In the context of lung physiology, what is the typical state of intrapleural pressure during normal breathing?

• A. Variable, depending on lung volume
• B. Positive relative to atmospheric pressure
• C. Negative relative to atmospheric pressure
• D. Equal to atmospheric pressure

Answer: (C)

Verified step by step guidance

1. Understand that intrapleural pressure is the pressure within the pleural cavity, which is the space between the visceral and parietal pleurae surrounding the lungs.
2. Recognize that during normal breathing, the intrapleural pressure is typically negative relative to atmospheric pressure. This negative pressure is crucial for keeping the lungs inflated.
3. During inspiration, the diaphragm contracts and the thoracic cavity expands, causing the intrapleural pressure to become more negative, which helps draw air into the lungs.
4. During expiration, the diaphragm relaxes and the thoracic cavity volume decreases, but the intrapleural pressure remains negative, although it becomes less negative compared to during inspiration.
5. The negative intrapleural pressure is maintained by the elastic recoil of the lungs and the surface tension of the pleural fluid, which prevents lung collapse and facilitates breathing.