To understand what is happening during normal breathing, let’s consider the pressure changes occurring in the respiratory tract and the effects that they have on the system.As the animal breathes in the diaphragm moves caudally and the chest wall moves out a little. This expands the chest cavity and causes a drop in pressure in the pleural space. This then pulls the lung open. As the lung is expanded, the airways inside the lung are also pulled open and the pressure inside them drops. This low pressure in the intrathoracic airways then sucks air down the trachea and generates a lower pressure inside the upper airway in the neck and throat. But the pressure around the extrathoracic airway is atmospheric pressure (which is higher than the pressure inside) so this means that the upper airway outside the chest will collapse on inspiration. That is why animals have a muscle to keep their vocal folds open when they are inspiring (called cricoarytenoidalis dorsalis). If that muscle is not working (as in laryngeal paralysis) the vocal folds get pulled together on inspiration and cause an upper airway obstruction.If a dog has a collapsing trachea affecting the intrathoracic portion, then inspiration is fine cos the floppy dorsal membrane gets pulled open. But when he tries to breathe out, it all snaps shut and you get the classic “goose honk” cough. If the extrathoracic trachea is affected then the dog will have signs of an upper airway obstruction on inspiration. If both are affected then the dog will have trouble on both inspiration and expiration.