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Pleural Effusions
Pleural Effusions
Healthy Pleural Space & Pleural Fluid
We draw a lung and label the visceral pleura, which is directly in contact with the lung tissue (recall that "viscera" = organ) and, the parietal pleura, which is the outer, more fibrous covering. Although not shown here, be aware that the parietal and visceral pleura are continuous with each other.
Pleural Details
The visceral and parietal pleura are separated by a thin layer of fluid that resides in the pleural space (which is exaggerated for our purposes).
Both the visceral and parietal pleura comprise a thin layer of mesothelial cells and underlying connective tissue.
Capillaries and lymphatic vessels lie in the connective tissue; the lymphatic vessels open to the pleural space through the parietal pleura; these openings are called stomata.
The capillary and lymphatic vessels are vital to the production and reabsorption of pleural fluid.
The majority of the pleural fluid is formed by the parietal pleura; this is a low-protein ultrafiltrate that comes from the parietal capillaries.
The majority of pleural fluid resorption occurs via drainage by parietal lymphatic vessels (via the stomata).
The normal volume of fluid within the pleural space is 10-20 mL, (the volume depends on a person's size), and that it has a low protein concentration.
Thus, disorders that alter the formation of ultrafiltrate or reabsorption can produce pleural effusions.
EFFUSIONS
Transudate effusions are the result of increased hydrostatic pressure and/or decreased plasma oncotic pressure, which will alter the formation of ultrafiltrate from the capillaries (as described by the Starling equation).
Because this occurs in the presence of intact vessels, the fluid is generally clear.
Key causes include heart failure (which increases hydrostatic pressure), cirrhosis, and nephrotic syndrome (which reduces plasma oncotic pressure).
Notice that none of these etiologies involves direct damage to the lung parenchyma.
Exudative effusions, on the other hand, are the result of damage to the lung tissue.
Exudative effusions occur when there is increased capillary permeability, for example, due to inflammation, which allows larger molecules to leak into the pleural space.
In contrast to the fluid in transudate effusions, exudative effusions produce fluid that is cloudy with proteins and cell contents.
Key causes include pneumonia, pulmonary embolism, cancers, Tuberculosis, and other infections of the lungs.
Specific types of effusions based on their contents:
- Hemothorax (blood in the pleural space; beware of clotting)
- Chylothorax (exudate rich in triglycerides)
- Chyliform, aka, pseudo-chylothroax (rich in cholesterol)
- Empyema, which is pus (think of infections)