Q. How do dogs pant so fast without hyperventilating?

A. Panting dogs take shallower breaths.

When people (and dogs) breathe too fast, they can lower the level of carbon dioxide in the blood to the point that they feel lightheaded or even faint, according to Gordon Mitchell, a professor who teaches respiratory physiology at the UW-Madison School of Veterinary Medicine.

Without the ability to sweat like humans, dogs need to pant to increase the amount of moisture evaporating from their bodies. That evaporation carries away some of their body heat.

But dogs are built to pant just right. The mechanics of their lungs and chest set a precise rate for panting that minimizes the amount of work while maximizing cooling power.

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"It's an all-or-nothing switch," Mitchell said. "They pant at this perfect rate until they're cool. And then when they're hot again, they start panting again at that same rate."

Panting differs from normal breathing in another key way: Panting dogs aren't completely filling their lungs.

"They lower the depth of each breath, so less of the air they are breathing is moving in and out of the deep part of their lungs where most of that gas exchange takes place," Mitchell said.

The more frequent breaths still move enough moisture for internal air conditioning. But because less oxygen in each breath is exchanged for carbon dioxide, gas ratios don't get out of whack and dogs (unlike people) avoid feeling faint.

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