Why do dogs sometimes breathe with their mouth open and tongue lolling out?

Breathing is the process by which air is taken in the lungs through the nostrils. But the process of breathing quick short breaths with the mouth open and the tongue lolling out as done by dogs is called panting. This is puzzling process. Many people are unaware about this intricate process so let us describe it with an interesting example.

Assume that a ferocious dog is chasing a rabbit in the countryside. But the dog’s sprint is no match for the rabbit’s dodges and dashes. The dog is unable to catch the rabbit in this race but continues to chase it regardless. If this chasing game is being played in the mid-day heat of summer the chances are that the rabbit might die even before it is caught by that ferocious dog. The exertion of running to save its life would have raised the temperature of the rabbit’s body as well as its brain to a dangerous level. The nerve cells of the brain are very sensitive to temperature. An increase in 4 degrees Celsius above the normal immediately affects brain functions. E.g. when a child has a high temperature it suffers bouts of spasms because the nerves of the muscles do not receive impulses from the brain properly.

The rabbit which has been running in leaps and bounds to throw off its deadly pursuer has to bring down the temperature of its body. To do this it is necessary to hide under the shade now and then. If the dog does not give it a chance to rest awhile, the death of that bugs-bunny is a foregone conclusion! The responsibility for this poor creature’s death would rest solely on Nature itself because it has discriminated against the rabbit by giving the dog a far more efficient cooling system.

The underlying message of this example is worth keeping in mind: It is essential that body temperature of all the mammals should always remain normal. The temperature should never be allowed to rise beyond the limit. Brain can not bear much heat so some animates like human beings keep their body temperature under control by means of perspiration. Evaporation of sweat automatically brings down heat. On the other hand, some animals like dog depend upon panting in addition to sweating to bring their body temperature at the normal level. They get rid of excessive heat by panting with the mouths open.

There is an intricate maze like network of hollow bones in the dog’s nose. It may appear unbelievable but it is a fact that the surface area of the membrane covering the internal surface of this network of hollow bones in the dog’s nose is as much as the surface area of dog itself! This membrane contains numerous blood vessels. Outer surface of the dog’s nose is covered with soft skin. One gland constantly supplies water to this skin. (That explains why a dog’s nose is always wet.) Porous skin continuously evaporates that water so that heated blood flowing through the internal web of the nose becomes cool. Dog also has a gland producing saliva, which increases the quantity of saliva in accordance with the increase in body temperature. This helps dog to release its heat by means of exhalation. As the body temperature rises the dog increases its rate of exhalation correspondingly which appears to us in the form of the dog’s panting. Finally, note that when its temperature is absolutely normal (38.30 degrees Celsius), blood flowing in its artery leading towards the moth and nose flows at an average rate of 700 milliliters per minute. But when its body temperature rises after some hard physical effort the blood flow in that artery increases up to 1200 milliliters per minute. The ultimate objective of this complex process is to keep its brain which is situated near the nose relaxed.

Additional reading:
Dog anatomy (Wikipedia)

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