Introduction
The surface of the inner wall of all of the body cavities is lined by a serous membrane which consists of a single layer of flat epithelium with a thin underlying propria (connective tissue). Within the thoracic cavity, this is known as the pleura. The visceral pleura which coats the outer surface of the lung is derived from the splanchnic mesoderm. The parietal pleura lining the thoracic cavity is derived from somatic mesoderm. The pleural cavity is a potential space between the two areas of pleural membrane, which normally are adhesed to each other.
Pleural Membranes
Structure of the Pleural Membranes
Each lung is placed within a separate layer of membrane, thus there are two pleural sacs. The space between the two sacs is known as the mediastinum, and is almost in the midline of the thorax. The pleura covering the surface of the lung is known as pulmonary pleura or visceral pleura. This becomes continuous with the mediastinal pleura, as it wraps around the lung. The diaphragmatic and costal pleura are continuous with the mediastinal pleura. Together, these three membranes are the parietal pleura.
Between the parietal and visceral pleura is the intrapleural space, or pleural cavity. This contains a small amount of serous fluid which establishes adhesion between the layers and allows smooth movement between the lung and chest wall, and between individual lobes of the lungs. The heart is invested in a parietal and visceral layer of pericardiumwith the pericardial space separating the two membranes. Cranially, the costal and mediastinal pleura adjoin to form a 'dome' of pleura which extends to the thoracic inlet. This is known as the cupulae pleurae. This region is only protected by soft tissue, and thus is vulnerable to injury.
Within the pleural sac encasing the right lung, there is a fold which encases the vena cava. It is known as the Plica Venae Cavae. The pleural membranes are larger than the lungs which they encase, thus there are areas where the facing surfaces of parietal pleura touch each other. These are known as pleural recesses. One example is the costodiaphragmatic recess, the space between the costal and diaphragmatic pleura. The volume of the recess varies to a point due to the phase of inspiration/expiration.
Pleural Membrane Function
The Pleural Cavity is filled with a small amount of serous fluid which forms a thin film of liquid between the pleural layers. This is vital in that it prevents separation of the two pleural layers and lubricates the surface, so the lungs can move easily within the thoracic cavity. The pleural fluid also provides surface tension, keeping the lung suitably close to the wall of the thorax, despite the lungs not being directly fixed to it. The pleurae thus allow the volume of the lungs to change with the volume of the thoracic cavity, enabling ventilation. The left and right pleural sacs are separate so that in the event of one being punctured, the other lung can continue to function effectively.
Function[edit]
The pleural cavity, with its associated pleurae, aids optimal functioning of the lungs during breathing. The pleural cavity also contains pleural fluid, which allows the pleurae to slide effortlessly against each other during ventilation. Surface tension of the pleural fluid also leads to close apposition of the lung surfaces with the chest wall. This relationship allows for greater inflation of the alveoli during breathing. The pleural cavity transmits movements of the ribs muscles to the lungs, particularly during heavy breathing. During inspiration the External Intercostals contracts, and so does the Diaphragm. This cause the expansion of the chest wall, that rise the volume of the lungs. A negative pressure is thus created and inhalation occurs.
The pleural space is located anatomically between the visceral membrane, which is firmly attached to the lungs, and the parietal membrane which is firmly attached to the chest wall (rib cage and intercostal muscles). The pleural space contains pleural fluid. This fluid holds the two membranes together by surface tension, as much as a drop of water between two sheets of glass prevents them from separating. Because of this, when the intercostal muscles move the ribcage outward, the lungs are pulled out as well, dropping the pressure in the lungs and pulling air into the bronchi, during inhalation
