The substance that reduces surface tension in the alveoli is:

A. A pneumothorax occurs when there is a collapse of the lung due to air entering the pleural cavity, but this is not directly related to the lungs dropping below atmospheric pressure.
B. Air flows out of the lungs when the pressure inside the lungs is higher than atmospheric pressure (e.g., during exhalation).
C. When the pressure in the lungs drops below atmospheric pressure, it creates a vacuum effect, causing air to flow into the lungs during inhalation. This is how normal breathing works.
D. Bronchiolar constriction can cause respiratory distress, but it is not directly linked to a drop in lung pressure. Instead, constriction happens due to factors like asthma or allergies, not pressure changes.

A. Epistaxis refers to nosebleed, which is unrelated to lung function or expansion.
B. Effusion refers to the accumulation of fluid (such as pleural effusion, where fluid collects in the pleural space around the lungs), not incomplete lung expansion.
C. Atelectasis is the collapse or incomplete expansion of a lung or part of a lung, which can occur due to blockage, pressure, or injury.
D. Adhesion refers to abnormal fibrous connections between tissues, which can occur in the body after injury or surgery, but it does not refer to incomplete lung expansion