So to begin, let’s review the 5 levels of the respiratory tract. They are the upper airway (from nose to main stem bronchi), the small airways, the pulmonary parenchyma, the pleural space, and the chest wall and diaphragm. It is vitally important to work out which level is affected because the stabilisation, diagnostic approach, differential diagnoses, treatment and prognosis are so different for each group. It is actually possible to differentiate them largely on physical examination with a bit of practice.By the way, in my book, the upper airway extends all the way to the carina because there is no difference to the animal between an obstruction in the larynx and an obstruction of the distal trachea.