Bleeding

Bleeding, technically known as haemorrhaging (Brit.) or hemorrhaging (US) is the loss of blood or blood escape from the circulatory system. 
Bleeding can occur internally, where blood leaks from blood vessels inside the body, or externally, either through a natural opening such as the vagina, mouth, nose, ear or anus, or through a break in the skin.
Desanguination is a major blood loss. Exsanguination is losing enough blood to cause death. A person does not have to lose all of their blood to die. People can die from losing half to two-thirds of their blood.
Typically, a healthy person can endure a loss of 10–15% of the total blood volume without serious medical difficulties, and blood donation typically takes 8–10% of the donor's blood volume.
The World Health Organization (WHO) established a scale to measure different degrees of blood loss, from level 0 to 4. Level 0 indicates no blood loss, whilst level 4 represents the highest level of hemorrhaging, with a risk of fatality.