During inhalation, what happens to intrathoracic pressure?

• A. It increases
• B. It stays the same
• C. It decreases (becomes more negative), enabling air to enter
• D. It fluctuates randomly

Answer: (C)

Inhalation works by making the space inside the chest cavity more negative. When the diaphragm contracts and the chest wall expands, the volume of the thoracic cavity increases. This expansion lowers intrathoracic pressure to a more negative value than the outside atmosphere, creating a pressure gradient that pulls air into the lungs. Airflow continues until the pressure inside the lungs matches atmospheric pressure. So intrathoracic pressure decreases (becomes more negative), enabling air to enter.