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HOW COME? Why we feel out of breath when exercising

How come we can feel out of breath if we exercise harder or run faster than usual? asks a reader.

Feeling "out of breath" can seem puzzling. After all, we're not running on Mars, where oxygen makes up less than 1 percent of the Red Planet's vanishingly thin air. On Earth, one of every five gas molecules whizzing through our (much denser) air is oxygen. And the faster we move, the more oxygen-rich air we take in.

Slumped on the couch watching TV, you might breathe in and out every five seconds. With each (shallow) breath, your body might process two cups of air. Turn off the TV and go out for a leisurely bike ride, and your breathing rate increases to about 35 to 40 times a minute. Every 1.5 seconds, each one of your (deeper) breaths will pull in about eight cups of air. Result: In 10 minutes as a couch potato, you might have used 240 cups of air. But in those 10 minutes of bike-riding, your air intake soared, to more than 3,000 cups.

How come? On a bike ride, muscles (especially leg muscles) demand extra O2. The heart, a muscle too, also requires more oxygen, as heart rate increases to meet the demand.

Switch to running full tilt, and even the 8-cup breaths you took while riding your bike or walking briskly may not supply oxygen fast enough. If your leg muscles are accustomed to, say, slow jogging, they won't be as good at extracting oxygen from blood when you speed up. And if your heart isn't already well-conditioned from very vigorous exercise, it can't pump as efficiently. Your lungs, meanwhile, must pull in more oxygen and push out more carbon dioxide. So as your body's oxygen requirements spike, you may find yourself out of breath.

(If you don't have a breathing problem such as asthma, or a heart condition, you may also have approached your maximum heart rate. With each beat your heart muscle contracts to push out blood, then expands to fill up again. There's a limit to how fast the heart can beat, since there must be enough time for it to refill with blood between contractions. Since maximum heart rate drops as we get older, this formula provides an estimate: 220 minus your age.)

The good news: Regular vigorous exercise gradually conditions your heart, which can pump extra blood with each beat. Lung function strengthens, and oxygen sent to working muscles with each breath increases. Leg muscles get more efficient at burning fuel, and better at pumping blood back to the heart. So you can run faster, with the same breaths per minute.

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