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Name: Nicole S.
Status: Student
Age: 20s
Location: N/A
Country: USA
Date: 2001-2002

How come dogs do not hyperventilate when they pant?

This is a very interesting questions. I got help on it from many different vets across the country, and we're still not sure we have an answer. Here's what we have so far.....

Panting does not involve full alveolar inflation and O2/CO2 exchange. Most of the air movement in panting is in the larger bronchioles, bronchi and trachea and this is called their "dead space" , where there is no air exchange. Dogs breathe this column of dead space air up and down when they pant.

From a comparative physiology text, "Animal Physiology, Adaptation and Environment" by Knut Schmidt-Nielsen, 1980.

"We know from our own experience that humans sweat to increase cooling by evaporation. Dogs, in contrast, have few sweat glands, and they cool primarily by panting - a very rapid, shallow breathing that increases evaporation from the upper respiratory tract. Some animals use a third method for increasing evaporation: They spread saliva over their fur and lick their limbs, thus achieving cooling by evaporation.... Panting has two obvious disadvantages. One is that increased ventilation easily causes an excess loss of carbon dioxide from the lungs, which can result in severe alkalosis; the other is that increased ventilation requires muscular work, which in turn increases the heat production and thus adds to the heat load. The tendency to develop alkalosis can in part be counteracted by shifting to a more shallow respiration (smaller tidal volume) at an increased frequency, so that the increased ventilation takes place mostly in the dead space of the upper respiratory tract. Nevertheless, heavily panting animals regularly become severely alkalotic, and thus they do not fully utilize the possibility of restricting the ventilation to the dead space.... The increased work of breathing during panting would be a considerable disadvantage were it not for the interesting fact that the muscular work, and thus heat production, can be greatly reduced by taking advantage of the elastic properties of the respiratory system. When a dog begins to pant, its respiration tends to shift rather suddenly from a frequency of 30 to 40 respiration's per minute to a relatively constant high level of about 300 to 400. A dog subjected to a moderate heat load does not pant at intermediate frequencies; instead, it pants for brief periods at the high frequency, alternating with periods of normal slow respiration. The meaning of this becomes clear when we realize that the entire respiratory system is elastic and has a natural frequency of oscillation, like other elastic bodies. That is, on inhalation, much of the muscular work goes into stretching elastic elements, which on exhalation bounce back again, like a tennis ball bouncing. To keep the respiratory system oscillating at its natural frequency (the natural resonant frequency) requires only a small muscular effort. As a consequence, the heat production of the respiratory muscles is small, adding only a little to the heat load...

It has been estimated that if the panting were to take place without the benefit of a resonant elastic system, the increased muscular effort of breathing at the high frequency of panting would generate more heat than the total heat that can be dissipated by panting...."

Phillip Raclyn, DVM CVA

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