Haemostasis and Coagulation

Key points

• • Haemostasis stops bleeding in stages: the vessel narrows, platelets plug the gap, and a mesh of fibrin seals it.
• • Platelets stick to the damaged wall, are activated, and clump together to form a soft plug, which is primary haemostasis.
• • The clotting cascade, a chain of proteins called clotting factors, then converts fibrinogen into fibrin, which strengthens the plug, in secondary haemostasis.
• • The cascade has intrinsic and extrinsic pathways that meet in a common pathway, tested by the aPTT and the PT.
• • Natural brakes and the breakdown of clot (fibrinolysis) keep clotting in check, so it seals the wound without blocking the vessel.