The human body has around ten pints of blood in it—or “units,” as doctors prefer. Women tend to have less blood than men and children have less blood than adults, but in all cases, a healthy person can lose around 15 percent of their blood without much effect. (Women commonly lose that much in childbirth.) At around 30 percent blood loss, though—three to four units—the body starts to go into compensatory shock to protect its vital organs. The heart rate increases to make up for low blood pressure, breathing gets faster and shallower, and capillaries and small blood vessels constrict to keep
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