Blood loss normally begins to pose a serious medical threat when an individual loses 30 to 40 percent, as part of what the American College of Surgeons considers a Class 3 hemorrhage. At this point, a blood transfusion is often necessary, according to Medical Daily.
An individual usually begins to feel symptoms when they experience Class 2 hemorrhaging and lose between 15 to 30 percent of their total blood volume, explains Medical Daily. However, some individuals begin to feel faint when they experience 8 to 10 percent blood loss during Class 1 hemorrhaging. Once an individual reaches Class 4 hemorrhaging, with more than 40 percent blood loss, they are at risk of organ failure and slipping into a coma due to the strain on their body's circulatory systems.