Dry rales can be produced for a variety of reasons. When there is inflammation in the trachea and main bronchus, the mucous membrane of the wall becomes edematous, congested, with increased secretions, and even spasm of the bronchial smooth muscle, so that the lumen of the trachea and main bronchus becomes narrower. In addition, dry rales may also be produced by the presence of superfluous organisms (tumors) or foreign bodies in the trachea or main bronchi, or by the presence of more and thicker secretions, resulting in narrowing or partial obstruction of the lumen. In a few cases, the lumen is narrowed due to compression of the trachea or main bronchus by a mass or lymph node outside the trachea or main bronchus.