Which of the following best describes the diaphragm's role in inhalation?

• A. It relaxes to allow air into the lungs.
• B. It contracts to cause a decrease in intrathoracic pressure.
• C. It has no role in breathing.
• D. It contracts to increase intrathoracic pressure.

Answer: (B)

The diaphragm is the primary muscle driving inhalation. When it contracts, it moves downward, expanding the vertical dimension of the thoracic cavity. This expansion lowers intrathoracic pressure, creating a pressure gradient that pulls air into the lungs. So contracting to cause a decrease in intrathoracic pressure is the correct description.

If the diaphragm relaxed, the thoracic cavity would shrink, increasing pressure and pushing air out rather than in. If it contracted to increase intrathoracic pressure, air would be harder to draw in. The diaphragm does play a real role in breathing, so saying it has no role isn’t accurate.