Which statement about the diaphragm's action during breathing is true?

• A. It contracts on Inspiration
• B. It relaxes during inspiration
• C. It expands on Inspiration
• D. It expands on Expiration

Answer: (A)

During breathing, the diaphragm moves the thoracic cavity to create the pressure changes that draw air into the lungs. Inhalation happens when the diaphragm contracts, pulling downward and flattening from its dome shape. This increases the volume of the thoracic cavity, lowers intrathoracic pressure, and allows air to flow in. That’s why the statement that the diaphragm contracts on inspiration is true. If the diaphragm relaxed during inspiration, it wouldn’t descend to enlarge the chest, so inspiration wouldn’t effectively draw air in. The diaphragm doesn’t actively expand during inspiration; its contraction is the mechanism that expands thoracic volume. During expiration, the diaphragm relaxes and rises back to its dome shape, decreasing thoracic volume and helping push air out.