During expiration, what elastic structure is responsible for pushing upwards?

The diaphragm, a large, dome-shaped muscular structure located beneath the lungs, plays a crucial role in the mechanics of breathing. During expiration, especially passive expiration, the diaphragm relaxes from its contracted position, which decreases the volume of the thoracic cavity. This relaxation allows the elastic recoil of the lung tissues, as well as the elastic properties of the chest wall, to assist in expelling air from the lungs.

The action of the diaphragm and the passive recoil of the chest wall create a slight increase in pressure within the thoracic cavity relative to the atmospheric pressure, aiding the process of air being pushed out of the lungs. Thus, the diaphragm's role in expiration is vital as it coordinates with these elastic structures to ensure effective breathing.

In contrast, while alveoli are critical in gas exchange and have elastic recoil properties, they do not exert an active force to push air out. The bronchioles primarily facilitate airflow but do not directly influence expiration in the same manner as the diaphragm. The chest wall also plays a role but is more about providing structural support rather than actively pushing air upwards. This makes the diaphragm the key elastic structure responsible for the movement associated with expiration.