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 has to stay around the damaged area The blood has natural brakes that slow the clotting reactions away from the injury. This helps stop the clot from spreading unnecessarily through the vessel. Once the vessel has healed enough, an enzyme called plasmin breaks down the fibrin mesh. This process is called fibrinolysis . A clot can form where it is not needed Thrombosis means an unwanted clot has formed inside a blood vessel. The clot can partly or completely block blood flow. A deep vein thrombosis, or DVT, is a clot in a deep vein, usually in the leg. It can cause pain and swelling in one leg, but those symptoms can have other causes too. If part of a DVT breaks away and travels to the lungs, it can cause a pulmonary embolism. Sudden breathlessness or chest pain can be warning symptoms and need emergency assessment. Blood tests look at different parts of clotting A full blood count can show how many platelets are in the blood, but it cannot tell whether those platelets are working normally. Other tests, including PT and APTT, look at some of the chemical steps the blood uses to make a stable clot. No single test checks the whole process. Clinicians interpret the results alongside the kind of bleeding or clotting problem the person has had and the rest of the clinical picture.

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.