Why a limb suffers when its blood supply stops
A tourniquet works by squeezing the limb hard enough to close off blood vessels, including the arteries that deliver oxygen-rich blood to tissues. That can prevent a life-threatening loss of blood, but the muscles, nerves, and skin below the device are left without their normal supply of oxygen and nutrients. As that deprivation continues, cells become injured; some damage may not be reversible.
The roughly two-hour figure is therefore a rough warning about prolonged restriction, not a countdown to a reliably safe release time. The actual risk depends on factors such as how much tissue is affected and the conditions around the injury. Serious harm can develop sooner, and a limb is not guaranteed to be unharmed just because less time has passed.
Why rescuers do not simply loosen it
Releasing a tourniquet can allow severe bleeding to start again. It may also send substances released by injured tissue back into circulation. In an emergency, weighing those risks requires medical judgment; loosening the device to check the limb can turn controlled bleeding into an immediate crisis.
Once a tourniquet is in place for major bleeding, keep it visible and note when it was applied if you can. Do not loosen or remove it yourself. Emergency medical professionals can manage the bleeding, assess the limb, and decide what treatment is needed. The practical tradeoff is stark: restricting circulation can threaten tissue, but uncontrolled blood loss can threaten life within minutes.
