The body has natural mechanisms that act to control bleeding.
If a blood vessel is severed, the end of the vessel may contract and decrease the size of the opening through which blood can escape the vessel.
This contraction is temporary and full bleeding will occur when the vessel relaxes. Contraction can often lead to a false impression of the severity of the wound. During care under fire, aggressive use of tourniquets is needed to ensure that, as the body starts to relax, the patient does not bleed out.
The body’s primary defense against blood loss is clotting.
When a blood vessel is damaged or cut, platelets (thrombocytes) in the blood attach themselves to the damaged part of the blood vessel and begin plugging the opening.
Fibrinogen in the blood changes to fibrin and reinforces the platelets. An insoluble clot forms that plugs the torn or cut blood vessel until the vessel is repaired. If the blood vessel is large and the damage is severe, a clot may not form in time to stop the bleeding.
The body may also react to damage by immobilizing the injured part.
The casualty with an injured leg may fall or lie down and remain still. The muscles become more rigid to decrease pain and the casualty will tend to avoid moving the injured body part. This natural splinting reaction helps to reduce blood loss by restricting activity of the body part. The more active the body part, the greater the blood flow in the injured part.