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Normally, the pressure within the pleural cavity is slightly less than the atmospheric pressure, which is known as negative pressure. [1] When the pleural cavity is damaged or ruptured and the intrapleural pressure becomes greater than the atmospheric pressure, pneumothorax may ensue. Intrapleural pressure is different from intrathoracic pressure.
The pleural cavity, or pleural space (or sometimes intrapleural space), is the potential space between the pleurae of the pleural sac that surrounds each lung.A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication between the membranes, and also to create a pressure gradient.
Transpulmonary pressure is the difference between the alveolar pressure and the intrapleural pressure in the pleural cavity. During human ventilation, air flows because of pressure gradients. P tp = P alv – P ip. Where P tp is transpulmonary pressure, P alv is alveolar pressure, and P ip is intrapleural pressure.
Alveolar pressure (P alv) is the pressure of air inside the lung alveoli. When the glottis is opened and no air is flowing into or out of the lungs, alveolar pressure is equal to the atmospheric pressure, that is, zero cmH 2 O .
Once air enters the pleural cavity, the intrapleural pressure increases, resulting in the difference between the intrapulmonary pressure and the intrapleural pressure (defined as the transpulmonary pressure) to equal zero, which cause the lungs to deflate in contrast to a normal transpulmonary pressure of ~4 mm Hg. [28]
Doing the same at 5500 m, where the atmospheric pressure is only 50 kPa, the intrapulmonary air pressure falls to 25 kPa. Therefore, the same change in lung volume at sea level results in a 50 kPa difference in pressure between the ambient air and the intrapulmonary air, whereas it result in a difference of only 25 kPa at 5500 m.
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Relaxing the diaphragm during expiration allows the lungs to recoil and regain the intrapleural pressure experienced previously at rest. Elastic recoil is inversely related to lung compliance . This phenomenon occurs because of the elastin in the elastic fibers in the connective tissue of the lungs, and because of the surface tension of the ...