Science Puzzle
Why Does a Cut Stop Bleeding?
Cut your finger and it bleeds. Yet a small cut usually stops bleeding by itself within a few minutes, and a scab forms. Blood is a liquid, and a liquid would normally keep running out of a hole.
What stops the bleeding?
The Answer
The blood seals the hole itself. Tiny cell fragments called platelets stick together to plug the gap, then a net of protein threads grows through the plug and turns it into a solid clot.
First, the damaged blood vessel tightens, which slows the flow. Next, platelets go to work. A small drop of blood holds hundreds of thousands of them, and they normally slide along smoothly. When they touch the torn edge of a vessel, they turn sticky, change shape and grab onto the wound and onto each other, building a plug.
Then comes a chain of chemical reactions. A protein that's always dissolved in your blood, called fibrinogen, is turned into long, tough threads of fibrin. The threads weave through the platelet plug like a net, trapping red blood cells and making the clot firm. Over the next hours it dries into a scab, while new skin grows underneath.
If one link in that chain is missing, bleeding takes far longer to stop. That's what happens in haemophilia, where one of the clotting proteins doesn't work properly. Queen Victoria carried the gene and passed it into several of Europe's royal families.
The principle: Blood clotting. Platelets plug a damaged blood vessel and a chain of reactions builds a net of fibrin threads through the plug, turning the liquid blood at a wound into a solid clot.