How does blood clot after an injury?
When you cut a finger, the bleeding stops by itself within minutes. This animation follows what happens under the skin: the injured blood vessel narrows, platelets stick together to form a soft plug, a chain reaction of clotting factors produces thrombin, and threads of fibrin knit the plug into a firm clot. Once the vessel has healed, the clot is quietly dissolved.
What you will learn
- Why an injured blood vessel narrows straight away
- How platelets are activated and clump together to form a plug (primary haemostasis)
- How the clotting factors amplify one another to generate thrombin (secondary haemostasis)
- How thrombin turns fibrinogen into fibrin, which binds the plug into a stable clot
- That the clot is broken down again as the vessel repairs itself (fibrinolysis)
Read the transcript
Nick your finger and it bleeds. But within a few minutes, it stops all by itself. So who plugs the leak?
Under the skin, a tiny blood vessel has been torn open. First, the vessel squeezes tight to slow the leak.
Riding in your blood are platelets, tiny, sticky cell fragments. When they touch the torn wall, they switch on. They sprout spiky arms, grab each other, and call for backup. Within seconds, they pile up into a soft plug.
But that plug is flimsy, so a chain reaction starts. Clotting factors switch each other on, like dominoes. Each one tips a bigger one, until there's a flood of thrombin. Thrombin snips a dissolved protein, fibrinogen, into sticky threads called fibrin.
The threads weave a net over the plug, trapping red cells. That tangled net is the clot. On your skin, it dries into a scab. Underneath, the vessel slowly mends and the clot is quietly dissolved.
So when a cut stops bleeding, thank a crowd of sticky platelets and a net spun in seconds.
Animation by Raghav C (i-animate.com). Content reviewed by Dr Deepak Chandra, Consultant Haematologist.