The intrapleural pressure is negative both during inspiration and expiration because
**Core Concept**
Intrapleural pressure is the pressure within the pleural space, which is the thin gap between the lungs and the chest wall. It is a crucial factor in lung expansion and is influenced by the balance between the elastic recoil of the lungs and the chest wall.
**Why the Correct Answer is Right**
The intrapleural pressure is negative due to the elastic recoil of the lungs, which is greater than that of the chest wall. This results in a downward force on the lungs, creating a negative pressure within the pleural space. During both inspiration and expiration, the intrapleural pressure remains negative because the elastic recoil of the lungs is constant. The diaphragm and other muscles of respiration help to increase the volume of the chest cavity, but they do not change the intrapleural pressure significantly.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a clear explanation for the negative intrapleural pressure.
**Option B:** This option is incorrect because it is incomplete and does not address the elastic recoil of the lungs.
**Option C:** This option is incorrect because it suggests that the intrapleural pressure is positive during expiration, which is not true.
**Clinical Pearl / High-Yield Fact**
Remember that the intrapleural pressure is a critical factor in lung expansion, and any changes in this pressure can lead to respiratory problems such as pneumothorax or pleural effusion.
**Correct Answer:** C. The elastic recoil of the lungs is greater than that of the chest wall, resulting in a negative intrapleural pressure.