Intrapleural pressure is the pressure within the pleural cavity, a space between the two membranes surrounding the lungs. This space contains a small amount of fluid that helps the layers glide smoothly over one another during breathing. This pressure results from the interaction between the lungs and the chest wall, allowing the lungs to inflate and deflate with the movement of the chest.
The Role of Intrapleural Pressure in Respiration
The pressure within the pleural space is normally negative, meaning it is lower than atmospheric pressure. This negative pressure is the result of two opposing forces. The lungs have a natural elasticity that causes them to pull inward, while the chest wall has its own elasticity that causes it to spring outward.
These conflicting forces pull the visceral pleura (attached to the lungs) and the parietal pleura (attached to the chest wall) away from each other. This creates a vacuum in the fluid-filled pleural space. This negative pressure, which is around -4 mmHg at rest, effectively “sticks” the lungs to the inside of the chest wall, preventing them from collapsing.
When you take a breath in (inspiration), your diaphragm and intercostal muscles contract, pulling the rib cage up and out and expanding the thoracic cavity. As the chest wall expands, it pulls the parietal pleura with it. This increases the volume of the pleural space, making the intrapleural pressure even more negative (dropping to approximately -6 mmHg). This increased suction pulls the lungs outward, causing them to expand and draw air in.
Quiet expiration is largely a passive process. The diaphragm and intercostal muscles relax, allowing the chest wall to return to its resting position. This decreases the volume of the thoracic cavity and the pleural space. As a result, the intrapleural pressure becomes less negative, returning to about -4 mmHg. This reduction in outward pull allows the natural elastic recoil of the lungs to push air out.
These fluctuations in intrapleural pressure drive changes in the pressure inside the lungs (intrapulmonary pressure). The difference between atmospheric and intrapulmonary pressure dictates the flow of air into and out of the lungs. The negative intrapleural pressure ensures the lungs remain expanded to some degree even between breaths.
Consequences of a Compromised Pleural Space
When the integrity of the pleural space is broken, the pressure dynamics that allow for normal breathing are disrupted, leading to a pneumothorax. This condition is the presence of air in the pleural cavity. This can happen if an injury punctures the chest wall from the outside or if an injury to the lung tissue allows air to leak out from the inside.
The introduction of air into the pleural space eliminates the negative pressure that holds the lungs against the chest wall. As air rushes in, the intrapleural pressure rises until it becomes equal to atmospheric pressure. Once this negative pressure is lost, the lung’s inherent elastic recoil takes over, causing it to collapse.
The extent of the collapse depends on the amount of air that enters the pleural space. A small pneumothorax might only cause minor symptoms, while a large one can lead to significant respiratory distress. Causes include traumatic injuries like a fractured rib or a puncture wound. A spontaneous pneumothorax can also occur without an obvious injury, often from a ruptured air blister (a bleb) on the lung’s surface.
In some cases, a tension pneumothorax can develop, where an injury creates a one-way valve effect, allowing air to enter the pleural space with each breath but not escape. This causes a progressive buildup of air and pressure that not only collapses the lung but can also shift the heart and major blood vessels, becoming a life-threatening emergency.
Other Conditions Affecting Intrapleural Pressure
Other substances can accumulate in the pleural space and disrupt lung function by altering intrapleural pressure. One common condition is a pleural effusion, the buildup of excess fluid in the pleural cavity, which can result from infections like pneumonia, heart failure, or cancer. The excess fluid takes up space, increasing pressure within the cavity and compressing the adjacent lung tissue, making it difficult to expand fully.
A similar condition is a hemothorax, the accumulation of blood in the pleural space. This is most often the result of trauma to the chest that damages blood vessels. Like a pleural effusion, the blood increases pressure in the pleural space, compressing the lung and hindering its expansion. A large hemothorax can lead to significant blood loss in addition to respiratory compromise.
These conditions differ from a pneumothorax. In a pneumothorax, the problem is the loss of negative pressure, allowing the lung’s own elasticity to collapse it. In pleural effusions and hemothorax, the issue is the addition of a substance that actively compresses the lung from the outside. The underlying pressure dynamic is one of compression rather than a loss of suction.