The lungs are protected by layered membranes collectively known as the pleura. The pleura functions as a protective lining for the chest cavity and a smooth covering for the lungs, ensuring that the continuous motion of breathing occurs without friction. The outer layer is the parietal pleura, which serves as the interface between the respiratory system and the protective rib cage.
Defining the Parietal Pleura: Location and Structure
The parietal pleura is a thin, serous membrane forming the outer boundary of the pleural sac surrounding each lung. It is distinct from the inner visceral pleura, which adheres directly to the lung surface. The parietal layer is firmly attached to the internal surfaces of the chest wall, providing a fixed lining for the entire thoracic cavity.
The outer membrane is subdivided into four distinct regions:
- The Costal pleura lines the inner surfaces of the ribs, sternum, and intercostal muscles.
- The Diaphragmatic pleura covers the superior surface of the diaphragm.
- The Mediastinal pleura runs along the lateral walls of the mediastinum, the central compartment housing the heart and major blood vessels.
- The Cervical pleura (cupula) extends superiorly into the neck, covering the apex of the lung above the first rib.
The parietal pleura is composed of simple squamous epithelial cells (mesothelium) supported by connective tissue. This layer receives a rich supply of somatic nerves, primarily from the intercostal and phrenic nerves. This somatic innervation makes the parietal pleura highly sensitive to stimuli like pain, temperature, and pressure, unlike the visceral pleura, which is largely insensitive to pain.
The Mechanics of Respiration and Pleural Function
The parietal pleura works with the visceral pleura to facilitate pulmonary ventilation. The layers are separated by the pleural cavity, which contains only 10 to 20 milliliters of pleural fluid. This fluid enables the layers to glide over one another without friction during the breathing cycle.
The fluid creates strong surface tension, causing the parietal pleura (fixed to the chest wall) to cling tightly to the visceral pleura (covering the lung). This mechanical coupling maintains the necessary negative pressure within the pleural space.
This negative intrapleural pressure acts as a suction force, preventing the lungs from collapsing due to their natural elastic recoil. When the chest wall expands during inhalation, the parietal pleura is pulled outward. The surface tension immediately drags the visceral pleura and lung tissue with it, allowing air to rush in.
The parietal pleura regulates the volume of pleural fluid through its vascular and lymphatic systems. The fluid is a plasma filtrate that continuously forms from systemic microvessels embedded within the parietal pleura. It is also the main site for fluid absorption.
Specialized openings called stomata lead directly into the lymphatic vessels, which drain the fluid and cellular debris from the pleural space. Approximately 75% of the fluid is reabsorbed through these channels. This maintains a dynamic equilibrium that keeps the fluid volume low and stable for optimal respiratory function.
Common Conditions Involving the Parietal Pleura
Disruptions to the pleural system often involve the parietal pleura, leading to noticeable symptoms. Pleurisy (pleuritis) is inflammation of the pleural membranes. This condition causes sharp, localized chest pain that intensifies with deep breathing, coughing, or sneezing.
The intense pain of pleurisy is directly attributable to the somatic nerve supply of the parietal pleura. When the inflamed layers rub against each other, the pain-sensitive nerves transmit the sharp sensation.
Another frequent condition is pleural effusion, the pathological accumulation of excess fluid in the pleural space. This occurs when fluid production exceeds the rate of lymphatic drainage, a function controlled by the parietal pleura. Effusions can stem from systemic conditions, such as heart failure, or from local inflammation, like pneumonia, which increases permeability.
A pneumothorax, or collapsed lung, represents a breach in the integrity of the pleural space, allowing air to enter. Air accumulation eliminates the surface tension created by the pleural fluid, breaking the mechanical link between the parietal and visceral pleura. Without the parietal layer pulling the lung outward, the lung recoils and collapses.