The present invention is generally directed to a respiratory delivery system (e.g. an inhaler) used to discharge an appropriate substance therefrom for inhalation by a user. The respiratory delivery system generally has at least one airflow inlet, at least one outlet (e.g., mouthpiece, nasal mask), at least one airflow passage extending therebetween, and at least one ejection actuator for discharging an appropriate substance into the airflow. The respiratory delivery system also generally includes an airflow regulation assembly that can adjust the size of a passage through which substantially all of the airflow is directed to achieve/maintain a certain flow rate through the inhaler.