Platelets and Hemostasis Quiz

Platelets and Hemostasis

• Platelets (thrombocytes) are minute discs, measuring 1 to 4 micrometers in diameter.
• Formed in bone marrow from megakaryocytes, which are large cells of the hematopoietic series.
• Megakaryocytes fragment into platelets within the bone marrow or soon after entering the blood, particularly as they pass through capillaries.
• Normal platelet concentration in blood ranges between 150,000 and 400,000 per microliter, with an average of 250,000/µL.

Platelet Function

• When platelets encounter a damaged blood vessel (especially collagen), they swell, become irregular, and extend pseudopods.
• Their contractile proteins release granules containing bioactive factors causing them to adhere to collagen and von Willebrand factor.
• Platelets release ADP and enzymes (thromboxane A2), activating more platelets and making them stickier.

Hemostasis

• Hemostasis is the process of stopping bleeding by forming blood clots in the injured vessel wall.
• The rest of blood remains fluid in the unaffected parts of the vascular system.

Mechanism of Hemostasis

• Local vasoconstriction: Narrowing of the blood vessels to reduce blood flow to the injury site
• Temporary hemostatic plug formation: Platelet activation, adherence and aggregation forms a temporary plug to stop bleeding.
• Definitive hemostatic plug formation (blood coagulation): Blood clotting process leading to a permanent plug mediated by a complex cascade of protein interactions.
• Repair of damaged blood vessel: Healing of the damaged blood vessel.

Clotting Mechanism

• Two pathways: intrinsic and extrinsic.
• Both result in activation of thrombin, which converts fibrinogen to fibrin, forming a mesh.
• Platelets are essential in the clotting process adhering to fibrin strands forming a stable platelet plug.

Fibrinolytic System

• The liver produces plasminogen (an inactive protein).
• Thrombin and tissue plasminogen activator (tPA) convert plasminogen into plasmin (active enzyme).
• Plasmin degrades fibrin, dissolving the clot.
• tPA inhibitor (antiplasmin) and PAI-1 (plasminogen activator inhibitor) control fibrinolysis to ensure that clots aren't dissolved prematurely.

Variations in Platelet Count

• Thrombocytopenia: Low platelet count (below 150,000/µL), potentially leading to bleeding. This condition can be caused by various factors: bone marrow not producing enough platelets, destruction of platelets by the body, or splenic retention of platelets.
• Thrombocytosis: High platelet count (above 400,000/µL), which may indicate an underlying condition such as inflammatory disorders, malignancy or post splenectomy.

Disorders of Hemostasis

• Vitamin K deficiency: This inhibits formation of prothrombin and factors VII, IX, and X in the liver, leading to bleeding.
• Hemophilia: Genetic bleeding disorder, commonly transmitted from females (carrier) to males. It results from a deficiency in factors crucial for blood clotting.