During inhalation, which action expands the thoracic cavity and draws air into the lungs?

• A. The abdominal muscles contract to raise the rib cage
• B. The diaphragm and intercostal muscles contract, compressing the thoracic cavity and pushing air out
• C. The diaphragm relaxes and intercostals contract, drawing air into the lungs
• D. The diaphragm and intercostal muscles contract, expanding the thoracic cavity and drawing air into the lungs

Answer: (D)

Inhalation is an active process where the chest cavity expands to let air flow into the lungs. This happens when the diaphragm contracts and moves downward, increasing the vertical space, and the external intercostal muscles lift the ribs to widen the chest in front-to-back and side-to-side dimensions. As the thoracic cavity enlarges, the air pressure inside the lungs drops below atmospheric pressure, so air is drawn in until pressures equalize. Abdominal muscles aren’t involved in normal inhalation; if they contract, they raise internal pressure and push the diaphragm upward, which actually reduces thoracic volume and aids expulsion of air. Exhaling, on the other hand, involves the diaphragm relaxing and the intercostal muscles aiding in reducing thoracic volume to push air out.