The diameter of the bronchioles plays an important role in air flow. The bronchioles change diameter to either increase or reduce air flow. An increase in diameter is called bronchodilation and is stimulated by either epinephrine or sympathetic nerves to increase air flow. A decrease in diameter is called bronchoconstriction, which is the tightening of the smooth muscle surrounding the bronchi and bronchioles due to and stimulated by histamine, parasympathetic nerves, cold air, chemical irritants, excess mucus production, viral infections, and other factors to decrease air flow. Bronchoconstriction can result in clinical symptoms such as wheezing, chest tightness, and dyspnea, which are common features of asthma, chronic obstructive pulmonary disease (COPD), and chronic bronchitis.