has less in vivo significance in normal physiological circumstances than the extrinsic cascade) is initiated when contact is made between blood and exposed negatively charged surfaces.

The extrinsic pathway is initiated upon vascular injury which leads to exposure of tissue factor, TF (also identified as factor III), a subendothelial cell-surface glycoprotein that binds phospholipid.

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Fibrinogen FibrinThrombin

Prothrombin

XaVa

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Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF(Thromboplastin)

Extrinsic Pathway

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Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

IXaVIIIa

XIa

XIIa

Intrinsic pathway

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Fibrinogen FibrinThrombin

Prothrombin

Xa

VIIa

TF

Extrinsic Pathway

IXaVIIIaCa++

XIa

XIIa

Intrinsic pathway

XIIIa

Soft clot

FibrinHard clot

Ca++

phospholipid

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The coagulation factors are generally serine proteases: enzymes circulate as inactive zymogens (inactive enzyme precursor)

Contact factors : Prekallikrein (PK), High molecular weight kininogen (HMWK),

Factor XII, Factor XI Labile factors :Factor V, Factor VIII Vitamine K dependent factors :Factors II, VII, IX and X, as well as Protein S, Protein C Co-factors : Factor VIII, Factor V, Factor III (tissue factor), Ca++

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Haemostatic response: Summary

Injury Blood vessels : Vasoconstriction Platelets : adhesion, release, change in shape,

aggregation Coagulation factors : formation of fibrin : stabilizes

platelets aggregation. Hard clot : factor XIIIa Finally, the clot must be dissolved. (fibrinolysis)

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Regulators of Coagulation

1. Protein C system : physiological anticoagulant. With protein S, they degrades FVa

and FVIIIa.2. Antithrombin : degrades thrombin, FIXa, FXa, FXIa, and FXIIa.3. Tissue factor pathway inhibitor (TFPI) : limits the action of tissue factor (TF).

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Fibrinolysis

In fibrinolysis, a fibrin clot, is broken down.The main component of fibrinolytic system is Plasmin which is produced in an inactive form, plasminogen, in the liver.

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Tests of Hemostasis: Screening tests:

Bleeding Time - To test Platelet function(adhesion , aggregation) & vascular function.

Clotting Time : tests plts count & function and coagulation. ProthrombinTime (PT): tests Extrinsic pathway. Activated Partial Thromboplastin Time (APTT): test Intrinsic

pathway. Thrombin clotting Time (TCT): common pathway.

Specific tests: Factor assays Tests of thrombosis – TCT, FDPs Platelet function studies:

Adhesion, Aggregation, Release tests. Blood film and bone Marrow examination.

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Disorders of Hemostasis

Vascular disorders – Scurvy, easy bruising, Henoch-Schonlein purpura.

Platelet disorders Quantitative - Thrombocytopenia Qualitative - Platelet function disorders – Glanzmans

Coagulation disorders Congenital - Haemophilia (A, B), Von-Willebrands Acquired - Vitamin-K deficiency, Liver disease

Thank you

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