During inhalation, which statement about the diaphragm is true?

• A. It relaxes and moves upward to expand the thoracic cavity
• B. It contracts and moves downward to expand the thoracic cavity
• C. It remains perfectly relaxed
• D. It contracts and moves upward to compress the thoracic cavity

Answer: (B)

During inhalation, the diaphragm contracts and moves downward, flattening from its dome shape. This increases the vertical size of the thoracic cavity, lowering intrathoracic pressure so air can flow into the lungs. If the diaphragm relaxed or moved upward, the chest volume would not expand and inhalation wouldn’t occur. It doesn’t remain perfectly relaxed, and contracting upward would compress the chest and push air out rather than in.