Define the different volumes in the lung, and describe how they are measured.
4.
Explain how static lung mechanics determines lung volumes.
5.
Define lung compliance.
6.
Explain how surfactant affects lung compliance, and describe its importance in maintaining unequal alveolar volumes.
o achieve its primary function of gas exchange, air must be moved in and out of the lung. The mechanical properties of the lung and chest wall determine the ease or difficulty of this 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 how the lungs work both normally and in the presence of disease, inasmuch as 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.
Pressures in the Respiratory System
In healthy people, the lungs and chest wall move together as a unit. Between these structures is the pleural space, which under normal conditions is best thought of as a potential (or virtual) space. Because the lungs and chest wall move together, changes in their respective volumes are equal during inspiration and exhalation. Volume changes in the lungs and chest wall are driven by changes in the surrounding pressure. In accordance with convention, pressures inside the lungs and chest wall are referenced in relation to atmospheric pressure, which is considered 0. Thus a negative pressure in the pleural space is a pressure that is lower than atmospheric pressure. Also in accordance with convention, pressures across surfaces such as the lungs or chest wall have been defined as the difference between the pressure inside and the pressure outside the surface. The pressure differences across the lung and across the chest wall are defined as the transmural (across a wall or surface) pressures. For the lung, this transmural pressure is called the transpulmonary (or translung) pressure (PL), and it is defined as the pressure difference between the air spaces (alveolar pressure [PA]) and the pressure surrounding the lung (pleural pressure [Ppl]):
Equation 21.1
The transmural pressure across the chest wall (Pw) is the difference between pleural (inside) pressure (Ppl) and the pressure surrounding the chest wall (Pb), which is the atmospheric pressure or body surface pressure:
Equation 21.2
The pressure across the respiratory system (Prs) is the sum of the pressure across the lung and the pressure across the chest wall:
Equation 21.3
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 end inspiration and at the end of exhalation, which are periods of time when there is no airflow, alveolar pressure (PA) is the same as atmospheric pressure (Pb), and there is no pressure gradient (Pb − PA = 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 H2O. 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.