Are you awash with fluid, or close to dehydration? It's an important question for people with conditions like kidney failure, and now it can be answered with a whiff of your breath. Doctors normally have to take a blood sample to get an accurate measure how much water is sloshing around someone's body. The patient first drinks a measured amount of heavy water, in which deuterium atoms replace normal hydrogen. The heavy water spreads evenly throughout the body's fluids, so doctors can calculate how much fluid there is by analysing the concentration of deuterium in a blood sample. Doctors monitoring dialysis patients can do this to keep track of fluid build-up.
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