Platelets

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Definition

Platelets

Platelets Size: 1 – 4 Micro meter in diameter Normal Count: 1, 50000 – 300,000 / cu mm or micro literLive span: 10 days Platelet membranes contain receptors for

Collagen ADP Vessel wall Von Willebrand factor Fibrinogen Glycoprotein

Platelets cytoplasm contain Actin Myosin Thrombasthenin Glycogen Lysosomes Granules

I. Dense granules Contains non protein substances e.g. Serotonin, ADP, Ca and other nucleotides.

II. Alpha granules:Secrete proteins e.g. Clotting factors, platelet derivedgrowth factor (PDGF), Von Willebrand factor.

Regulation of Platelet Production Thrombopoietin Erythropoietin Adrenaline Injury Hypoxia Smoking

Thrombocytopenia

Decreased Number of platelets in the blood Causes.1. Idiopathic 2. Secondary

a. Pancytopenia b. Increased destruction of platelets c. Splenomegaly d. Leukemia

Thrombocytosis

Increase no of platelets in the blood Causes. 1. After acute haemorrhage 2. Splenectomy 3. Polycythemia vera

Hemostasis “Arrest of bleeding” is called hemostasis. It involves three steps 1. Vascular spasm

a. Inherent vascular response b. Sympathetically induced vasoconstriction

2. Formation of platelet plug a. Platelet adhesion b. Platelet activation c. Platelet aggregation

3. Clot formation a. Intrinsic system b. Extrinsic system

Activated Platelets

Platelet plug Formation

Factors involved in Platelet Function

Whenever the vessel is damaged, the net result is formation of prothrombin activator.

Prolthrombin activator is formed in two ways a. The extrinsic pathway b. The intrinsic pathway

Intrinsic Pathway for initiating Clotting

Intrinsic pathway

Extrinsic Pathway for initiating Clotting

Extrinsic Pathway

Erythrocytes trapped in the fibrin

meshwork of a clot (Clot formation)

Roles of Thrombin

Concurrent Platelet Aggregation and Clot Formation

Hemostasis

Factors involved in Clotting

Role of Factor XII

Fibrinolytic system Plasminogen Activator Plasminogen Plasmin

Fibrin Fibrin Degradation Products

FDPs. Prevent platelet aggregation Inhibit thrombin formation Prevent fibrin polymerization Plasmin also digests factors I,II,V, VIII and XII.

Endogenous factors involved in Fibrinolysis and Anticoagulation

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