Until you rehydrate, your body temperature rises slightly and blood thickness and heart rate go up slightly, he says. At the same time, your kidneys conserve water by not producing as much urine. It never cools, and people are chronically bothered by the heat. He adds that babies are especially vulnerable because the control mechanisms aren't well-developed. What happens when people get extremely dehydrated from heat isn't well-documented because scientists can't ethically overheat people, Dantzler says.
People become disoriented and lose the ability to salivate.
At this point, victims' temperatures are 104 and beyond. They can't swallow, and they have to be given fluids intravenously to survive. What happens is, you don't get adequate circulation to the body surface, and body temperature rises explosively. The body's first defense against the heat is sweat. All that sweat has to be replaced, and the best thing to replace it with is water, he says.