Stridor is caused by the opening of collapsed or fluid-filled alveoli during inspiration.

Wheezing is high-pitched sounds heard during expiration when there is a blockage.

Rhonchi are low-pitched sounds heard during expiration, although they can be heard during inspiration when there is a partial blockage of the larger airways.

The reduction of respiratory sounds occurs due to bronchospasm or other mechanisms that limit airflow, which may be present in asthma and COPD.