Blood Clotting

Blood Clotting

Blood clotting or coagulation is a very important biological process in many organisms in healing of the body or injuries; and of hemostasis. Coagulation is initiated almostinstantly after an injury to the blood vessel damages the endothelium (lining of the vessel).

Platelets immediately form a hemostatic plug at the site of injury; this is called primary hemostasis.Secondary hemostasis occurs simultaneously; proteins in the blood plasma, called coagulation factors, respond in a complex cascade to form fibrin strands which strengthen the platelet plug.

PlateletActivation
When damage happens, circulating platelets bind collagen with surface collagen-specific glycoprotein Ia/IIa receptors. The adhesion is strengthened further by the large, multimericcirculating proteins von Willebrand factor, which forms links between the platelets glycoprotein Ib/IX/V and the collagen fibrils. This adhesion activates the platelets.
Activated platelets release thecontents of stored granules into the blood plasma. The granules include ADP, serotonin, platelet activating factor (PAF), vWF, platelet factor 4 and thromboxane A2 (TXA2) which in turn activate additionalplatelets. The granules contents activate a Gq-linked protein receptor cascade resulting in increased calcium concentration in the platelets' cytosol.
The calcium activates protein kinase C which inturn activates phospholipase A2 (PLA2). PLA2 then modifies the integrin membrane glycoprotein IIb/IIIa, increasing its affinity to bind fibrinogen. The activated platelets changed shape from sphericalto stellate and the fibrinogen cross-links with glycoprotein IIb/IIIa aid in aggregation of adjacent platelets.
Coagulation Cascade
This event of secondary hemostasis has two pathways, the contactactivation pathway (formerly known as the intrinsic pathway) and the tissue factor pathway (formerly known as the extrinsic pathway) that lead to fibrin formation. It was previously thought that...