The question of what organs reside in the pleural cavity has a precise anatomical answer: the cavity itself is not filled with solid organs. Instead, the pleural cavity is a thin, fluid-filled space located within the larger thoracic cavity, or chest. It represents a “potential space” that exists between two layers of protective membrane surrounding the lungs.
Defining the Pleural Cavity: The Potential Space
The pleural cavity is an anatomical term for a space that is normally collapsed, much like an empty balloon that has not been inflated. This “potential space” exists between two layers of serous membrane known as the pleura. The outer layer, called the parietal pleura, is firmly attached to the inner surface of the chest wall, the top of the diaphragm, and the mediastinum, which is the central compartment of the chest.
The inner layer, the visceral pleura, is a continuous membrane that tightly adheres to the entire surface of each lung. These two layers are separated only by a microscopic film of fluid in a healthy state. The pleural cavity is defined by the boundaries of these two membranes, existing as a narrow cleft between the chest wall and the organ it protects.
The Lungs: Structures Enclosed by the Pleura
While the lungs are the primary organs contained within the overall pleural sac, they are technically outside the confines of the pleural cavity itself. This unique spatial relationship is often described using the analogy of a fist pushing into a partially inflated balloon. The fist represents the lung, and the two layers of the balloon represent the visceral and parietal pleura.
The lung is covered directly by the visceral pleura, which extends into the deep fissures that divide the lung into lobes. This arrangement ensures that the lung surface is completely protected and can move smoothly against the inner chest wall. The lungs perform the physiological task of gas exchange, taking oxygen from the air and transferring it to the bloodstream. The integrity of the pleural membranes is necessary to facilitate the continuous, unrestricted movement required for this respiratory function.
The Role of Pleural Fluid
The true content of the pleural cavity is a thin film of serous fluid, which is constantly produced and reabsorbed by the pleural membranes. In a healthy human, this fluid volume is minimal, often measuring only about 10 to 20 milliliters across the entirety of the space. This fluid serves two important mechanical functions necessary for effective respiration.
First, the fluid acts as a lubricant, allowing the visceral and parietal pleura to glide over one another with minimal friction during the cycle of inhalation and exhalation. Second, the fluid creates surface tension, which causes the two pleural layers to stick together, similar to two panes of wet glass. This adhesive force ensures that when the chest wall expands during inhalation, the lungs are pulled along with it to inflate.