I do not understand what is the purpose for asking for a code for such a broad category of system
The bronchial tree is an essential part of the respiratory system. It consists of several interacting structures, such as the bronchi,bronchioles, and alveoli. The bronchial tree provides a system for the trachea to service the lungs. It is important to note, however, that like the trachea, the bronchial tree does not include the lungs. The bronchial tree begins with the primary bronchi and ends with the alveoli.
Only the events /conditions/diseases occurring there can be coded individually.
By and large the bronchial secretions can be exudative or purulent.
With your info, it could go for 466.0 for bronchial exudates, and 466.1x for bronchioles exudates( finer subdivisions of the Bronchial tree)