Nitric oxide is asign of asthma and other lung diseases, such as chronic obstructive pulmonarydisease, cystic fibrosis and bronchiectasis (the abnormal widening of airways).

Due to the fact that the lung tissue that attaches itself to the walls of the bronchioles has a certain "elasticity", the walls are pulled outwards, thereby expanding the bronchioles.

It may occur in previously normal lungs or be superimposed on underlying bronchitis or other respiratory disease, e. g. bronchiectasis or carcinoma.