How Your Body Stops Bleeding

When a small blood vessel is damaged, it narrows to slow blood loss. Platelets gather to make a temporary plug, then fibrin strengthens it. As the vessel heals, the body breaks the fibrin down.

A damaged blood vessel starts to narrow When a blood vessel is damaged, the body starts a process called haemostasis , which helps stop blood loss. The vessel wall, platelets and clotting proteins all take part. The damaged vessel narrows, so less blood flows through the injured area. This narrowing is called vasoconstriction . Platelets make the first plug Platelets are small cell fragments in the blood. They stick to the damaged area, become stickier and attract more platelets. They join together to make a platelet plug . This plug forms quickly, but it is not very strong on its own. Fibrin makes the plug stronger Several clotting proteins work together. One of the later steps produces thrombin . Thrombin changes a dissolved blood protein into strands of fibrin . These connected chemical steps are called coagulation . The fibrin strands form a mesh through the platelet plug, making it stronger while the vessel heals. The clot must stay local and later be removed The blood has natural brakes that slow clotting reactions away from the injury. This helps keep clot formation focused around the damaged area rather than spreading through intact parts of the vessel. Fibrin is temporary. As repair progresses and the seal is no longer needed, fibrinolysis breaks fibrin down. The enzyme plasmin cuts the fibrin mesh into smaller fragments. If clotting happens inappropriately inside a blood vessel, this is called thrombosis . A thrombus can obstruct blood flow, which is the opposite problem from failing to form an adequate haemostatic seal. Blood tests look at different parts of haemostasis A full blood count can show how many platelets are in the blood, but it cannot tell whether those platelets are working normally. Prothrombin time (PT) and activated partial thromboplastin time (APTT) screen selected parts of the coagulation system that generates fibrin. They do not test platelet plug formation, and normal results do not rule out every cause of unusual bleeding. Clinicians interpret these results alongside the bleeding pattern, medicines, examination and other evidence. More specialised tests may be used when the clinical question requires them. When bleeding needs emergency help If a wound is bleeding heavily, apply firm pressure using a clean cloth or dressing. If an object is embedded in the wound, do not remove it or press directly onto it. Apply pressure around the object instead. Call your local emergency services immediately if the bleeding is severe or cannot be controlled. Continue applying pressure while emergency help is arranged.

Stopping bleeding takes several coordinated steps. The damaged vessel narrows, platelets make a quick plug, fibrin makes the plug stronger, and the body keeps the clot local before breaking it down as the vessel heals.