The visceral and parietal pleurae are normally flush against each other, so that the lungs are stuck to the chest wall in the same manner as two wet pieces of glass sticking to each other. The intrapleural space contains only a film of fluid secreted by the two membranes. The pleural cavity in a healthy person is thus potential rather than real; it can become real only in abnormal situations when air enters the intrapleural space. Since the lungs normally remain in contact with the chest wall, they expand and contract along with the thoracic cavity during respiratory movements.
Air enters the lungs during inspiration because the atmospheric pressure is greater than the intrapulmonary, or intra-alveolar, pressure. Since the atmospheric pressure does not usually change, the intrapulmonary pressure must fall below atmospheric pressure to cause inspiration. A pressure below that of the atmosphere is called a subatmospheric pressure, or negative pressure. During quiet inspiration, for example, the intrapulmonary pressure may decrease to 3 mmHg below the pressure of the atmosphere. This subatmospheric pressure is shown as -3 mmHg. Expiration, conversely, occurs when the intrapulmonary pressure is greater than the atmospheric pressure. During quiet expiration, for example, the intrapulmonary pressure may rise to at least +3 mmHg over the atmospheric pressure.
The lack of air in the intrapleural space produces a sub-atmospheric intrapleural pressure that is lower than the intrapul-monary pressure (table 16.1). There is thus a pressure difference across the wall of the lung—called the transpulmonary (or transmural) pressure—which is the difference between the intrapulmonary pressure and the intrapleural pressure. Since the pressure within the lungs (intrapulmonary pressure) is greater than that outside the lungs (intrapleural pressure), the difference in pressure (transpulmonary pressure) keeps the lungs against the chest wall. Thus, the changes in lung volume parallel changes in thoracic volume during inspiration and expiration.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.