Q.Which blood corpuscles are necessary for starting the process of blood clotting?

Thrombocytes (platelets) are small, nucleus-free cell fragments broken off from megakaryocytes in bone marrow, numbering about 2.5-4.5 lakh per mm3 of blood; a drop below normal (thrombocytopenia) causes internal bleeding (haemorrhage). Platelets secrete clotting factors, physically seal small vessel ruptures by clumping into a platelet plug (thrombus), and secrete serotonin, which locally constricts the damaged vessel.

Blood clotting (coagulation) converts liquid blood to a solid clot and can be triggered by contact with a foreign surface (intrinsic pathway) or with damaged tissue (extrinsic pathway); both work through a step-wise cascade of twelve clotting factors (numbered I-XIII, factor VI unused). The extrinsic pathway starts with tissue damage releasing tissue thromboplastin; the intrinsic pathway starts with vessel-wall damage trigger …