The reader understands the mechanical properties of the lung and the chest wall during breathing.
- Describes the generation of a pressure gradient between the atmosphere and the alveoli.
- Describes the passive expansion and recoil of the alveoli.
- Defines the mechanical interaction of the lung and the chest wall, and relates this concept to the negative intrapleural pressure.
- Describes the pressure-volume characteristics of the lung and the chest wall, and predicts changes in the compliance of the lung and the chest wall in different physiologic and pathologic conditions.
- States the roles of pulmonary surfactant and alveolar interdependence in the recoil and expansion of the lung.
- Defines the functional residual capacity (FRC), and uses his or her understanding of lung-chest wall interactions to predict changes in FRC in different physiologic and pathologic conditions.
- Defines airways resistance and lists the factors that contribute to or alter the resistance to airflow.
- Describes the dynamic compression of airways during a forced expiration.
- Relates changes in the dynamic compliance of the lung to alterations in airways resistance.
- Lists the factors that contribute to the work of breathing.
- Predicts alterations in the work of breathing in different physiologic and pathologic states.
Air, like other fluids, moves from a region of higher pressure to one of lower pressure.
Therefore, for air to be moved into or out of the lungs, a pressure difference between the atmosphere and the alveoli must be established. If there is no pressure difference, no airflow will occur.
Under normal circumstances, inspiration is accomplished by causing alveolar pressure to fall below atmospheric pressure. When the mechanics of breathing are being discussed, atmospheric pressure is conventionally referred to as 0 cm H2O, so lowering alveolar pressure below atmospheric pressure is known as negative-pressure breathing. As soon as a pressure difference sufficient to overcome the resistance to airflow offered by the conducting airways is established between the atmosphere and the alveoli, air flows into the lungs. It is also possible to cause air to flow into the lungs by raising the pressure at the nose and mouth above alveolar pressure. This positive-pressure ventilation is generally used on patients unable to generate a sufficient pressure difference between the atmosphere and the alveoli by normal negative-pressure breathing. Air flows out of the lungs when alveolar pressure is sufficiently greater than atmospheric pressure to overcome the resistance to airflow offered by the conducting airways.
During normal negative-pressure breathing, alveolar pressure is made lower than atmospheric pressure. This is accomplished by causing the muscles of inspiration to contract, which increases the volume of the alveoli, thus lowering the alveolar pressure according to Boyle law. (See Appendix II: The Laws Governing the Behavior of Gases.)
Passive Expansion of Alveoli
The alveoli are not capable of expanding themselves. They only expand passively in response to an increased distending pressure across the alveolar wall. This increased transmural ...