How does blood clot?

When a blood vessel is injured, the body launches a rapid, multi-step process to stop the bleeding. First, the vessel narrows to reduce blood flow. Then tiny cell fragments called platelets rush to the wound, stick to the damaged surface and to each other, forming a temporary plug. At the same time, a cascade of clotting proteins in the blood is activated, culminating in the formation of long strands of a protein called fibrin. This fibrin weaves through the platelet plug like a net, trapping blood cells and hardening into a stable clot that seals the wound while the tissue heals. Disorders such as haemophilia occur when this clotting cascade does not work properly.