Pleural effusion pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Prince Tano Djan, BSc, MBChB [2]; Nate Michalak, B.A.



The lungs are surrounded by two membranes, the outer parietal pleura is attached to the chest wall and the inner visceral pleura is attached to the lung. [1] The two layers are continuous with one another. In between the two is a thin space known as the pleural cavity or pleural space. This is filled with pleural fluid; a serous fluid produced by the pleura. Each pleural membrane is made up of a layer of mesothelial cells,[2] basement membrane , connective tissue, microvessels and lymphatics.[2] The parietal pleura have lymphatic stomata, of 2 to 10 µm in diameter that open onto the pleural space. The pleural fluid lubricates the [[pleura]l surfaces and allows the layers of pleura to slide against each other easily during respiration. It also provides the surface tension that keeps the lung surface in contact with the chest wall. During quiet breathing, the cavity normally experiences a negative pressure (compared to the atmosphere) which helps to adhere the lungs to the chest wall, so that movements of the chest wall during breathing are coupled closely to movements of the lungs. The pleural membrane also helps to keep the two lungs away from each other and air tight, thus if one lung is punctured and collapses due to an accident, the other pleural cavity will still be air tight, and the other lung will work normally.

The parietal pleura has different names depending on its location,[3] namely costal pleura, diaphragmatic pleura; cervical pleura, and mediastinal pleura. The visceral pleura covers the surface of lungs, including the interlobar fissures. There is no anatomical connection between the left and the right pleural cavities so in cases of pneumothorax, the other hemithorax will still be able to function normally. The parietal pleura is supplied with blood from systemic circulation (intercostals, internal thoracic and musculophrenic) its veins join the systemic veins in the neighboring parts of the chest wall; it contains sensitive nerves (derived from the intercostals nerves and from the phrenic nerve) and cells with a dense cilliary layer.

The visceral pleura is supplied with blood from bronchial artery and from the pulmonary artery which divides into a net work of very delicate capillaries. Its nerve supply is derived from the autonomic nerves innervating the lung and accompanying the bronchial vessels. The lymphatic drainage of parietal and visceral pleura differs from each other.[4] The mesothelial surface of parietal pleura is permeated by stomas that connect via lacunae to a lymphatic network in the adjacent submesothelial layers. Costal surface of the parietal pleura drains to parasternal and para vertebral nodes, while diaphragmatic surface drains to the tracheobronchial nodes. The visceral pleura are devoid of lacunas and stomas and the underlying lymphatic vessels appear to drain the pulmonary parenchyma rather than the pleural space.

The parietal pleura has been proposed as the more important pleura for pleural liquid turnover in the normal physiologic state in absence of disease.[2] Its microvessels are closer to the pleural surface and perfusion pressure is likely higher than the visceral pleura. It is approximately 30 to 40 µm thick. Pleural fluid is filtered across the parietal mesothelium in the top of the pleural cavity and removed by lymphatic stomatas in the more dependent mediastinal and diaphragmatic regions.[5] The pleural lymphatics act as a feedback system that regulates pleural liquid volume and its protein composition around a low volume set point.

Healthy individuals have less than 15 ml of fluid in each pleural space. Normally, fluid enters the pleural space from the capillaries in the parietal pleura, from interstitial spaces of the lung via the visceral pleura, or from the peritoneal cavity through small holes in the diaphragm. This fluid is normally removed by lymphatics in the visceral pleura, which have the capacity to absorb 20 times more fluid than is normally formed. When this capacity is overwhelmed, either through excess formation or decreased lymphatic absorption, a pleural effusion develops.[6] [7] [8] [9]


Pleural effusion results either from increased pleural fluid formation or decreased exit of fluid.

Increased Pleural Fluid Formation

  • Increased capillary permeability (e.g. infection, neoplasm)
  • Passage of fluid through openings in diaphragm (e.g. cirrhosis with ascites)

Decreased Fluid Exit

  • Reflects a reduction in lymphatic function.
  • There are intrinsic and extrinsic factors.
  1. Intrinsic
  2. Extrinsic
    • Limitation of respiratory motion (diaphragm paralysis, lung collapse), compression of lymphatics (pleural fibrosis, pleural granulomas), blockage (pleural malignancy), or acute increased in systemic venous pressure (superior vena cava syndrome). In chronic increase in systemic venous pressure, the lymphatics can adapt.

Postoperative Pleural Effusion in Patients Undergoing Cardiac Surgical Procedures:

  • Predominantly left-sided: suggestive of underlying pericarditis as a causative factor
  • Majority are small and not serious
  • Pathogenesis may relate to immunologic cause
  • Respond to steroids and prolonged latent period from injury to onset

Types of Fluids

Four types of fluids may accumulate in the pleural space:


  1. Goss CM (ed): Gray's anatomy of the human body, American ed 29. Philadelphia, Lea & Febiger, 1973, pg 66, isbn: 0812103777
  2. 2.0 2.1 2.2 Albertine KH, Wiener-Kronish JP, Staub NC (1984). "The structure of the parietal pleura and its relationship to pleural liquid dynamics in sheep". Anat Rec. 208 (3): 401–9. doi:10.1002/ar.1092080310. PMID 6721233.
  3. Goss CM (ed): Gray's anatomy of the human body, American ed 29. Philadelphia, Lea & Febiger, 1973, pg 66, isbn: 0812103777
  4. Courtice FC, Summonds WJ: Physiological significance of lymph drainage of the serous cavities and lungs, Physiol Rev 34:419, 1954
  5. Courtice FC, Summonds WJ: Physiological significance of lymph drainage of the serous cavities and lungs, Physiol Rev 34:419, 1954
  6. Irwin, Jame (1985). Internsive care medicine. Little, Brown and Company Boston/Toronto. p. 58-59. ISBN 0316747106. Unknown parameter |First= ignored (|first= suggested) (help)
  7. Courtice FC, Summonds WJ: Physiological significance of lymph drainage of the serous cavities and lungs, Physiol Rev 34:419, 1954
  8. Yoffey JM, Courtice FC, Lymphatics, lymph and lymphoid tissue. Cambridge, Mass, Harvard University press, 1956, p 510
  9. Broaddus VC, Light RW. Pleural Effusion. In: Murray & Nadel’s Textbook of Respiratory Medicine, 6th ed, Elsevier, 2016. p.1396.

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