With severe, potentially catastrophic bleeding an adult could easily lose 40% of their blood volume in 3-4 minutes and this could be even quicker when dealing with children.
Tourniquets are tight bands used to control bleeding by completely stopping the blood flow to a wound.
Tourniquets can arrest bleeding quite well and are certainly useful in cases of severe bleeding and should only used when bleeding cannot be stopped any other way,
Tourniquet application should happen in two cases:
If bleeding can't be stopped with direct pressure and elevation when both are applied immediately and simultaneously.
If there's any reason direct pressure can't be maintained, even if it's working.
Tourniquets work only on arm and leg injuries; you can't exactly wrap a tight band around a patient's neck and cinch it down to stop the flow of blood.
Always place on a single bone never a joint or double bone as it has to squeeze the artery or vein closed so a lot of pressure is needed and needs to be mainatined, a bit like squeezing a hose pipe to stop the water coming through.
Applying a tourniquet is not designed to be comfortable and will cause the casualty immense pain but you are trying to cut off their circulation in their effected limb, a bit like turning off a tap, keep turning the tourniquet until the blood stops flowing.
You may need to apply a secondary one to achieve this fully.
Tourniquet - after application
Once a tourniquet has been applied then it needs to remain in place as the clotting process will begin, if the tourniquet is removed the blood comes rushing through and know the clotting process out of the way and allow blood to escape again