Final answer:
The combining forms for the respiratory system are "pneumo-" and "pulmon-", indicating involvement with the lungs.
Step-by-step explanation:
The combining form that applies to a structure within the respiratory system is "pneumo-" or "pulmon-". For example, "pneumonia" involves the lungs, and "pulmonary" pertains to the lung area. Understanding these prefixes can be helpful in identifying terms related to the respiratory system.