What is the relationship between pleural pressure and lung expansion?

• A. Pleural pressure directly inhibits lung expansion
• B. Pleural pressure becomes less negative during lung expansion
• C. Pleural pressure lowers during deflation of the lungs
• D. Pleural pressure becomes more negative during lung expansion

Answer: (D)

The relationship between pleural pressure and lung expansion revolves around the concept of how pressure dynamics work within the thoracic cavity during the respiratory cycle. During lung expansion, such as when inhaling, the diaphragm and intercostal muscles contract, increasing the volume of the thoracic cavity. This increase in volume results in a decrease in the pleural pressure, making it more negative compared to atmospheric pressure.

When pleural pressure becomes more negative, it creates a strong gradient that allows air to flow into the lungs, leading to their expansion. This negative pressure essentially "pulls" the lungs outward, allowing them to inflate. Thus, as the lungs expand, pleural pressure decreases due to the increase in thoracic volume, which is why the statement regarding pleural pressure becoming more negative during lung expansion is correct.

In contrast, pleural pressure is not inhibited by lung expansion, does not become less negative during lung expansion (rather, it becomes more negative), and does not lower significantly during lung deflation, as that process involves a rebound of pressure back towards a less negative or even positive state relative to atmospheric pressure.