The respiratory bronchioles have single alveoli off their walls.
When these bronchioles are infected, they become inflamed, and breathing may become difficult.
These then branch to give rise to several orders of progressively smaller airways called bronchioles, the smallest of which are called terminal bronchioles.
Researchers chemically modified adrenaline to find molecules that would have the same effect in dilating the bronchioles but without the stimulation of the heart.
These medicines, properly known as bronchodilators, have a relaxing effect on the muscle surrounding the bronchioles.