Static Lung and Chest Wall Mechanics

To achieve its primary function of gas exchange, air must be moved into and out of the lung. The mechanical properties of the lung and chest wall determine the ease or difficulty of this bulk air movement. Lung mechanics is the study of the mechanical properties of the lung and chest wall (including the diaphragm , abdominal cavity , and anterior abdominal muscles) . Lung mechanics is important for understanding normal lung function, and how disease disrupts this normal function. Most lung diseases affect the mechanical properties of the lungs, chest wall, or both. In addition, death from lung disease is almost always due to respiratory muscle fatigue, which results from an inability of the respiratory muscles to overcome the altered mechanical properties of the lungs, chest wall, or both. Lung mechanics includes static mechanics (the mechanical properties of a lung whose volume is not changing with time) and dynamic mechanics (properties of a lung whose volume is changing with time). Dynamic mechanics of the lung and chest wall are described in Chapter 22 .

How a Pressure Gradient Is Created

Air flows into and out of the lungs from areas of higher pressure to areas of lower pressure. In the absence of a pressure gradient, there is no airflow. Thus, at the end of inspiration and at the end of exhalation, which are periods of time when there is no airflow, alveolar pressure (P _A ) is the same as atmospheric pressure (P _b ), and there is no pressure gradient (P _b − P _A = 0). Pleural pressure at these same times, however, is not 0. Before inspiration begins, the pleural pressure in normal individuals is approximately −3 to −5 cm H ₂ O. Therefore, the pressure in the pleural space is negative in relation to atmospheric pressure. This negative pressure is created by the inward elastic recoil pressure of the lung, and it acts to “pull the lung” away from the chest wall. The lung is not able, however, to pull away from the chest wall, inasmuch as the two function as a unit. Thus, the inward elastic recoil pressure of the lung is balanced by the outward recoil of the chest wall.

With the onset of inspiration, the muscles of the diaphragm and chest wall contract, which causes a downward movement of the diaphragm and an outward and upward movement of the rib cage. As a result, pleural pressure decreases during inspiration. This negative pleural pressure is transmitted across the lung tissue and results in a decrease in alveolar pressure. As alveolar pressure decreases below 0 (i.e., from atmospheric pressure to below atmospheric pressure), air moves into the airways when the glottis is open. As air flows into the airways to the alveoli, the pressure gradient along the airways decreases, and flow stops when there is no longer a pressure gradient between the atmosphere and the alveoli. The decrease in pleural pressure at the start of inspiration secondary to inspiratory muscle contraction is greater than the transmitted fall in alveolar pressure, and, as a result, transpulmonary pressure at the start of inspiration is positive (see Eq. 21.1 ). Positive transpulmonary pressure is necessary to increase lung volume, and lung volume increases with increasing transpulmonary pressure ( Fig. 21.1 ). Similarly, during inspiration, the chest wall expands to a larger volume. Because pleural pressure is negative in relation to atmospheric pressure during quiet breathing, the transmural pressure across the chest wall is negative (see Eq. 21.2 ).

On exhalation, the diaphragm moves higher into the chest, pleural pressure increases (i.e., becomes less negative), alveolar pressure becomes positive, the glottis opens, and gas again flows from a higher (alveolar) pressure to a lower (atmospheric) pressure. In the alveoli, the driving force for exhalation is the sum of the elastic recoil of the lungs and pleural pressure (see Chapter 22 ). This relationship between changes in pressure, changes in airflow, and changes in volume during inspiration and exhalation is displayed in Fig. 21.2 . During tidal volume breathing in normal individuals, the decrease in alveolar pressure at the start of inspiration is small (1–3 cm H ₂ O). It is much larger in individuals with airway obstruction because of the larger pressure drop that occurs across obstructed airways. Airflow stops in the absence of a pressure gradient, which occurs whenever alveolar pressure and atmospheric pressure are equal.