What is the cause of rales when auscultating lungs in a patient with pulmonary edema?

Rales, also known as crackles, are abnormal lung sounds that can often be heard during auscultation. In the context of pulmonary edema, rales are primarily caused by the presence of excess fluid in the alveoli. As air moves in and out of the lungs, the fluid creates a distinct sound, akin to the popping of bubbles.

When a patient inhales, the terminal airways, which were previously collapsed due to the fluid in the alveoli, begin to reopen or pop open. This occurs as the negative pressure created during inspiration overcomes the surface tension of the fluid lining the alveoli. The sudden opening of these airways results in the characteristic crackling sounds associated with rales.

Understanding that the sounds originate from the terminal airways is crucial in recognizing pulmonary edema as a respiratory condition, where fluid accumulation directly affects normal lung function and airflow dynamics.