Pleural pressure is a fundamental aspect of human breathing, representing the pressure within the pleural space surrounding the lungs. This pressure plays a significant role in maintaining lung function and enabling the continuous exchange of air during respiration. Understanding pleural pressure is a step towards appreciating the intricate mechanics that allow us to breathe effortlessly.
The Pleural Space and Pressure Basics
The pleural space is a thin, fluid-filled area located between two membranes: the parietal pleura, which lines the chest cavity, and the visceral pleura, which covers the lungs. These two pleural layers are continuous at the hilum of the lung, where blood vessels and airways enter. Within this potential space, there are a few milliliters of serous pleural fluid, acting as a lubricant to reduce friction between the lungs and the chest wall during movement.
Under normal physiological conditions, the pressure within this pleural space, known as pleural pressure, is negative relative to the atmospheric pressure. This negative pressure is a result of opposing forces: the lungs naturally tend to recoil inward, while the chest wall tends to expand outward. This elastic recoil creates a suction-like effect within the pleural cavity.
This subatmospheric pressure averages around -4 mm Hg (approximately -5 cm H2O) during quiet breathing, when neither inhaling nor exhaling. The negative pressure is what keeps the lungs expanded and prevents their collapse, as it pulls the lung surface outward against the chest wall. The difference between the alveolar pressure inside the lungs and the pleural pressure is called transpulmonary pressure, which directly influences lung volume.
Pleural Pressure’s Role in Respiration
Pleural pressure undergoes dynamic changes throughout the breathing cycle, facilitating the movement of air into and out of the lungs. During inspiration, the diaphragm contracts and moves downward, while the external intercostal muscles contract, pulling the rib cage upward and outward. This coordinated muscle action increases the volume of the thoracic cavity.
As the thoracic cavity expands, the adhesive forces of the pleural fluid cause the lungs to stretch and expand along with it. This expansion of lung volume leads to a decrease in the pleural pressure, making it more negative. Typically, during inspiration, pleural pressure can drop to about -8 cm H2O. This more negative pleural pressure creates a pressure gradient, where the pressure inside the lungs (intra-alveolar pressure) becomes lower than the atmospheric pressure.
Air then flows from the higher atmospheric pressure into the lower-pressure alveoli within the lungs. During normal expiration, the process is largely passive, as the diaphragm and intercostal muscles relax. The elastic recoil of the lung tissue causes the lungs to naturally contract, reducing the volume of the thoracic cavity.
This decrease in thoracic volume leads to an increase in pleural pressure, making it less negative, typically returning to around -5 cm H2O. The increased pleural pressure, combined with the elastic recoil of the lung, causes the intra-alveolar pressure to rise above atmospheric pressure. This pressure gradient then drives air out of the lungs until the pressure within the alveoli equalizes with the atmospheric pressure.
When Pleural Pressure Goes Awry
Disruptions to the balance of pleural pressure can have consequences for respiratory function. Pneumothorax occurs when air enters the pleural space. This can happen if there’s a breach in the lung or chest wall, causing pleural pressure to become less negative or even positive.
When pleural pressure is lost, the lung’s elastic recoil causes it to collapse, partially or completely. This lung collapse impairs breathing, as the affected lung can no longer effectively exchange gases. Patients with pneumothorax often experience symptoms such as chest pain and shortness of breath, with severity depending on the extent of collapse.
Pleural effusion involves the accumulation of excess fluid in the pleural space. This fluid buildup increases pleural pressure. The increased pleural pressure can compress the lung, reducing its volume and hindering its expansion during inspiration.
Pleural effusions can lead to a restrictive ventilatory defect, where the lungs cannot expand fully. This can result in decreased lung volumes and impaired gas exchange, potentially causing hypoxemia (low blood oxygen). While the chest wall may expand to accommodate the fluid, the compressed lung’s function is still compromised, making breathing more difficult.