The Pleura

The pleura consists of a double layer of glistening, semitransparent serousmembranes, which surround the pleural space.  (web ref) (web ref 02) lumen)The secreting surfaces face each other across the pleural space.  One of these layers, the visceral pleura, is intimately attached to the lung and follows the fissures, while the second component, the parietal layer, is intimately related to the thoracic cage mediastinum and diaphragm. There is no communication between the right and left pleural spaces. The pleura have elastic qualities in order to accommodate the changes in size with the expansion and contraction of the chest cavity.  During the respiratory cycle the two layers of pleura move in tandem gliding smoothly over each other, lubricated by a small amount of serous fluid. As the chest wall expands during inspiration, the parietal pleura moves with it, while the visceral pleura moves with the expansion of the lung, the two being held together by the negative pressure caused by capillary forces of the fluid that lies between the two smooth surfaces. (Try the experiment of putting a small film of water between two clean, smooth, and flat glass surfaces. The two cannot be easily separated, unless a small amount of air can be introduced between the opposing surfaces

The cells lining the pleura are called mesothelial cells.  They are responsible for the production of pleural fluid.  Although we describe two components to the pleura, it is really one structure into which the developing lung bud grows, similar to a fist pushing its way into a balloon.  At the hilum of the lung, the bronchovascular and lymphatic bundles acting as the fist, push their way into the pleura and become enveloped by the pleura, with the most distal progeny of the lung bud coming into intimate contact with the visceral pleura. 

Despite the apparent single embryologic origin of the visceral and parietal pleura, they are in fact functionally and structurally different.  The visceral pleura for example has no pain fibers whereas the parietal pleura is extremely sensitive to pain.  The visceral pleura receives its nerve supply through the autonomic nervous system, whereas the parietal pleural innervation is via the somatic intercostal nerves and the phrenic nerve. 

The blood supply of the visceral and parietal pleura is also different.  The parietal pleura is supplied only  by the systemic circulation from the  intercostal arteries while the visceral pleura is supplied by the bronchial arteries and the pulmonary arteries.  

The visceral pleura follows the lung surface and therefore will follow the fissures. The parietal pleura follows the chest wall and therefore will not follow the fissures of the lung.   The pleural cavity is larger than the total lung volume, allowing the lung to expand during respiration. It is particularly obvious in the pleural recesses at the inferior aspect of the thoracic cavity.

For the interventionalist it is important to remember how caudal the pleural space extends and therefore in planning a liver biopsy or drainage the low lying costophrenic sulcus that may fill with lung on inspiration must be taken into account.          

  

The visceral pleura is  subdivided into three distinct parts, including the  mediastinal, the diaphragmatic, and the costal pleura.
There is continued production of pleural fluid by the mesothelial cells into the pleural space.  There is also a continued drainage of the fluid by the sub pleural lymphatic system which lies between the visceral pleura and lung coursing into the interlobular septa.

Applied Anatomy

On a plain film of the chest the easiest method of identifying fluid is the lateral upright examination where fluid accumulates in the posterior pleural recesses and causes “blunting” of the posterior costophrenic angles. CT is far more sensitive in the detection of the fluid and to some extent in the characterization of the fluid.  When the fluid layers with a meniscus that is parallel to the floor and falls into expected dependent positions (just as water would in a glass on a flat table top) then the effusion can be expected to be free flowing.  If the fluid falls in a non-dependent fashion it is considered “loculated” and it is more likely to be complex. Under these circumstances the level is tilted.  Ultrasound however is better able to identify septations and other soft tissue components of the fluid collection. Thoracentesis however is still necessary to enable the chemical evaluation of the fluid.

When there is an accumulation of fluid in the pleural space (pleural effusion) or air (pneumothorax) the underlying lung gets pushed away from the chest wall and the harmonious interaction of chest wall pleura and lung is lost.  As the accumulation gets larger, the underlying lung collapses without the support mechanisms, and functional lung tissue is lost, with ensuing respiratory difficulty. The lungs are relatively forgiving in that they have enough reserve to accommodate effusions (even large) and moderate degrees of atelectasis, particularly when this occurs gradually, enabling compensatory mechanisms to evolve.

   

Tension pneumothorax

Tension pneumothorax is an acute life threatening event, where there is not only collapse of the lung from the pneumothorax but a pressure build up in the pleural space that causes the heart and great vessels to be compressed. This acute loss of support of the system, is life threatening and if not treated by emergency decompression will lead to death. It would only take a small open-ended needle placed into the pleural space to decompress the tension and save a life. Sometimes when a patient suddenly collapses, and the condition of tension pneumothorax is a clinical consideration, physicians emergently put a small needle into the chest wall even before the chest X-ray confirms their clinical suspicions. Many a life, hanging on with but a thread, has been saved by a quick thinking, astute, and an artful doctor, who with a single stick of a simple needle, turned the life on the thread back to health.

 

 

CLINICAL ASPECTS

Acute inflammatory conditions of the pleura, results in pleuritic chest pain. When the patient inspires the inflamed pleural surfaces rub against each other causing exquisite localized pain. The motion of the pleura becomes restricted by fibrinous exudates, and may cause a pleural “rub” – a squeaking sound caused by the restricted motion. As the pleural effusion enlarges the chances of contact of the two surfaces becomes less likely, and hence the pleural rub cannot be heard. Instead the large effusion compresses the adjacent lung causing a condition called “compressive atelectasis.”  Percussion of the fluid results in dullness and auscultation results in amplification of sound called egophony.

In the laboratory it is important to distinguish between a transudative effusion and an exudative effusion. This is done by evaluating the concentration of protein in the fluid. A transudate usually results from pulmonary congestion and contains limited protein and no fibrin, whereas an exudate is more complex and contains fibrin. Empyema is an infected pleural effusion and one of its hallmarks is the relatively low PH of the fluid.