The extent of bleeding from a wound will depend on the nature of the injury. A graze, even if it quite large, is shallow and the bleeding is only from tiny blood vessels called capillaries. These can contract and reduce blood loss, and clotting can occur fairly quickly. A deeper wound may cut a vein, in which case there is a more significant, but steady flow of blood. If a wound cuts an artery, then blood will spurt out under pressure every time the heart beats. Blood loss is large and rapid and potentially life-threatening.
