Breathing, while seemingly simple, involves an intricate interplay of pressures and anatomical structures. This complex system ensures that the lungs can efficiently perform their function of gas exchange.
Understanding Intrapleural Pressure
Between the outer surface of each lung and the inner wall of the chest cavity lies a narrow space known as the pleural cavity or intrapleural space. This space contains a very thin layer of fluid. The pressure within this fluid-filled space is called intrapleural pressure (IPP). Unlike the pressure inside the lungs, intrapleural pressure is consistently lower than the surrounding atmospheric pressure.
Forces That Create Negative Pressure
The persistent negativity of intrapleural pressure results from a dynamic balance between two opposing elastic forces: the natural tendency of the lungs to recoil inward and the chest wall’s inclination to spring outward. The lungs, rich in elastic fibers, continuously attempt to shrink, much like a deflating balloon. This inward pull is also influenced by the surface tension of the fluid lining the tiny air sacs within the lungs, which encourages collapse. Conversely, the chest wall, composed of bones and muscles, possesses elastic properties that cause it to naturally expand outwards. If the lungs were removed, the chest wall would spring out to a larger volume.
The thin layer of pleural fluid connects these two opposing forces. Similar to how two wet glass slides stick together, this fluid exhibits cohesive properties due to surface tension, acting like an adhesive. It effectively couples the outer surface of the lung (visceral pleura) to the inner surface of the chest wall (parietal pleura). This cohesive force prevents the lungs from fully recoiling and the chest wall from fully expanding, creating a constant pull that maintains the negative intrapleural pressure. Under normal conditions, this pressure typically ranges around -4 mmHg relative to atmospheric pressure when at rest.
Keeping Lungs Inflated
Negative intrapleural pressure maintains lung inflation and facilitates breathing. It creates a continuous suction that adheres the lungs to the chest wall, ensuring they expand and contract in unison with the thoracic cavity. Without this adhesive effect, the lungs would collapse due to their inherent elastic recoil.
During inspiration, the diaphragm contracts and flattens, while external intercostal muscles pull the rib cage upward and outward. This increases the volume of the thoracic cavity. Because the lungs are held against the expanding chest wall by negative intrapleural pressure, they are pulled open, and their volume increases. This expansion leads to a drop in pressure inside the lungs, drawing air in from the outside atmosphere. The negative intrapleural pressure becomes even more negative during inspiration, sometimes dropping to -8 mmHg, further enhancing the pull on the lungs.
What Happens When Pressure Changes
Maintaining negative intrapleural pressure is important for normal respiratory function; its disruption can lead to serious consequences. If intrapleural pressure loses its negativity or becomes positive, the lungs can no longer remain expanded. This occurs if air enters the pleural space, a condition known as a pneumothorax.
A pneumothorax happens when there is a breach in either the lung or the chest wall. For instance, a puncture wound or a rupture of small air sacs on the lung surface can allow air to leak into the pleural cavity. When air enters this space, the intrapleural pressure equalizes with atmospheric pressure, eliminating the suction that keeps the lung inflated. As a result, the lung, driven by its natural elastic recoil, collapses partially or completely. Symptoms can include sudden chest pain and shortness of breath.