The human body contains approximately 5 litres of blood. A healthy adult can lose up to half a litre of blood without harmful effects, but the loss of more than this can be threatening to life.

Haemorrhage from major blood vessels of the arms, neck, and thighs may occur so rapidly and extensively that death occurs in a few minutes. Haemorrhage must be controlled immediately to prevent excessive loss of blood.

Bleeding may occur externally following an injury to the outside of the body, or internally from an injury in which blood escapes into tissue spaces or the body cavity.

The signs and symptoms of excessive loss of blood are: weakness or fainting; dizziness; pale, moist, and clammy skin; nausea; thirst; fast, weak, and irregular pulse; shortness of breath; dilated pupils; ringing in the ears; restlessness; and apprehension.

The simplest and preferred method of controlling severe bleeding is to place a dressing over the wound and apply pressure directly to the bleeding site with the palm of the hand.

When there is a severely bleeding wound of an extremity or the head, direct pressure should be applied on a dressing over the wound with the affected part elevated.

When direct pressure and elevation cannot control severe bleeding, pressure should be applied to the artery that supplies the area.

A tourniquet should be applied to control bleeding only when all other means have failed.