The major organs of the respiratory system are the nose, mouth, pharynx, esophagus, trachea, chestwall and the lungs. Dogs, use their lungs, not only to capture introduce oxygen to the blood and to exhaust carbon dioxide to the environment, but also for cooling. In hot, humid, confined environments, older dogs are at greater risk to overheating.

The nose--dogs breathe in through their noses and out through their mouths. The nose secretes large volumes of water used in evaporative cooling. By comparison with the evaporative cooling done by the nose and the lungs, the evaporation from a dripping tongue is relatively minimal. Short-nosed (brachycephalic) dogs such as Pugs, are do not have the nasal anatomy for efficient cooling, and are therefore susceptible to overheating, especially in old age when cardiac and pulmonary capacity are less.

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