During which phase of respiration does the diaphragm contract and flatten?

• A. Expiration
• B. Inspiration
• C. Resting phase
• D. Exchange phase

Answer: (B)

The diaphragm's contraction and flattening occur during inspiration, which is the phase where air is drawn into the lungs. When the diaphragm contracts, it moves downward, increasing the thoracic cavity's volume. This reduction in pressure within the chest allows external air to flow in through the respiratory tract and into the lungs, facilitating gas exchange. The diaphragm plays a crucial role during this phase because its movement directly affects lung capacity and airflow.

In contrast, during expiration, the diaphragm relaxes and moves back up into a dome shape, thereby decreasing the thoracic cavity's volume, which pushes air out of the lungs. The resting phase refers to periods of passive breathing where minimal muscular effort is involved, while the exchange phase pertains to the actual transfer of gases (oxygen and carbon dioxide) occurring at the alveolar level in the lungs rather than a physical action of the diaphragm.