• The visceral and parietal pleura differ both anatomically and functionally. These differences determine the physiology of fluid transport across the pleural space.
• The gradient of partial pressure of gas between the parietal pleura and venous blood keeps the pleural space free of gas.
• Pleural effusions result when a breakdown in the balance between hydrostatic and cellular osmotic forces occurs.
• Upright chest radiography may detect pleural effusions larger than 300 ml. Decubitus views increase the sensitivity. Ultrasonography is more sensitive than chest radiography. Effusions as small as 2 ml may be detectable.
• Pleural fluid can be categorized as transudate or exudate by protein content, specific gravity, and pleural fluid to serum lactate dehydrogenase and protein ratios. Low pleural fluid glucose and low pH are characteristic of empyema.
• Evaluation of pleural white cell counts may help to differentiate bacterial infections from tuberculosis and neoplasm. Gram stains and cultures of pleural fluid are more reliable than sputum cultures for determining the etiology of an underlying pneumonia.
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