Smooth muscle in the lung encircles bronchiolar tubular passages that lead to the alveoli (you would need a microscope to see this, by the way).

when the bronchial tree relaxes more oxygen is taken into the body and more carbon dioxide is ... What muscle under the rib cage controls your breathing?

How is breathing affected if the smooth muscle of the bronchial tree relaxes?: More air flow. 6. Increases surface area of nasal mucous membrane: Nasal concha ...