Passive expiration is preceded by the:

a. recoil of the lungs

b. contraction of the internal intercostals muscles.

c. contraction of the diaphragm.

d. increase in atmospheric pressure.

e. increase in CO2 in the blood

The correct answer is option a. "recoil of the lungs".

Explanation:

Normal respiration is passive, which means that no energy is needed to release the air from the lungs. Passive respiration is preceded by the recoil of the lungs that happens without any effort because of the elasticity of the lungs tissue. After inspiration, the diaphragm and intercostal muscles relax and the thoracic cavity and lungs decrease in volume. This causes that the interpulmonary pressure rises above atmospheric pressure, creating an air pressure gradient that put the air out of the lungs without the need of energy.