What occurs in the respiratory system during forced expiration?

During forced expiration, the process involves actively expelling air from the lungs, which is characterized by an increase in intrapulmonary pressure. This pressure increase is primarily achieved through the contraction of the abdominal muscles and the internal intercostal muscles. When these muscles contract, they push up the diaphragm and help to push air out of the lungs more forcefully.

As the volume of the thoracic cavity decreases due to this muscle action, the air inside the lungs is compressed, which leads to intrapulmonary pressure rising above atmospheric pressure. Consequently, this pressure gradient causes air to flow out of the lungs.

The other options do not occur during forced expiration. For example, during this phase, the diaphragm does not flatten; instead, it moves upward as abdominal pressure increases. Furthermore, the expansion of the chest wall does not happen because the expiration is actively pushing air out rather than allowing it to expand. Lastly, intrapulmonary pressure decreasing would occur during inhalation, not expiration, because air moves into the lungs when the pressure in the thoracic cavity drops below atmospheric pressure.