Pressure-volume characteristics of the respiratory system

Pressure-volume characteristics of the respiratory system. This diagram shows the passive pressure-volume characteristics of the lung and chest wall in isolation, as well as the two integrated together as the respiratory system. Note that the resting position of the lungs is at a volume below the residual volume (RV), and the resting position of the chest wall is close to the total lung capacity (TLC). The schematics of the lung and chest wall show the forces exerted by the lungs and chest wall at the different volumes. At functional residual capacity (FRC), the forces exerted by the chest wall and the lungs are equal in intensity and opposite in direction. Thus, this is the resting position of the respiratory system. At volumes above FRC, the balance of forces is such that the respiratory system wants to get smaller; at volumes below FRC, the balance of forces pushes the system toward a higher volume, that is, back to FRC.