Mammals employ which type of pressure breathing?

• A. Ambient Pressure Breathing
• B. Positive Pressure Breathing
• C. Negative Pressure Breathing
• D. Zero Pressure Breathing

Answer: (C)

Mammals breathe by creating a pressure in the chest that is lower than the surrounding air, so air is drawn into the lungs. When the diaphragm contracts, it moves downward and the rib cage expands, increasing the volume of the thoracic cavity. This makes the pressure inside the lungs (intrapulmonary pressure) fall below atmospheric pressure, so air flows in until those pressures equalize. Expiration is then mostly passive as the chest tissues recoil, pushing air back out.

This negative pressure mechanism is distinct from positive pressure breathing, where air is pushed into the lungs by increasing airway pressure (as happens with certain ventilators or in some other animals under different conditions). Breathing at ambient or zero pressure wouldn’t generate the necessary pull of air into the lungs.