When a blood vessel is damaged, blood clotting (coagulation) quickly seals the break. This prevents further blood loss, helps maintain blood pressure and stops pathogens entering the body through the wound.The first cells to respond are platelets. These are small fragments of large cells called megakaryocytes, which are made in the bone marrow. Platelets have no nucleus and normally circulate freely in the plasma without sticking to anything, because the endothelium lining a healthy vessel is smooth.When the endothelium is damaged, the collagen fibres in the vessel wall underneath are exposed. Platelets stick to the exposed collagen and then to each other, forming a platelet plug. This plug alone can seal very small breaks. The platelets also release serotonin, which makes the smooth muscle in the vessel wall contract. The vessel narrows and blood flow to the damaged area is reduced.The platelets and damaged tissue also release a substance that starts a series of enzyme-controlled reactions in the plasma (the clotting cascade, covered in the next lesson). The end result is a mesh of insoluble protein fibres called fibrin. The fibrin mesh traps red blood cells and more platelets, forming a clot. On the skin, the clot dries and hardens to form a scab while the tissue underneath repairs.
Clotting should only happen where a vessel is damaged. A clot that forms inside an intact vessel can block blood flow, which is dangerous.
Clotting should only happen where a vessel is damaged. A clot that forms inside an intact vessel can block blood flow, which is dangerous.