Mechanisms of pleural liquid turnover in the normal state
- V Courtney Broaddus, MD
V Courtney Broaddus, MD
- Section Editor — Pleural Disease
- Professor of Medicine
- University of California San Francisco
The pleural space is bounded by the parietal and visceral membranes covered by a continuous layer of pleural mesothelial cells. Studies of pleural liquid dynamics in the normal pleural space are limited. Available data indicate that pleural fluid is formed from the systemic vessels of the pleural membranes at an approximate rate of 0.6 mL/h and is absorbed at a similar rate by the parietal pleural lymphatic system. Normally, the pleural spaces contain approximately 0.25 mL/kg of low protein liquid. Disturbances in either formation or absorption result in the accumulation of excess pleural fluid . (See "Mechanisms of pleural liquid accumulation in disease".)
The pleural space is a real, not potential, space that is approximately 10 to 20 micrometers wide and extends completely around the lung to the hilar root (image 1) [2-4]. The visceral pleura covers the lung and interlobar fissures; the parietal pleura covers the chest wall, diaphragm, and mediastinum. Each pleural interface has a surface area of approximately 1000 cm2. Each membrane is covered by a single cell layer of mesothelial cells and each (at least in humans and large mammals) is supplied by a systemic circulation.
The visceral and parietal pleural membranes differ in one important respect: only the parietal pleura has lymphatic stomata that open directly into the pleural space. Current concepts of pleural liquid turnover have stemmed in large part from considering the differential anatomy of the two pleural membranes .
Parietal pleura — The parietal pleura has been proposed as the more important pleura for pleural liquid turnover in the normal physiologic state . The reasons for this hypothesis lie in the proximity of the microvessels to the pleural space, the presence of the lymphatic stomata, and the consistent anatomy of the parietal pleura among species. The parietal pleural membrane overlies the intercostal fascial layer and ribs. It is approximately 30 to 40 micrometers thick, consisting of a superficial mesothelial layer and subpleural layer (picture 1) . Within the subpleural layer lies loose connective tissue in which run the intercostal arteries, nerves and lymphatics. The intercostal microvessels are about 10 to 12 micrometers from the pleural space.
The most interesting and unusual features of the parietal pleura are the lymphatic stomata, holes of 2 to 6 micrometers in diameter that open onto the pleural space (picture 2) [6,7]. These measurements were obtained in the resting state; the diameters likely increase with chest expansion during ventilation. The stomata have been demonstrated on the parietal pleural surface with scanning electron microscopy. Each stoma is formed by a gap in the otherwise continuous mesothelial cell layer, where the mesothelial cells join with the endothelial cells of the lymphatics. Each lymphatic joins others, forming a lake or lacuna (picture 3); from the lacunae, collecting lymphatics join intercostal trunk lymphatics, which travel to the parasternal and periaortic lymph nodes.
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