Coming off of the Potomac River last summer after two hours of kayaking on a muggy, 95-degree day, I thought I would collapse. My heart was racing, my skin was on fire and my thirst seemed unquenchable (though I couldn't test that feeling at the time: I was out of drinking water).

I was dehydrated and probably close to suffering heat stroke, even though I had rolled in the river several times to try to cool off.

What went wrong -- besides my not stowing another gallon or two to drink? Herewith, a mid-swelter-season refresher in Body Cooling 101:

"Short dips in cool water and running through water sprays during [exercise] briefly cools the skin and feels good," said Larry Kenney, professor of physiology and kinesiology at Pennsylvania State University. But those splashes "provide minimal whole-body cooling effects."

High air temperatures make your heart work harder than it otherwise would to pump blood to your skin to keep you cool. "When you hear about old people dying during heat waves, it is almost always from a heart attack," Kenney said. "Their hearts are overworked."

High humidity impedes the evaporation of sweat from the body, which also strains your heart.

"More sweat drips from the body than evaporates, and that makes the heart pump harder to transfer blood to the skin," Kenney said. When most of your sweat evaporates, as it does at lower humidity levels, heat is transferred away from the skin. "Without evaporation, you would only be able to exercise for 10 or 15 minutes without overheating."

Dehydration is a danger for exercisers at any temperature, whether on land or in water.

"Swimmers sweat even though they do not perceive it," Kenney notes. "Fluid lost in sweating [during any activity] must be replaced."

Heat exhaustion, characterized by nausea, dizziness, fatigue and headaches, often caused by dehydration. Heat stroke results from such a rapid rise in body temperature that the body stops sweating -- it loses the ability to regulate body temperature -- and that affects the nerves of the brain. "That's why the effects of heat stroke are mostly neurological -- confusion, not knowing where you are," Kenney said.

Does that mean you can't exercise outdoors in high heat and humidity? Not at all.

"You can do almost [any exercise] in moderation, even on the hottest days," said Kenney. (One caveat: Curtail outdoor exercise at any temperature when ozone levels are high -- Code Orange [unhealthy] or Red [very unhealthy] -- especially if you have a respiratory ailment like asthma.) Allow your body to gradually adjust to a heat wave. On the first day, exercise for 15 minutes, then 20 minutes on the second day, then a half-hour on the third day.

"When football players collapse during summer training camp, almost 100 percent of those cases occur on the first or second day of camp," Kenney notes.

Drink more fluids -- before, during and after exercise -- than you would in milder weather. That way, the 103 heat index is less likely to get you. And wear light-fitting, light-colored clothing designed to wick sweat away from the body. Cotton, though comfortable, isn't advised because it holds sweat.

No chat this week, but we still like to hear from you about your own ways of coping with the heat, or any moving experiences you'd like to discuss. E-mail us -- with your thoughts, questions, feedback -- at move@washpost.com.

-- John Briley