Question

which of the following occurs first, causing expiration?
multiple choice
intra - alveolar pressure increases.
air flows down its pressure gradient, out of the lungs.
elastic tissues of the lungs and thoracic cage recoil.
the diaphragm and external intercostal respiratory muscles relax.

Explanation:

During expiration, the first event is the relaxation of the diaphragm and external intercostal respiratory muscles. This relaxation reduces the volume of the thoracic cavity. When these muscles relax, it leads to the recoil of the elastic tissues of the lungs and thoracic cage (which is a subsequent step, not the first). As the volume decreases (due to muscle relaxation), intra - alveolar pressure increases (another subsequent step), and then air flows down its pressure gradient out of the lungs (the final step in the expiration process).

Answer:

The diaphragm and external intercostal respiratory muscles relax.