Q.Which of the following is not involved in blood clotting?
a. Fibrin
b. Calcium
c. Platelets
d. Bilirubin

Blood coagulation (clotting) is the mechanism that stops excessive blood loss after an injury by forming a solid plug at the site of damage. It begins the instant a vessel's inner endothelial lining is torn and the underlying connective tissue is exposed to blood: platelets stick to the exposed collagen fibres and release chemicals that recruit more platelets, quickly forming a temporary platelet plug. Substances from these clumped platelets and from the damaged tissue combine with clotting factors already in the plasma to generate a prothrombin-activator substance, which converts the inactive plasma protein prothrombin into its active enzyme form, thrombin - a step that requires both calcium ions and vitamin K. Thrombin then converts a second plasma protein, fibrinogen, into long insoluble threads of fibrin, which weave into a mesh that traps blood cells and platelets and physically seals the wound. As the mesh matures, the …