Question

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

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. As a result, the intra - alveolar pressure increases (due to Boyle's law: \(P_1V_1 = P_2V_2\), when volume \(V\) decreases, pressure \(P\) increases if temperature is constant). Then, elastic tissues of the lungs and thoracic cage recoil (because of the change in pressure and muscle relaxation), and finally, air flows down its pressure gradient out of the lungs.

Answer:

The diaphragm and external intercostal respiratory muscles relax.