CBSE Class 10 Answered

When bleeding occurs from a wound, the platelets gather at the wound and attempt to block the blood flow. For clotting to occur, calcium ions, vitamin K,  prothrombin and fibrinogen are important and help the platelets form a clot. A clot begins to form when the blood is exposed to air. The platelets sense the presence of air and begin to break apart. They react with the fibrinogen to begin forming fibrin, which resembles tiny threads. The fibrin threads then form a meshwork  that traps the blood cells within it. This mesh of blood cells hardens as it dries, forming a clot. A straw colored fluid called serum squeezes out during this clot formation.