Body membranes are thin sheets of tissue that cover surfaces, line cavities, and enclose organs. They serve various functions, including protection, secretion, and friction reduction. The pleural membrane is one such structure, playing a role in the respiratory system by surrounding the lungs. It is essential for the mechanics of breathing.
What is the Pleural Membrane?
The pleural membrane is a serous membrane, a type of tissue that lines body cavities and covers organs. It is a continuous, two-layered sac that surrounds each lung, defining a distinct compartment for each within the chest cavity.
It forms a protective sac around each lung, separating them from other structures in the thoracic cavity. There are two separate pleural cavities, one for each lung, which are not anatomically connected.
The Two Layers and Pleural Space
The pleural membrane consists of two layers: the parietal pleura and the visceral pleura. The parietal pleura forms the outer layer, attaching to the inner surface of the thoracic (chest) wall, the upper surface of the diaphragm, and the mediastinum (the central compartment of the chest containing the heart and other structures).
The visceral pleura is the inner layer, covering the surface of each lung and extending into the fissures that divide the lung into lobes. These two layers are continuous around the hilum, the area where blood vessels, nerves, and bronchi enter and exit the lung.
Between the parietal and visceral pleura lies a narrow gap known as the pleural cavity or pleural space. This potential space contains a small amount of pleural fluid, typically 10 to 20 milliliters. This fluid is secreted by the pleural layers.
How the Pleural Membrane Works
The pleural fluid within the pleural cavity serves as a lubricant, enabling the parietal and visceral pleura to glide smoothly over each other during the movements of breathing. This lubrication minimizes friction, preventing damage to the lung surfaces and the inner chest wall as the lungs expand and contract.
A negative pressure is maintained within the pleural cavity, slightly lower than atmospheric pressure. This creates a suction effect, pulling the two pleural layers into close apposition. This links the lungs to the chest wall and diaphragm.
During inhalation, the diaphragm contracts and flattens, and the chest muscles pull the rib cage upward and outward. Because of the adhesive force created by the negative pleural pressure, the lungs are pulled along with these movements, expanding and drawing air inward. This mechanism allows for efficient lung expansion and contraction, facilitating the exchange of gases.
The presence of separate pleural cavities for each lung also offers a protective benefit. Should one lung suffer an injury or infection, the enclosed nature of each pleural space helps to contain the issue, preventing it from spreading to the other lung.