During inspiration, air enters the lungs because of a/an_____ in intrathoracic pressure.

• A. Decrease
• B. Increase
• C. No change
• D. Both increase and decrease

Answer: (A)

During inspiration the key event is a drop in intrathoracic pressure. When the diaphragm contracts and the intercostal muscles lift the rib cage, the thoracic cavity expands and the pleural space becomes more negative. This makes the intrathoracic pressure fall below atmospheric pressure, which pulls the lungs outward and increases their volume. As the lung volume rises, the pressure inside the alveoli (intrapulmonary pressure) drops below atmospheric, creating a pressure gradient that draws air into the lungs until pressures equalize. If intrathoracic pressure didn’t decrease, air wouldn’t flow in; an increase would push air out instead.