How is the intrapleural pressure (PIP) characterized under normal conditions?

• A. Always positive
• B. Always negative
• C. Variable
• D. Always neutral

Answer: (B)

Intrapleural pressure, also referred to as pleural pressure, is a key physiological phenomenon that plays a crucial role in mechanics of ventilation. Under normal conditions, it is characterized as being negative relative to atmospheric pressure. This negative pressure is essential for keeping the lungs inflated and allowing for effective gas exchange.

The negative intrapleural pressure is generated by the elastic recoil of the lungs, which naturally tend to collapse, and the surface tension of the fluid lining the alveoli. It also results from the outward pull of the chest wall, which seeks to expand the thoracic cavity. This pressure helps to create a pressure gradient between the atmosphere and the alveoli during inspiration, facilitating airflow into the lungs.

While the intrapleural pressure can fluctuate during respiration (becoming less negative during inhalation and more negative during exhalation), it is consistently negative under normal conditions. This negative pressure is crucial for maintaining lung inflation and ensuring that the lungs do not collapse, allowing for efficient gas exchange.