Intrapleural Pressure Explained

Intrapleural pressure is the pressure within the pleural cavity, the narrow space between the visceral pleura (covering the lungs) and the parietal pleura (lining the chest wall). This pressure is normally subatmospheric, meaning it is lower than the atmospheric pressure outside the body. This negative pressure is crucial for keeping the lungs expanded.

Respiration Phases and Intrapleural Pressure

The intrapleural pressure changes dynamically during the different phases of respiration:

• End of Quiet Expiration: At the end of a normal, quiet expiration, the respiratory system is at a resting state, also known as Functional Residual Capacity (FRC). At FRC, the elastic recoil forces of the lungs (which tend to collapse inward) and the chest wall (which tends to expand outward) are in balance. This opposition of forces creates a negative pressure in the intrapleural space, typically around \(-5 \text{ cm H}_2\text{O}\) relative to atmospheric pressure. Statement A is correct.
• Start of Inspiration: Inspiration begins from the FRC state, where the intrapleural pressure is already subatmospheric (around \(-5 \text{ cm H}_2\text{O}\)). Statement B is correct.
• During Inspiration: During inspiration, the inspiratory muscles (like the diaphragm and external intercostals) contract, expanding the volume of the thoracic cavity. This expansion pulls the parietal pleura outward. Due to the cohesive force of the pleural fluid, the visceral pleura and thus the lungs are also pulled outward, increasing the volume of the intrapleural space. This increase in volume leads to a further decrease in pressure, making the intrapleural pressure more negative. During quiet inspiration, it typically becomes about \(-8 \text{ cm H}_2\text{O}\). Statement C is correct.
• During Expiration: During quiet expiration, the inspiratory muscles relax. The elastic recoil of the lungs and chest wall causes the thoracic volume to decrease, pushing air out. The intrapleural pressure becomes less negative, returning towards the FRC value of \(-5 \text{ cm H}_2\text{O}\). It remains subatmospheric throughout quiet expiration. Therefore, statement D, which says it attains a value above atmospheric pressure during expiration, is incorrect for quiet expiration.
• During Strong Inspiratory Efforts: Strong inspiratory efforts involve more forceful contraction of inspiratory muscles and recruitment of accessory muscles, leading to a greater expansion of the chest cavity. This results in a significantly more negative intrapleural pressure than during quiet inspiration (e.g., down to \(-15 \text{ cm H}_2\text{O}\) or lower). Therefore, statement E, which says the intrapleural pressure becomes positive during strong inspiratory efforts, is incorrect. Positive intrapleural pressure typically occurs during forced expiration or maneuvers like coughing or Valsalva.

Statement Analysis Summary

• Statement A: Correct. At the end of quiet expiration (FRC), lung recoil and chest wall recoil balance, and intrapleural pressure is subatmospheric.
• Statement B: Correct. Inspiration starts from FRC, where intrapleural pressure is subatmospheric.
• Statement C: Correct. During inspiration, the expanding chest makes intrapleural pressure more negative.
• Statement D: Incorrect. During quiet expiration, intrapleural pressure remains subatmospheric. It can become positive during forced expiration, but not typically "during expiration" generally.
• Statement E: Incorrect. Strong inspiration makes intrapleural pressure more negative, not positive.

Based on the analysis, statements A, B, and C are correct.