Life threatening external bleeding from either a major artery or vein, can quickly cause a loss of a lot of blood. The bleeding is likely to pulsate in time with the heart. This is more pronounced with an arterial bleed, but happens within venous bleeds too. With extremely heavy, potentially catastrophic bleeding an adult could easily lose 40% of their blood volume in 3-4 minutes. This could be even quicker when dealing with children.
If a casualty loses a substantial amount of blood – more than 40% of their blood volume, the body is no longer able to compensate for that amount of blood loss. They will develop shock. Shock can happen when there is a low volume of blood causing failure of the circulatory system, if untreated they will die.
Book a First Aid course and learn how to manage this and other life threatening injuies.