The combined compliance of the lung and chest wall of a healthy individual is measured and plotted as shown below. It is noted that the intrapleural pressure at the end of maximal inspiration is -8 cm H₂0 (marked x). Which of the following is the best estimate of the intrapleural pressure at the point marked by the black dot?

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11ec160b_3ce8_be2d_9249_c321badd56e9_MD0001_00 On the above pressure-volume curve, collapsing pressure is positive and expanding pressure is negative. The lungs have a tendency to collapse at all lung volumes; therefore, the entire lung curve is located in the positive region. In contrast, the resting chest wall has a tendency to expand at all but very high lung volumes; therefore, most of the chest wall curve is located in the negative region. The point where the collapsing force of the lungs is equivalent to the expanding force of the chest wall is the resting equilibrium of the respiratory system (ie, end-tidal expiration), represented by the black dot); the alveolar pressure at this point is equivalent to atmospheric pressure (ie, 0 cm H₂O) and the lung volume is the functional residual capacity (FRC). Compliance is defined as change in volume per change in pressure and is represented by the slope of the pressure-volume curve. A highly compliant container is able to stretch to accommodate large increases in volume with little change in pressure, demonstrating a steep compliance curve. Generally, lung compliance is greatest around the FRC and decreases at very high and very low lung volumes (note that surface tension causes compliance to differ during inspiration and expiration). The chest wall has low compliance at low lung volumes and becomes more compliant as lung volume increases with inspiration. The compliance of the respiratory system as a whole is the combined compliance of the lungs and chest wall (represented by the blue curve). Intrapleural pressures are not represented on the lung pressure-volume curve. The expanding force of the chest wall working in opposition to the collapsing force of the lungs creates negative intrapleural pressure throughout the respiratory cycle. Inspiration is driven by active expansion of the chest wall, which generates even greater intrapleural negative pressure and pulls the lungs outward. Intrapleural negative pressure peaks at maximal inspiration at a value of approximately -8 cm H₂O. During passive expiration the chest wall relaxes and the respiratory system returns to its equilibrium position, in which intrapleural pressure is approximately -5 cm H₂O. Educational objective: The lungs generate a collapsing force and the chest wall generates an expanding force; the point at which these opposing forces are equivalent is the resting equilibrium of the respiratory system, where alveolar pressure is equal to atmospheric pressure (ie, 0 cm H₂O) and lung volume is the functional residual capacity. The opposing forces create negative intrapleural pressure throughout the respiratory cycle; intrapleural pressure at resting equilibrium (ie, end-tidal expiration) is approximately -5 cm H₂O. __________ References: Physiology, lung compliance. (http://www.ncbi.nlm.nih.gov/pubmed/32119404)

The Combined Compliance of the Lung and Chest Wall of a Healthy