The ventilatory apparatus consists of the lungs and the surrounding chest wall. The chest wall includes not only the rib cage but also the diaphragm and the abdominal wall. The lungs fill the chest so that the visceral pleura is in contact with the parietal pleura of the chest cage. Consequently, the lungs and the chest wall act in unison. From a mechanical point of view, it may be regarded as a pump with elastic, flow-resistive and inertial properties. During inspiration, the diaphragm contracts, and the abdominal contents are pushed downwards. When the chest wall expands, the lungs follow.

As opposed to inspiration, expiration is passive during quiet breathing. It is caused by the elastic recoil of the lung and chest, which tend to return to their equilibrium position, FRC, after being expanded during inspiration. Only during increased ventilation, as during exercise or in patients with increased expiratory resistance of the airways, expiration is facilitated by contraction of expiratory muscles.

Mechanically, the lung and chest wall operate in series with one another. However, the lungs are not directly attached to the chest wall. The visceral and parietal pleuras are coupled together by a thin layer of liquid which fills the intrapleural space. The liquid allows the lungs to slide against the internal wall of the chest during breathing and to follow the change in thoracic configuration. Pleural pressure (Ppl) is the pressure that can be measured in the the liquid-filled space between lung and chest.

Dymanics of a breath
During inspiration, the diaphragm contracts, the chest wall and the lungs expand. As lungs are pulled further away from their resting position (which is below RV), Ppl becomes more subatmospheric. Volume of lungs is increased, therefore, gas in the lungs is decompresed, and pressure in the alveoli (Palv) drops below atmospheric pressure. The created negative pressure gradient between airways and atmosphere generates air flow to the lungs. As inspiration proceeds, lungs are filling up with air, and the pressure gradient and air flow gradually decrease. At the end of inspiration air flow stops because Palv is equal to atmopheric pressure (no pressure gradient). At the onset of expiration, the diaphragm relaxes, elastic recoil of the respiratory system compresses the gas in the lungs, therefore, increasing Palv. The positive pressure gradient between the lungs and the atmosphere is reversed and air from the lungs is pushed out to the atmosphere. As lung volume decreases, Ppl slowly returns to its resting level. At the end of expiration, i.e at FRC, air flow and Palv are 0 (ml/sec and cmH2O, respectively), and Ppl is about -5 cmH20 (Fig. 24).
Fig. 24. Dynamics of a breath. The onset of the diaphragmatic activity designates the onset of inspiration. Simultaneously with the diaphragmatic contraction, pleural pressure becomes more subatmospheric, lungs expand, and alveolar pressure decreases below atmospheric.  The pressure gradient creates air flow to the lungs, and lung volume increases. With time of inspiration air flow becomes slower, and alveolar pressure starts to return to atmospheric pressure. At the end of inspiration (indicated by broken line), the diaphragm relaxes. Recoil of the chest wall compresses air in the alveoli: alveolar pressure increases above atmospheric (O cmH2O). Therefore, in contrast to inspiration, during expiration the pressure gradient creates air flow from the lungs to atmosphere. Consequently, lung volume decreases and pleural pressure returns to its resting position. The time course of changes in pleural pressure during inspiration and expiration (indicated by thick line) depends on contraction of the diaphragm and airway resistance. The dotted area in the graph shows the amount of pleural pressure necessary to overcome airway resistance. Note at the beginning of expiration some diaphragmatic activity is still present.
Elastic properies of the respiratory system

To evaluate the elastic properties of the respiratory system (chest wall, lungs), we measure changes in the recoil pressure of each separate structure for a given change in lung volume. Lung volumes can be measured by means of the spirometry. For the respiratory system, pressures are measured, using manometers, in cmH2O, as a difference from atmospheric pressure. "Negative pressure" indicates a pressure below atmospheric, and "positive pressure" indicates a pressure above atmospheric. The recoil pressure of a structure is defined as the pressure difference between the inside and outside the structure (transmural pressure). The recoil pressure of the chest wall, trans-chest-wall pressure (Pcw) is the difference between Ppl and the pressure at the body surface. Ppl can be measured using a flexible balloon introduced to the esophagus, since esophageal pressure happens to follow closely Ppl. The body surface is under atmospheric pressure (PB).

Pcw = Ppl - PB

The recoil pressure of the lungs, transpulmonary pressure (Pl) is measured from the difference between Palv and Ppl. (When there is no air flow, closed nose and mouth, Palv and the pressure measured at the mouth are the same.)

Pl = Palv - Ppl

The recoil pressure of the total respiratory system, transrespiratory system pressure (Prs) is measured as the difference between Palv and PB

Prs = Palv - PB

Hence, Prs is the sum of the pressure generated by its two components, lung and chest:

Prs = PL + PCW

At functional residual capacity (FRC) the elastic recoil pressures of the lung and the chest wall are equal in magnitude but opposite in direction: the algebraic sum of the two (Prs) equals 0 (Fig. 25). Therefore, FRC represents the equilibrium position of the total respiratory system. At any volume above FRC, Prs exceeds atmospheric pressure. Therefore, the net recoil pressure at such volumes favors a decrease in lung volume. With the airway open to the atmosphere, lung volumes above FRC can be maintained only by the action of the muscles of inspiration.

At any lung volume below FRC, the respiratory system recoil pressure is less than atmospheric pressure, and the net recoil effect is directed toward increasing lung volume. Lung volumes below FRC can be maintained with the airway open only by the opposing action of the expiratory muscles. Even at residual volume, the lung maintains its tendency to collapse and the chest wall is solely responsible for the outward recoil of the respiratory system (Fig. 25).

 Fig. 25.  Static recoil pressure-voulem curves of the chest wall (CW), total respiratory system (CW+L), and lungs (L). The slopes of these curves represent compliance, distensibility, of each of these structures. The recoil pressurea of any structure is the difference of pressures inside and outside this stricture. The recoil pressure of CW is measured from the difference between pleural pressure and pressure at the body surface. The recoil pressure of CW+L is measured from the difference between alveolar pressure and pressure at body surface. The recoil pressure of L, transpulmonary pressure, is the difference between the alveolar pressure and pleural pressure. Pleural pressure is measured by introducing a compliant baloon into the esophagus, and alveolar pressure is measured at the mouth, by means of manometers (when there is not flow, alveolar pressure is equal to pressure at the mouth).  The resting position of the CW+L is at FRC. At FRC, the recoil pressure of CW (5 cmH2O less than atmospheric) is balance by the recoil pressure of the lungs (5 cmH2O above atmospheric). The resting position of CW is about 75% of TLC, and that of the lungs is less than residual volume (RV).
Static pressure-volume relationships

The compliance of the lungs or the chest wall, or of the total respiratory system, is a parameter which refers to the ease with which each of these structures can be distended. The standard procedure for measuring the respiratory system compliance in humans is to determine the static pressure-volume relationship while lung volume is decreases step by step from the total lung capacity. Compliance is expressed as the volume change in the lung for a unitary change in pressure. The slopes of the pressure-volume curves in Fig. 25 indicate compliance of the chest wall, total respiratory system, and the lungs. Chest wall compliance is the change in lung volume divided by the change in Pcw. Respiratory system compliance is the change in lung volume divided by the change in Prs. Lung compliance is the change in lung volume divided by the change in Pl.

At FRC, Prs must be zero because the system is at rest. This stable condition is caused by the inward recoil of the lungs (Pl is about + 5 cmH2O) which is balanced by the outward recoil of the chest wall (Pcw is about - 5 cmH2O). This means that the lungs are above their resting volume, and the chest is below its rest volume. When lung volume is at about 75% of total lung capacity, Pcw is 0: Ppl=pressure at the body surface, and the total pressure across the respiratory system equals Pl.

When the chest is opened during thoracic surgery, air enters the pleural space because Ppl is less than atmospheric pressure. The condition is called pneumothorax. The lungs collapse, and the chest wall expands towards its resting position at about 75% of total lung capacity. A traumatic or spontaneous pneumothorax may be a life-threatening emergency since the lungs are uncoupled from the chest wall.