Shortly after birth, once the first breaths are taken, a puppy has a fully functional respiratory system. The respiratory system is basically composed of the nares (nostrils), nasal cavity, sinuses within the skull, pharynx (back of the mouth), larynx (voice box), trachea (windpipe), bronchi (the branches of the trachea going into the lungs), and the lungs. Dogs have right and left lungs, just like humans. Both sides of the lungs are further divided into sections, or lobes. Inside the lungs, the bronchi divide into smaller and smaller tubes, called 'bronchioles,' much like branches of a tree divide into smaller and smaller branches. At a microscopic level, the bronchioles end in small structures called 'alveoli'. It is here that the blood makes contact with the individual cells in the lungs and oxygen is exchanged for carbon dioxide. Alveoli are supplied by a vast network of microscopic blood vessels known as capillaries.
The normal breathing process
As a dog inhales, fresh air moves through the nose (or mouth), pharynx, and larynx to the trachea. The trachea carries the air to the bronchi, which in turn supply the lungs. Air exchange occurs in the alveoli and the used air follows the opposite path of new air: passing into the bronchi, into the trachea, through the larynx and pharynx, finally exiting through the nose or mouth.
Breathing is relatively simple and is accomplished by the actions of the rib muscles (intercostals) and the movement of a great internal muscle called the diaphragm. The diaphragm muscle separates the chest, containing the heart and lungs, from the abdomen which holds the intestines, stomach, liver, bladder, etc. As this great muscle moves toward the abdomen, it creates a negative pressure and pulls fresh air and oxygen into the lungs, causing the dog to breathe in (inhale). The chest cavity surrounding the lungs is a vacuum, thus allowing the lungs to inflate easily when the dog inhales. When the muscle moves forward (towards the animal's head), it causes the lungs to compress and force air out (exhale), thus ridding the body of used air.
Functions of the respiratory system
The dog's respiratory system serves two purposes. First, it is the exchange mechanism by which the body's carbon dioxide is replaced with oxygen. It is also a unique cooling system. Since dogs do not have sweat glands (except on their feet), they cannot perspire to lower their body's temperature like humans do. To cool their body they must breathe harder (pant). By breathing faster, warm air is exchanged from the body for the cooler outside air. Additionally, moisture within the respiratory system evaporates, further cooling these surfaces. Therefore, the lungs function both to exchange carbon dioxide for oxygen and to cool the body.