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Learning Objectives
The student will be able to describe the normal physiology of pleural fluid formation and reabsorption.
The student will be to explain the diverse pathophysiological mechanisms responsible for development of pleural effusions.
The student will be able to define the categorization of parapneumonic pleural effusions and the stages of pleural space infections.
The student will be able to recognize the common symptoms and signs of pleural diseases, and the general principles of their treatment.
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Diseases of the pleural space are common in clinical medicine. One of the most frequent manifestations of pleural disease is a pleural effusion. Consequently, knowledge of the pathophysiological basis for disorders that culminate in pleural effusions is important for their timely diagnosis and proper treatment. Additional information regarding the laboratory evaluation of various types of pleural effusions is presented in Chap. 19.
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PHYSIOLOGY OF THE PLEURAL SPACE
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The pleural space is a low-pressure environment between the parietal pleura that covers the inner surface of the ribs and thoracic musculature, and the visceral pleura covering the lungs' external regions. These have a combined surface area of about 4,000 cm2. The forces regulating pleural fluid formation by the parietal and visceral pleura are summarized by the Starling's equation (as established in Chaps. 7 and 19):
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Notably, the protein reflection coefficient σ defines the barrier function of the pleural membranes, such that decreases in σ typified by inflammatory conditions correlate with increased permeability to protein (see also Chap. 28). The intravascular concentration of albumin (MW ≈ 65 KDa) is the primary determinant of serum colloid osmotic pressure.
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The normal entry rate of pleural fluid into the pleural space in humans is considered to be approximately 0.5 mL/h or 12 mL/day, and this pleural fluid derives from microvascular filtration governed by Starling's forces across both the parietal pleura and visceral pleura. Although a degree of fluid reabsorption occurs within both the parietal and visceral pleura membranes, actual pleural fluid reabsorption from the pleural space itself occurs via lymphatic stomata in the parietal pleura (Fig. 29.1). Thus, at any time the total volume of pleural fluid in normal individuals is 0.1-0.2 mL/kg, or approximately 10-20 mL in a 70 kg subject. These parietal pleural lymphatic stomata are fenestrated openings in the mesothelial cell layer that average 10-12 μm in diameter, and are especially prevalent in dependent regions of the pleural space, especially on the diaphragmatic surface and in the mediastinal regions (Fig. 29.2).
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