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The expiratory muscle force controls the volume of air left in the lungs after a normal exhalation, known as the residual volume (RV). When the lung volume decreases, the expiratory muscles shorten, leading to a decrease in muscle force. This decrease in lung volume also causes an increase in the outward recoil pressure of the chest wall. The residual volume is reached when the expiratory muscle force is not enough to further reduce the volume of the chest wall.

The functional residual capacity (FRC) is the lung volume at the end of a normal exhalation. It is determined by the balance between the inward recoil pressure generated by the lung parenchyma (elastic recoil) and the outward recoil pressure generated by the chest wall. When the chest wall muscles are weak, the FRC decreases because the elastic recoil of the lungs is greater than the force generated by the chest wall muscles. On the other hand, in the presence of airway obstruction, the FRC increases due to premature closure of the airways, trapping air in the lungs.

An analysis of the pressure-volume curves of the lung, chest wall, and respiratory system reveals several important observations. At the resting lung volume (FRC), the inward recoil pressure of the lung is counteracted by the outward recoil pressure of the chest wall. These opposing forces are equal and opposite, resulting in relaxed muscles. Therefore, the transmural pressure across the respiratory system (Prs) at FRC is 0. At total lung capacity (TLC), both lung pressure and chest wall pressure are positive, requiring a positive transmural distending pressure. The resting volume of the chest wall, without the presence of the lungs, is approximately 60% of TLC, where the transmural pressure is 0. Above 60% of TLC, the chest wall recoils inward and positive transmural pressure is necessary, while below 60% of TLC, the chest wall tends to recoil outward.

The lungs alone reach their smallest size when the transpulmonary pressure is 0. However, even when the transpulmonary pressure is 0, the lungs still contain air due to the surfactant's ability to lower surface tension. The transmural pressure of a healthy lung flattens at pressures higher than 20 cm H2O because the lung's elastic limits have been reached. Further increases in transmural pressure result in minimal volume changes and low compliance. The connective tissue (collagen, elastin) of the lung limits further distention. Excessive pressure can cause the alveoli near the lung surface to rupture, leading to air leakage into the pleural space, known as a pneumothorax. In situations such as pneumothorax or during thoracic surgery when the chest is opened, the lungs and chest wall no longer function as a single unit. The lungs recoil until the transpulmonary pressure reaches 0, while the chest wall expands until the transchest wall pressure is 0.

The relationship between transpulmonary pressure, pleural pressure, alveolar pressure, and elastic recoil pressure is illustrated in Figure 21.7. The alveolar pressure is the sum of the pleural pressure and the elastic recoil pressure of the lung (Pel):

Equation 21.4

Since transpulmonary pressure (PL) equals the sum of alveolar pressure (PA) and pleural pressure (Ppl),

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Fig.21.1 .FVC,forcedvitalcapacity;PL,transpulmonary(ortranslung)pressure(PL);Prs,thepressureacrosstherespiratorysystem;Pw,thetransmuralpressureacrossthechestwall;RV,residualvolume;TLC,totallungcapacity.
lung (i.e., RV) is controlled by the expiratory muscle force. Decreasing lung volume results in shortening of the expiratory muscles, which, in turn, results in a decrease in muscle force. The decrease in lung volume is also associated with an increase in the outward recoil pressure of the chest wall. RV occurs when expiratory muscle force is insufficient to further reduce chest wall volume.
FRC, or the volume of the lung at the end of a normal exhalation, is determined by the balance between the elastic recoil pressure generated by the lung parenchyma to become smaller (inward recoil) and the pressure generated by the chest wall to become larger (outward recoil). When the chest wall muscles are weak, FRC decreases (lung elastic recoil > chest wall muscle force). In the presence of airway obstruction, FRC increases because of premature airway closure, which traps air in the lung (see
A number of important observations can be made from an examination of the pressurevolume curves of the lung, chest wall, and respiratory system (Fig. 21.6 ). At the resting volume of the lung (FRC), the elastic recoil of the lung acts to decrease lung volume, but this inward recoil is offset by the outward recoil of the chest wall, which acts to increase lung volume. At FRC, these forces are equal and opposite, and the muscles are relaxed. As a result, the transmural pressure across the respiratory system (Prs) at FRC is 0. At TLC, both lung pressure and chest wall pressure are positive, and both require positive transmural distending pressure. The resting volume of the chest wall, in the absence of the lungs, is the volume at which the transmural pressure for the chest wall is 0, and it is approximately 60% of TLC. At volumes greater than 60% of TLC, the chest wall recoils inward and positive transmural pressure is needed, whereas at volumes below 60% of TLC, the chest wall tends to recoil outward.
The lungs alone are smallest when transpulmonary pressure is 0. The lungs, however, are not totally devoid of air when transpulmonary pressure is 0 because of the surface tension–lowering properties of surfactant (see the section “ ”). The transmural pressure for a healthy lung alone flattens at pressures higher than 20 cm H2O because the elastic limits of the lung have been reached. Thus further increases in transmural pressure produce little change in volume, and compliance (see the section ”) is low. Further distention is limited by the connective tissue (collagen, elastin) of the lung. If further pressure is applied, the alveoli near the lung surface can rupture, and air can escape into the pleural space. This is called a pneumothorax. In a pneumothorax or when the chest is opened, as during thoracic surgery, the lungs and chest wall no longer function as a single unit. The lungs recoil until transpulmonary pressure is 0; the chest wall then increases in size until trans chest wall pressure is 0.
The relationship between transpulmonary pressure and pleural, alveolar, and elastic recoil pressures is depicted in
Fig. 21.7 . Alveolar pressure is the sum of the pleural pressure and elastic recoil pressure (Pel) of the lung:
Equation 21.4
Because transpulmonary pressure (PL) = PA + Ppl,