Static Mechanics Of The Respiratory System

The first step in understanding respiratory mechanics is understanding the determinants of static lung volumes. Static lung volumes refer to conditions of no flow (no ventilation). The lungs are elastic structures and their volume depends on the pressure difference between the inside and outside, similar to a balloon. Figure 2 illustrates the important pressure differences determining lung volume. Alveolar pressure (PA) is the pressure inside the lungs. With an open airway and no

Diaphragm

Diaphragm

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FIGURE 1 During inspiration (I) the diaphragm contracts and flattens, and external intercostal muscles raise the ribs to expand the thoracic volume. During expiration (E) the diaphragm relaxes to its domed shape and external intercostal muscles pull the ribs down to decrease thoracic volume.

FIGURE 2 Three transmural pressures in the respiratory system should be considered: (1) transrespiratory system, Prs = Pa — Pb; (2) transpulmonary pressure, Pl = Pa — Ppl; (3) trans-chest wall, Pcw = Pa — Ppl. Pa, alveolar pressure; Pb, barometric pressure; Ppl, intrapleural pressure. The importance of these pressure differences for determining lung volume are described in the text.

FIGURE 2 Three transmural pressures in the respiratory system should be considered: (1) transrespiratory system, Prs = Pa — Pb; (2) transpulmonary pressure, Pl = Pa — Ppl; (3) trans-chest wall, Pcw = Pa — Ppl. Pa, alveolar pressure; Pb, barometric pressure; Ppl, intrapleural pressure. The importance of these pressure differences for determining lung volume are described in the text.

flow, Pa equals atmospheric or barometric pressure (Pb). Intrapleural pressure (Ppl) is the pressure outside the lungs, so static transpulmonary pressure (Pl) and lung volume are determined only by Ppl with open airways and no air flow.

The intrapleural space is a sealed space filled with a very thin layer (10-20 ^m) of pleural fluid. This provides lubrication between the visceral pleura surrounding the lungs and the parietal pleura lining the chest wall, and allows the lungs to move easily against the chest wall surface during volume changes. Ppl is determined by the elastic properties of the lungs and chest wall, which vary with volume, as described later. Ppl is negative relative to barometric pressure (i.e., subatmospheric) at normal lung volumes because the lungs tend to collapse inward from the chest wall. However, the pleural surfaces cannot separate because the intrapleural space is sealed. The situation is similar to two pieces of glass with a layer of water between them: The panes cannot be pulled apart easily, but they can slide over one another. Therefore, the positive Pl distending the lungs results from a negative pressure outside the lungs and zero pressure inside the lungs. The lungs expand whenever inside pressure exceeds outside pressure (Pa > Ppl). If the lungs were exposed to atmospheric pressure outside the thorax, then the comparable volumes could be achieved by blowing the lungs up with equivalent positive pressure.

Compliance

Figure 3 illustrates the pressure-volume relationship of a lung and how this changes under different conditions. The curves demonstrate three important features: (1) The pressure-volume relationship is nonlinear and changes with volume, (2) the air curves show hysteresis, i.e., a difference between inflation and deflation, and (3) the curves are different for inflation with air and saline. These observations can all be described in terms of compliance, which quantifies the volume change for a given pressure change in distensible elastic structures. Compliance is the slope of the pressure-volume curve:

C = AV/AP, where A V is the change in lung volume and AP is the change in pressure distending the lungs.

Absolute values of compliance depend on body size because lung volume changes with size, but pressures do not. (In physical terms, volume is an extensive, or size-dependent, property whereas pressure is an intensive, or size-independent, property.) Specific compliance, or compliance per unit lung volume, can be used to compare lungs of different size. Elastance is a measure of the stiffness of a structure and is defined as the inverse of compliance.

Compliance of the air-filled lungs shown in Fig. 3 could be measured by blowing up a lung outside the body. However, compliance can be measured physiologically by estimating intrapleural pressure from esophageal pressure (Pes). The entire thoracic contents outside of the lung are essentially in equilibrium with Ppl. Therefore, Pes measurements at different static lung volumes can be used to estimate Ppl and measure compliance in vivo.

The air inflation compliance curve in Fig. 3 is nonlinear. Volume does not increase on inflation until a critical pressure level is achieved. A critical opening

Saline

Air with surfactant

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