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.
Stage 1: Thirst
Water Lost: Two percent of body weight. For a 170-pound person, that’s 3 pounds. You might lose this much sweat by kickboxing for an hour in a hot room without a drink.
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.
Stage 2: Fainting
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.
Stage 3: Organ Damage
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.
Stage 4: Death
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.
Research help for this story came from Stavros Kavouras, the director of the Hydration Science Lab at the University of Arkansas.
This article was originally published in the March/April 2017 issue of Popular Science, under the title “The Thirsty Body.”