A dog breathes only through the nose. This is the misconception of a lot of dog owners. It is also believed that dogs cant breathe through the mouth as the mouth is only used by the dog to pant and not to breathe. Panting though is a form of heavy breathing. Dogs have no sweat glands and when they are hot or when their heart rate is elevated they will disperse the body heat by hanging out their tongue to pant. Yes, dogs breathe through their noses but not all the time. Like humans dogs breathe both through the nose and the mouth. Have you noticed how your throat gets dry when you breathe through your mouth because your nose is stuffed or blocked because of a cold? Pretty much the same thing happens to a dog. To prove that dogs also breathe through the mouth try holding out your hand on the dogs mouth and you will feel the dogs warm breath. Although in most cases breathing is done through the nose breathing can be done using both the mouth and the nose at the same time. This breathing technique that is known as circular breathing or cyclical breathing is a learned thing that is most commonly used by wind instrument players. A dog that has been doing a strenuous exercise or one that is panting is breathing in this manner.
The nasal cavity is composed of mucus membranes that serve as filters that stop dirt and germs from getting into the lungs. The same is true in canines although canine noses are far more sensitive than humans. Dogs have heightened sense of smell because their olfactory bulbs are at least four times bigger than what we humans have. A dog breathes through the nose as by doing so they can sniff not only interesting smells but they can also track and scent a prey. Because of this ultrasensitive sense of smell, dogs are predisposed to breathing through their noses when sleeping. Seldom would you see sleeping dog breathing through the mouth. By breathing through the nose a sleeping dog can detect the detect the presence of predators. Canine noses are different from human noses in design. Notice the slits on the sides of a dogs nose. This is where exhaled air is released so that it would not taint or pollute the air being inhaled into the nostrils. The nostrils are opened and widened when the dog is tracking or trying to sniff a scent.
A dogs nose is very sensitive. Breeds with long noses are known to breathe easier than short nosed breeds. The Pug, boxer, Shih Tzu and Boston Terrier are only some of the brachycephalic breeds that are noted to have breathing problems. As mentioned a dog would breathe through the nose and through the mouth. But when you noticed the dog breathing only through its mouth there may be some nasal problems. Dogs have the propensity to sniff and it is possible that the nasal area is blocked by a foreign object. The dog may also have nasal tumor. The dog may have stenotic nares. These are obstructed or abnormally narrow and constricted nostrils that makes breathing through the nose difficult. A dog with this type of disorder would breathe through the mouth as breathing through the nose would naturally be unproductive. This disorder can be detected easily as the dog would make wheezing sounds when it breathes through the nose. This disorder can be easily corrected with surgery.