Wandering around the desert is a good way to get dangerously dehydrated, but severe dehydration can also occur in more moderate conditions if you're exercising a lot, or just not drinking enough water.
Water makes up about 55 to 65 percent of your body. It’s a crucial ingredient in the chemistry that helps your brain think, your blood flow, and your muscles move. But what happens after you sweat through a spin class, spend a day at the beach, or simply ignore your thirst? Dehydration is different for everybody—it depends on how much you’re exercising, the temperature around you, and how much you typically sweat—but it can get dangerous quick.
Water Lost: Your urine is more than 90 percent water (and your poop is about three-quarters). Each time you flush, the sewer system ferries your waste—along with 1.6 to 7 gallons of perfectly good H2O used to swirl it down your porcelain throne—to a treatment plant designed to make it taste delicious.
Effects: When thirst kicks in, your body clings to all remaining moisture. Your kidneys send less water to your bladder, darkening your urine. As you sweat less, your body temperature rises. Your blood becomes thicker and sluggish. To maintain oxygen levels, your heart rate increases.
Water Lost: Four percent of body weight. For a 170-pound person, that’s 7 pounds. This is roughly equivalent to riding a bike for three hours in extreme heat without rehydrating, or going without water for two days.
Effects: Your blood is so concentrated that the resulting decrease in blood flow makes your skin shrivel. Your blood pressure drops, making you prone to fainting. You’ve basically stopped sweating, and without this coolant, you start to overheat.
Water Lost: Seven percent of body weight. For a 170-pound person, that’s 12 pounds. You might lose this much sweat doing hot yoga for eight hours without rehydrating.
Effects: Your body is having trouble maintaining blood pressure. To survive, it slows blood flow to nonvital organs, such as your kidneys and gut, causing damage. Without your kidneys filtering your blood, cellular waste quickly builds up. You’re literally dying for a glass of water.
Water Lost: Ten percent of body weight. For a 170-pound person, that’s 17 pounds. This is like going for five days, or running for 11 hours in 90-degree weather, without rehydrating.
Effects: You need to drink some water—stat! If it’s hot out, your uncontrollable body temperature means your vital organs risk overheating; liver failure will probably kill you. But if conditions are mild, toxic sludge builds up in your blood, and your coroner’s report will more likely read: kidney failure.
This article was originally published in the March/April 2017 issue of Popular Science, under the title “The Thirsty Body.”