- The study of the respiratory system during conditions in which there is no gas flow is termed statics.
- Both the chest wall and the lungs possess elastic properties.
- The chest wall and lungs have a resting or unstressed volume. To change the volume from the unstressed level requires force. After release of the force, the chest wall and lungs return to their resting volumes.
- The major lung volumes—TLC, FRC, and RV—are determined primarily by the balance of forces exerted by the chest wall, lungs, and ventilatory muscles.
- The pleural plays a crucial role in linking the forces exerted by the lungs and chest wall.
- The transmural pressure is the pressure across a wall of an enclosed structure. The transmural pressure is the pressure inside the structure minus the pressure outside the structure. Positive transmural pressures are associated with distending forces that increase the volume of a structure. Negative transmural pressures are associated with collapsing forces that decrease the volume of a structure.
- The TLC is the volume of the lung at the end of a maximal inspiration.
- The RV is the volume of the lung at the end of a maximal expiration.
- The FRC is the volume of the lung at the end of a relaxed expiration.
- In older adults, airway closure is a contributing factor in determining the RV.
- The tension generated by a muscle is related to the length of the muscle at the time that it is stimulated by a neurological impulse. The longer the muscle at the time of neurological stimulation, the greater the tension generated.
- The diaphragm is the major inspiratory muscle.
- Compliance is a characteristic of a structure that relates the transmural pressure change required to achieve a given change in volume.
- The compliance of the respiratory system primarily depends on the elastic properties of the lungs and chest wall in healthy individuals.
- The volume of the lung depends on the compliance of the respiratory system and the transpulmonary pressure.
- FRC can be measured with the helium dilution or nitrogen washout techniques. Both of these tests may underestimate FRC in people with airways disease that impairs communication of regions of the lungs with the central airway. Measurement of FRC with a body plethysmograph is less affected by airways disease.
- The action of surfactant to reduce surface tension in the alveolus is critical in increasing respiratory system compliance and in minimizing alveolar collapse during exhalation.
- Surface forces are the primary cause of the hysteresis seen in the pressure–volume curves of the lungs during the respiratory cycle.