Blood. The very word can prompt syncope (fainting) in hemophobics, or those affected with an irrational fear of the sight of the stuff. Then again, it might be perfectly rational: Blood carries nutrients in our bodies, supports us with oxygen, and protects against infection. Keeping it inside our veins and arteries is ideal, and losing it would understandably provoke some anxiety.
So how much blood does an average adult carry around? And how much can they afford to lose?
For a helpful visual on the former, try heading to the dairy aisle of the grocery store and picking up a gallon of milk. That’s roughly how much blood you’re harboring at any given moment.
Specifically, it’s more like 1.2 to 1.5 gallons, according to Dr. Daniel Landau, a hematologist and oncologist who spoke with LiveScience in 2016. For some adults, that equates to about eight to 10 percent of their body weight; that total blood volume remains stable from roughly the age of 6 on. Babies carry far less blood—about a cup, or the same as your average cat.
Lose some of that and your body begins to notice the absence. That 1.2 to 1.5 gallons is equivalent to 4.5 to 5.5 liters: Blood donors typically give about one pint, or a half-liter, at a time without any serious effects. But if an injury or other calamity prompts you to lose three to four pints, you’re at a Class 3 hemorrhage and a blood transfusion is in order. More than that and your heart can’t maintain blood pressure. (The reason we turn pale when losing blood is because the body is attempting to use vasoconstriction to divert what’s left to critical organs.)
A 200-pound individual will carry far more blood than a 100-pound person and could conceivably lose more of it without coming closer to death. However, researchers have cautioned that blood volume is not totally related to body weight and can also be influenced by body composition. If a person has more fat than lean tissue, or vice versa, it will affect the vascular system.
Using an online blood volume calculator, it’s possible that André the Giant, who weighed about 525 pounds, toted around 21 liters of blood—or four times the amount a normal person would have, not accounting for varying body fat percentages. That's impressive, but André was not among the largest of mammals, no matter what his promoters may have claimed. Consider instead that a blue whale’s 400-pound heart pumps 220 liters through its massive frame, or roughly 58 gallons.
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