A constriction in any part of the air passages from the glottis to the alveoli causes an additional resistance to the passage of air. This results in an increased effort on the part of the respiratory musculature to ventilate the lungs. When obstruction is present within the thorax, respiration modifies the degree of obstruction, since the lumens of the bronchi enlarge on inspiration and constrict during expiration.1 This variation in size of the smaller bronchi has been demonstrated by roentgenograms of the lungs, made after injection of iodized poppyseed oil.2 In the extrathoracic trachea and larynx, however, the lumen is not exposed to varying intrathoracic pressures; it is, moreover, protected from alteration in its size by strong cartilaginous rings.
In asthmatic dyspnea, inspiration is short and forcible, and expiration is prolonged. The long-drawn-out character of expiration may be explained by the constriction of the bronchi during the expiratory