Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance

by Alex Hutchinson

A 150-pound person typically carries around about forty liters of water, and that total is fixed to within less than a liter (one exception is the fluctuations that occur throughout a woman’s menstrual cycle, which can add and then subtract more than two liters of retained water). When you fail to replace lost fluids, you start craving a drink, and your kidneys begin reabsorbing fluid that would otherwise become urine. If that’s not enough to restore your internal balance, fluid will start draining out of your cells and into your veins and arteries to maintain the necessary volume of blood pumping through your body. These adjustments will buy you some time, but eventually your blood will get so concentrated that your brain will start shrinking as fluid is sucked out by osmosis, tearing delicate cerebral veins and ultimately killing you. According to calculations by U.S. Army researchers in a wilderness medicine textbook, you might last, in theory, for about seven days without water under ideal indoor conditions before reaching this critical point. If you’re lost in a hot desert and travel only by night, your expected survival plummets to twenty-three hours; if you also travel during the day, it’s sixteen hours.