This is what happens to your body as you die of dehydration

Caution: this article may cause thirst

footprints in desert
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. Moyan Brenn via Flickr

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

dehydration in the body
Stage 1: Thirst Michael Brandon Myers

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

dehydration in the body
Stage 2: Fainting Michael Brandon Myers

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

dehydration in the body
Stage 3: Organ Damage Michael Brandon Myers

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

dehydration in the body
Stage 4: Death Michael Brandon Myers

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.”

Claire Maldarelli
Claire Maldarelli

is the Science Editor at Popular Science. She has a particular interest in brain science, the microbiome, and human physiology. In addition to Popular Science, her work has appeared in The New York Times, Scientific American, and Scholastic’s Science World and Super Science magazines, among others. She has a bachelor’s degree in neurobiology from the University of California, Davis and a master’s in science journalism from New York University's Science, Health, and Environmental Reporting Program. Contact the author here.