The thoracic cavities house the lungs and heart. Proper lung function requires a protective, low-friction environment for continuous expansion and contraction. This environment is created by the pleura, which forms a thin space surrounding each lung. This article explores the normal contents of the pleural cavity, the physiological mechanisms they enable, and the consequences when the space is compromised by abnormal contents.
Defining the Pleural Space and its Linings
The pleural cavity is described anatomically as a “potential space.” This means that under normal conditions, the cavity is a very narrow gap, approximately 10 to 20 micrometers wide, separating two membranes. These membranes, collectively known as the pleura, are thin layers of serous tissue that form a closed sac around each lung.
The pleura is composed of two layers: the visceral pleura and the parietal pleura. The visceral pleura is the inner layer, adhering tightly to the surface of the lung, even dipping into the fissures between the lobes. The parietal pleura is the outer layer, lining the inside of the chest wall, the top of the diaphragm, and the sides of the mediastinum.
These two layers are continuous at the root of the lung. The primary role of this structure is to couple the movements of the chest wall to the lung tissue. This coupling, facilitated by the space’s contents, allows the lungs to be pulled open as the chest expands during inhalation.
The Essential Contents: Pleural Fluid and Pressure
The primary contents of the pleural cavity are pleural fluid and a specific pressure differential. Pleural fluid is an ultrafiltrate of plasma, derived from blood vessels, and is a clear, straw-colored liquid. The fluid is continuously produced by the parietal pleura and reabsorbed by the lymphatic system, maintaining a constant, small volume.
The total volume of pleural fluid is remarkably small, typically ranging from about 8 to 10 milliliters (mL) or approximately 0.1 to 0.2 mL per kilogram of body weight. This thin film acts as a lubricant, allowing the visceral and parietal pleura to slide smoothly past one another without friction as the lungs move. The fluid also contains hyaluronic acid, which contributes to its lubricating properties.
The pressure within the space, known as the intrapleural pressure, is more important than the fluid’s volume. This pressure is normally negative, meaning it is lower than the pressure within the lungs and the atmosphere. At rest, the intrapleural pressure is typically around -4 millimeters of mercury (mm Hg) relative to the atmosphere. This negative pressure is created by the opposing elastic forces of the chest wall pulling outward and the lung tissue recoiling inward.
The negative pressure acts like a suction cup, holding the lungs to the chest wall and preventing them from collapsing. When the chest wall expands during inspiration, the intrapleural pressure becomes even more negative, dropping to about -8 mm Hg. This lower pressure pulls the lungs open, creating a pressure gradient that draws air into the airways, enabling the mechanics of breathing.
When the Cavity Fills: Common Abnormal Conditions
When the balance of fluid and pressure is disrupted, the pleural cavity can fill with abnormal substances, leading to pathology. A condition called pleural effusion occurs when there is an excessive accumulation of fluid in the space. This fluid can be watery, known as a transudate, often caused by systemic pressure changes from conditions like heart failure or liver cirrhosis.
Alternatively, the fluid can be an exudate, which is thicker and rich in protein, often resulting from inflammation, infection, or malignancy (such as pneumonia or cancer). The excess fluid compresses the lung, impairing its ability to fully expand and causing shortness of breath.
A pneumothorax is another major disruption, defined as the accumulation of air or gas in the pleural space. Air can enter the cavity from a defect in the lung tissue or a puncture through the chest wall, instantly destroying the negative pressure. Without this pressure, the lung’s natural elastic recoil causes it to collapse, severely limiting breathing.
A hemothorax refers to the presence of blood, most commonly caused by trauma or injury to the chest. The blood accumulation not only compresses the lung like a pleural effusion but can also lead to significant blood loss. Pus accumulation, known as empyema, typically results from a severe infection, like pneumonia, spreading into the space.