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How Much Blood Is in the Human Body?

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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.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
Can You Really Go Blind Staring at a Solar Eclipse?
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A total solar eclipse will cut a path of totality across the United States on August 21, and eclipse mania is gripping the country. Should the wide-eyed and unprotected hazard a peek at this rare phenomenon?

NASA doesn't advise it. The truth is, a quick glance at a solar eclipse won't leave you blind. But you're not doing your peepers any favors. As NASA explains, even when 99 percent of the sun's surface is covered, the 1 percent that sneaks out around the edges is enough to damage the rod and cone cells in your retinas. As this light and radiation flood into the eye, the retina becomes trapped in a sort of solar cooker that scorches its tissue. And because your retinas don't have any pain receptors, your eyes have no way of warning you to stop.

The good news for astronomy enthusiasts is that there are ways to safely view a solar eclipse. A pair of NASA-approved eclipse glasses will block the retina-frying rays, but sunglasses or any other kind of smoked lenses cannot. (The editors at MrEclipse.com, an eclipse watchers' fan site, put shades in the "eye suicide" category.) NASA also suggests watching the eclipse indirectly through a pinhole projector, or through binoculars or a telescope fitted with special solar filters.

While it's safe to take a quick, unfiltered peek at the sun in the brief totality of a total solar eclipse, doing so during the partial phases—when the Moon is not completely covering the Sun—is much riskier.

WOULDN'T IT BE EASIER TO JUST TELL YOUR KIDS THEY WILL GO BLIND?

NASA's website tackled this question. Their short answer: that could ruin their lives.

"A student who heeds warnings from teachers and other authorities not to view the eclipse because of the danger to vision, and learns later that other students did see it safely, may feel cheated out of the experience. Having now learned that the authority figure was wrong on one occasion, how is this student going to react when other health-related advice about drugs, alcohol, AIDS, or smoking is given[?]"

This story was originally published in 2012.

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If Beer and Bread Use Almost the Exact Same Ingredients, Why Isn't Bread Alcoholic?
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If beer and bread use almost the exact same ingredients (minus hops) why isn't bread alcoholic?

Josh Velson:

All yeast breads contain some amount of alcohol. Have you ever smelled a rising loaf of bread or, better yet, smelled the air underneath dough that has been covered while rising? It smells really boozy. And that sweet smell that fresh-baked bread has under the yeast and nutty Maillard reaction notes? Alcohol.

However, during the baking process, most of the alcohol in the dough evaporates into the atmosphere. This is basically the same thing that happens to much of the water in the dough as well. And it’s long been known that bread contains residual alcohol—up to 1.9 percent of it. In the 1920s, the American Chemical Society even had a set of experimenters report on it.

Anecdotally, I’ve also accidentally made really boozy bread by letting a white bread dough rise for too long. The end result was that not enough of the alcohol boiled off, and the darned thing tasted like alcohol. You can also taste alcohol in the doughy bits of underbaked white bread, which I categorically do not recommend you try making.

Putting on my industrial biochemistry hat here, many [people] claim that alcohol is only the product of a “starvation process” on yeast once they run out of oxygen. That’s wrong.

The most common brewers and bread yeasts, of the Saccharomyces genus (and some of the Brettanomyces genus, also used to produce beer), will produce alcohol in both a beer wort
and in bread dough immediately, regardless of aeration. This is actually a surprising result, as it runs counter to what is most efficient for the cell (and, incidentally, the simplistic version of yeast biology that is often taught to home brewers). The expectation would be that the cell would perform aerobic respiration (full conversion of sugar and oxygen to carbon dioxide and water) until oxygen runs out, and only then revert to alcoholic fermentation, which runs without oxygen but produces less energy.

Instead, if a Saccharomyces yeast finds itself in a high-sugar environment, regardless of the presence of air it will start producing ethanol, shunting sugar into the anaerobic respiration pathway while still running the aerobic process in parallel. This phenomenon is known as the Crabtree effect, and is speculated to be an adaptation to suppress competing organisms
in the high-sugar environment because ethanol has antiseptic properties that yeasts are tolerant to but competitors are not. It’s a quirk of Saccharomyces biology that you basically only learn about if you spent a long time doing way too much yeast cell culture … like me.

This post originally appeared on Quora. Click here to view.

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