Question

Why does air rush out of the lungs during exhalation?

1. Because the air pressure in the chest cavity is equal to outside (atmospheric) pressure.
2. Because the size or volume of the chest cavity has gotten smaller and so air pressure is increased.
3. Because the person pushed the air out.
4. Because the size or volume of the chest cavity has enlarged and so the air pressure is increased.

Answer 1

Answer B: Because the size or volume of the chest cavity has gotten smaller and so air pressure is increased.

The chest cavity or thoracic cavity always has negative pressure which prevents the airways of the lungs from collapsing. During inhalation, the lung volume is increased due to the contraction of the diaphragm and intercostal muscles, and results in the expansion of the thoracic cavity. As the volume inside the lungs increases, the pressure is decreased. This decrease of pressure in the chest cavity results in the reduced air pressure inside the lungs (as compared to the atmospheric pressure). This pressure difference between the atmosphere and the thoracic cavity allows air to rush into the lungs, and the process of inhalation occurs.

During the process of exhalation, the air is pushed out of the lungs. In this process, the intercostal muscles relax, returning the chest wall to its original position. The diaphragm also relaxes during exhalation and moves in upward direction into the thoracic cavity. This results in the reduced volume or size of the chest cavity and increase of pressure within the chest cavity relative to the atmosphere. Due to this pressure difference between the thoracic cavity and the atmosphere, air rushes out of the lungs. This movement of air out of the lungs (exhalation) is considered as a passive process because it does not involve any muscles contraction to expel the air out of the lungs.