What primarily causes crackles (rales) in the lungs?

Crackles, also known as rales, are primarily caused by air passing through fluid in the small airways and alveoli of the lungs. This sound occurs when the fluid, which might be due to pulmonary edema, infections like pneumonia, or inflammation, disrupts the airflow, creating the distinctive snapping or crackling noises during inhalation. The presence of fluid causes the small airways to close during expiration and then reopen during inspiration, leading to these sounds.

In contrast, mucus in the larger airways tends to produce more of a wheezing sound or rhonchi, while narrowing of the upper airways typically leads to stridor or other high-pitched sounds during breathing. Severe bronchoconstriction can cause wheezing but does not typically generate crackles, as the primary issue involves the narrowing of the air passages rather than fluid in the small airways. Thus, the presence of fluid passing through air effectively accounts for the crackling sound heard in certain respiratory conditions.