The intrapleural space is a narrow, fluid-filled region situated between the lungs and the inner chest wall. This space is lined by the pleura, a double-layered membrane where the visceral layer covers the lung surface and the parietal layer lines the thoracic cavity. The pressure within this space, known as intrapleural pressure (\(P_{ip}\)), is negative, meaning it is lower than atmospheric pressure. This sub-atmospheric pressure is the fundamental mechanism that keeps the lungs inflated and prevents them from collapsing during normal breathing.
The Opposing Elastic Recoil of the Thorax
The negative intrapleural pressure stems from two opposing elastic forces within the chest cavity. The lungs contain elastic fibers that give them a natural tendency to recoil inward, similar to a stretched rubber band. If isolated, this inward force would cause the lungs to collapse completely.
Working against this inward pull is the chest wall, composed of the rib cage, muscles, and diaphragm. The chest wall has inherent elasticity that causes it to naturally spring outward, expanding beyond its resting state if isolated from the lungs.
At functional residual capacity (the volume of air remaining after a normal passive exhale), these two opposing forces reach equilibrium. The lungs pull inward while the chest wall pulls outward, attempting to separate the two pleural layers. This constant separation attempt creates a slight vacuum in the thin intrapleural space, resulting in the negative pressure, which is around \(-4\) millimeters of mercury (\(\text{mmHg}\)) relative to the atmosphere at rest.
How Pleural Fluid Maintains Adhesion
A thin layer of serous pleural fluid ensures that the negative pressure created by opposing elastic forces is effectively transmitted to the lungs. This fluid acts as a lubricant, allowing the visceral and parietal pleura to slide smoothly past each other during respiratory movements, minimizing friction.
The pleural fluid maintains a strong physical connection between the lung and the chest wall through adhesion and cohesion. This fluid’s strong surface tension effectively links the two layers, similar to how two wet glass slides are difficult to pull apart but slide easily over one another. This mechanism ensures that when the chest wall moves outward, the lungs are passively pulled along with it.
This fluid bond transmits the outward pull of the chest wall to the lung surface, counteracting the lung’s inward elastic recoil. The negative intrapleural pressure is a constant manifestation of this mechanical linkage, ensuring the lung remains expanded and adheres to the thoracic cavity wall throughout the respiratory cycle.
Changes in Pressure During Inhalation and Exhalation
The negative intrapleural pressure fluctuates dynamically with each breath. During inhalation, the diaphragm contracts and flattens, and the external intercostal muscles pull the rib cage upward and outward. This expansion of the thoracic cavity increases the volume of the intrapleural space.
According to Boyle’s law, increasing the volume decreases the pressure, causing the intrapleural pressure to become even more negative. During quiet inspiration, the pressure drops from its resting value of about \(-4\) \(\text{mmHg}\) to approximately \(-6\) \(\text{mmHg}\). This more negative pressure increases the force that pulls the lungs open.
Quiet exhalation is passive, driven by the relaxation of the inspiratory muscles and the recoil of the elastic lung tissue and chest wall. As the thoracic volume decreases, the intrapleural space is compressed, causing the negative pressure to return toward its resting value of \(-4\) \(\text{mmHg}\). The pressure remains negative throughout the entire cycle.
Loss of Negative Pressure and Lung Collapse
Maintaining the negative intrapleural pressure is essential for lung volume. A condition known as pneumothorax occurs when air enters the intrapleural space, often due to trauma or the rupture of air sacs on the lung surface. This breach allows the intrapleural pressure to quickly equalize with the atmospheric pressure, rising to \(0\) \(\text{mmHg}\).
When the intrapleural pressure becomes zero or positive, the mechanical link between the chest wall and the lung surface is lost. The lung’s natural inward elastic recoil, previously opposed by the negative pressure, becomes unopposed. The lung immediately recoils inward, resulting in a partial or complete collapse, a state called atelectasis.
Simultaneously, the chest wall’s unopposed elastic tendency causes it to spring slightly outward. The collapsed lung can no longer participate in gas exchange.