Which of the following occurs during the expiration phase of breathing?

During the expiration phase of breathing, several physiological processes occur that contribute to the expulsion of air from the lungs.

The activation of the muscles of expiration is a crucial aspect of this phase. While expiration can occur passively (with the diaphragm and intercostal muscles relaxing), active expiration involves the engagement of muscles such as the abdominal muscles and the internal intercostals to actively push air out of the lungs. This is particularly important when larger volumes of air need to be expelled, such as during vigorous activities like exercise or speaking.

As the muscles contract during active expiration, the volume of the thoracic cavity decreases, which in turn increases the pressure within the lungs. According to Boyle's law, when the volume of a container decreases, the pressure within that container increases. This increase in pressure relative to atmospheric pressure leads to air being expelled from the lungs.

Additionally, the vocal folds can remain closed at certain points during expiration, especially when phonation is occurring or during certain speech sounds, contributing to the control of airflow.

Thus, all these elements—activation of expiration muscles, increased lung pressure, and potential closure of the vocal folds—occur collectively during expiration, supporting the correct answer that encompasses all these phenomena.