Physiology of the pleural space

Abstract

The pleural cavity is created between the 4^th and 7^th week of embryologic development. These embryonic components of visceral and parietal pleurae develop different anatomic characteristics with regard to vascular, lymphatic, and nervous supply. There are two layers: a superficial mesothelial cell layer facing the pleural space and an underlying connective tissue layer. The pleura might present inflammatory response and maintenance of the pleural fluid is observed. The latter function is especially important in the mechanical coupling of the lung and chest wall. Fluid is filtered into the pleural space according to the net hydrostatic oncotic pressure gradient. It flows downward along a vertical pressure gradient, presumably determined by hydrostatic pressure and resistance to viscous flow. There also may be a net movement of fluid from the costal pleura to the mediastinal and interlobar regions. In these areas, pleural fluid is resorbed primarily through lymphatic stomata on the parietal pleural surface. In the current review we will present the physiology of the pleural space in a step by step manner.

The pleural cavity is created between the 4-7 weeks of embryologic development and is lined by the splanchnopleurae and somatopleurae. These embryonic components of visceral and parietal pleurae develop different anatomic characteristics. Both pleurae have two layers. A superficilial mesothelial cell layer facing the pleural space and an underlying connective tissue layer (1-10). There is a relationship of the functions of the pleural membranes—local inflammatory response and maintenance of the pleural fluid. The fluid in the pleural space transmits transpleural forces involved in normal respiration. It flows downward along a vertical pressure gradient presumably determined by hydrostatic pressure. There a net movement of fluid from the costal pleura to the mediastinal (from lymphatic stomata on the parietal pleura surface).

A primitive body cavity with stretchable mesothelial cells endows the subsequently developed internal organs a great flexibility to expand. The lung is maintained in inflated state by the mechanical coupling between the lung and the chest wall. Normal mesothelial cells are fragile in the air (11-20). The activated mesothelial cells are resilient and rich in organelles and enzymes. Fluid and electrolytes permeate freely between normal mesothelial cells (Starling’s law) with the endothelium as the main barrier. Proteins and cells are removed mainly from the preformed stomas and the lymphatic lacuna present in the lower mediastinum—portions of the diaphragm (21-30). This removal of the pleura fluid and particles by the lymphatic route is enhanced by the respiratory movements. Kampmeier’s foci are conglomerates of activated mesothelial and lymphoreticular cells with central capillary and lymphatic vessels. They impede direct from the pleural cavity into chest wall and mediastinum (31-40).

Pneumothorax refers to air in the pleural sac. It may occur in the absence of known pulmonary disease or as a result of some thoracic or lung disorder. Secondary, it occurs with rupture of any pulmonary lesion situated close to the pleural surface allows inspired air to gain access to the pleural cavity. There are several possible complications of pneumothorax. A ball-valve leak may create a tension pneumothorax that shifts the mediastinum. Compromise of the pulmonary circulation may follow and may even be fatal. If the leak seals and the lung are not reexpanded within a few weeks, enough scarring may occur so that it can never be fully reexpanded. With prolonged collapse, the lung becomes vulnerable to infection, as does the pleura cavity when communication between it and the lung persists (32,41-48).

Pneumothorax tends to be recurrent. This is understandable when it complicates other pulmonary disease because the predisposing condition remains (Figures 1,2,3,4,5,6,7,8,9,10,11).

Figure 1.

Pleura pathology.

Figure 2.

Pleura pathology (continue).

Figure 3.

Pleura recesses.

Figure 4.

Physiology of the pleura.

Figure 5.

Mechanism of breathing showing: inspiration-exhalation.

Figure 6.

Mechanism of breathing showing: (A) inspiration; (B) exhalation.

Figure 7.

Types of pneumothorax.

Figure 8.

Closed (A) and open pneumothorax (B).

Figure 9.

Pneumothorax; inspiration and exhalation.

Figure 10.

Tensioned pneumothorax.

Figure 11.

Thorax anatomies.