According to the American Society of Hematology, a healthy person can lose approximately 10 to 15 percent of the body's total blood volume without experiencing serious medical problems. Blood volume in an individual depends on gender, size, age and other factors; however, generally, an average-size woman has approximately 9 pints of blood in her body, while an average-size man has about 12 pints.
According to the American Red Cross, roughly 1 pint is taken during a blood donation, or approximately 8 to 10 percent of a person's blood volume. The organization states that one blood donation can potentially save the lives of three people.
LifeSource notes that car-accident victims who suffer major blood loss can require blood transfusions of 50 pints or more. Bleeding, technically called hemorrhaging, is broken into four classes of severity by the American College of Surgeons. Class I hemorrhaging involves less than a 15 percent loss of total blood volume and typically results in no change in vital signs. Class II hemorrhaging involves a 15 to 30 percent loss of total blood volume and usually affects the victim's heart rate and blood pressure. Class III hemorrhaging is a loss of 30 to 40 percent of blood volume and usually sends the victim into shock. Class IV hemorrhaging involves a blood loss of greater than 40 percent, often requiring resuscitation to prevent death.