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Dogs Panting
Name: Nicole S.
Status: Student
Age: 20s
Location: N/A
Country: USA
Date: 2001-2002
Question:
How come dogs do not hyperventilate when they pant?
Replies:
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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Update: June 2012
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