How Your Body Stops Bleeding

A cut ought to be a serious problem: a hole in a pressurised, constantly moving fluid system. That it usually is not comes down to a fast, two stage sealing process that starts within seconds of injury and finishes with a stable, self limiting seal.

A cut should be a bigger problem than it usually is Blood is a pressurised, constantly moving fluid, pushed around your body by the heart roughly once a minute. A break in a blood vessel ought to be a serious, ongoing leak. That it usually is not, for anything short of a major injury, comes down to a fast and remarkably well controlled sealing process called clotting. Stage one: the platelet plug Platelets are small cell fragments circulating in your blood. The moment a vessel wall is damaged, platelets nearby detect the change and become sticky, clumping together at the site of injury within seconds. This forms a loose plug, enough to slow the bleeding quickly, but not yet strong enough to hold indefinitely. Stage two: the fibrin mesh A loose clump of platelets would eventually be washed away by the very blood flow it is trying to stop. To make the seal permanent, dissolved clotting proteins in the plasma are activated in a rapid chain reaction, ending with one called fibrin. Fibrin strands weave through and around the platelet plug, forming a mesh that traps blood cells. This reinforces the whole structure into a stable clot, which you see and feel on the skin's surface as a scab. Why clotting has to stop as well as start A clotting system that only switched on would eventually block healthy vessels throughout the body. Alongside the proteins that build a clot, the blood also contains proteins that break one down once it is no longer needed. Separate signals also keep the clotting process contained to the site of injury, rather than spreading unchecked through the bloodstream. Clotting, in other words, is not simply an on switch. It is a tightly balanced system that starts, stops, and later dismantles itself once healing is underway. When the balance shifts either way Sometimes the system does not seal well enough, producing easy bruising or bleeding that takes longer than expected to stop. Sometimes it activates too readily, or in the wrong place, producing an unwanted clot inside a vessel that was never actually injured. This process, called thrombosis, can block blood flow to whatever that vessel supplies. Both directions matter, and doctors assess for either depending on what a person describes. What a doctor checks when this system is affected Looking. Unusually easy bruising, or small red or purple spots that do not fade under pressure, can suggest the clotting process is not sealing properly. Special tests. A blood test measuring clotting time shows how quickly the platelet and fibrin process forms a stable seal. Blood tests can also measure clotting proteins and platelet numbers directly when a bleeding or clotting disorder is suspected. When this system is affected When clotting does not seal properly, people may notice easy bruising or prolonged bleeding from small cuts. When clotting activates somewhere it should not, people may notice swelling, pain or colour change in a limb, most often a sign a clot has formed within a vein. Unexplained bruising and limb swelling are each explored further in future lessons.

Clotting is a two stage, self limiting process, not a simple on/off switch: platelets form a fast, temporary plug, fibrin turns it into a stable seal, and separate proteins later break the clot down once healing is complete.