The pleural cavity is a specialized, fluid-filled space that is fundamental to the mechanics of breathing. This space, which is not an open void but rather a potential area between two membranes, surrounds each lung within the thoracic cavity. It allows the lungs to expand and contract smoothly without creating damaging friction against the ribcage and other structures. The primary purpose of this system is to ensure the lungs remain tethered to the chest wall, allowing them to follow the movement of the diaphragm and rib muscles during respiration.
Anatomy of the Pleura and Cavity
The pleural cavity is formed by two thin, smooth membranes known collectively as the pleura, which are composed of a layer of simple squamous cells supported by connective tissue. The inner layer, called the visceral pleura, adheres tightly to the entire surface of the lung, extending into the deep fissures that divide the lung lobes. This layer is relatively insensitive to pain, as its nerve fibers primarily detect stretch rather than sharp stimuli.
The outer layer is the parietal pleura, which lines the internal surfaces of the chest wall, the top of the diaphragm, and the sides of the mediastinum (the central compartment of the chest). Unlike its counterpart, the parietal pleura is highly sensitive to pain, temperature, and pressure, receiving its nerve supply from the intercostal and phrenic nerves. The pleural cavity is the narrow space between these two layers, normally containing only 10 to 20 milliliters of fluid in a healthy adult.
This small volume of serous fluid is continuously produced by the parietal circulation and reabsorbed by the lymphatic system. Because the two layers are pressed closely together, the pleural cavity is described as a “potential space.” It only becomes an actual space when air or excess fluid accumulates pathologically. This closed system, with its two membranes and minimal fluid, is critical for respiratory function.
Essential Functions of the Pleural System
The dual structure of the pleura and its fluid serve two primary physiological roles that enable breathing. The first function is lubrication, where the thin film of pleural fluid acts like oil, allowing the two pleural surfaces to glide smoothly over one another. As the lungs inflate and deflate, this lubricating action prevents the intense friction that would otherwise occur between the moving lung surface and the stationary chest wall.
The second function is mechanical coupling, facilitated by the fluid’s surface tension and the subatmospheric pressure maintained in the space. This negative pressure acts like a suction cup, pulling the visceral pleura against the parietal pleura. When the muscles of respiration contract and expand the chest wall, the parietal pleura moves outward. The surface tension of the fluid ensures the visceral pleura and the attached lung tissue are forced to follow.
This mechanism links the lungs to the skeletal movements of the chest, allowing the lungs to expand and draw air in as the chest cavity volume increases. Without this mechanical linkage, the lungs would naturally recoil and collapse inward due to their inherent elasticity. The continuous negative pressure maintained within the pleural space overcomes this natural elastic recoil, ensuring proper inflation.
Common Clinical Conditions Involving the Pleural Space
Disruptions to the pleural space can lead to several common clinical conditions, often resulting in pain and difficulty breathing. A pneumothorax, or collapsed lung, occurs when air or gas enters the pleural cavity. This air entry immediately breaks the surface tension and eliminates the negative pressure, causing the lung to pull away from the chest wall and partially or fully collapse. Pneumothorax can result from chest trauma or happen spontaneously, sometimes due to underlying lung diseases.
Another common issue is a pleural effusion, the excessive accumulation of fluid in the pleural space. Normally, pleural fluid production and reabsorption are constantly balanced, but this balance is upset in an effusion. Effusions are classified into two types: transudative, caused by pressure imbalances in blood vessels (often seen in conditions like congestive heart failure), and exudative, caused by inflammation, infection, or malignancy, resulting in fluid with a higher protein content.
The accumulation of this excess fluid prevents the lung from fully expanding, leading to shortness of breath. Pleurisy, or pleuritis, involves the inflammation of the pleura itself. This inflammation causes the normally smooth membranes to become rough. When they rub against each other during breathing, it results in a sharp, localized chest pain that often worsens with deep inhalation. Viral infection is the most frequent cause, though many other underlying conditions can be responsible.