Inhalation occurs when which of the following happens?

• A. The diaphragm and intercostal muscles relax and cause an increase in intrathoracic pressure.
• B. The diaphragm and intercostal muscles ascend and cause an increase in intrathoracic pressure.
• C. The diaphragm and intercostal muscles contract and cause a decrease in intrathoracic pressure.
• D. The diaphragm ascends and the intercostal muscles contract, causing a decrease in intrathoracic pressure.

Answer: (C)

Inhalation happens when the chest cavity expands because the diaphragm and intercostal muscles contract. The diaphragm moves downward and the rib cage lifts up, increasing the volume of the thoracic cavity. This expansion lowers intrathoracic pressure below atmospheric pressure, so air is drawn into the lungs until the pressures equalize. If these muscles relaxed or the diaphragm moved upward, intrathoracic pressure would rise and air would be pushed out instead, which describes exhalation. So the mechanism is the contraction of the diaphragm and intercostal muscles that decreases intrathoracic pressure.