During quiet breathing the distribution of inspired gas is governed mainly by the pressure-volume curve of the lung and the vertical pleural pressure gradient. The non-linear relationship between pressure and volume results in a larger volume increase for a given increment in pressure at low compared with high lung volumes. The pleural pressure increases down the pleural space by approximately 0.2 to 0.4 cmH 2O/cm distance. This results in a lower transpulmonary pressure (approximated by airway pressure minus pleural pressure) in dependent than in upper lung regions. Therefore dependent lung regions are positioned on the lower steeper part of the pressure-volume curve (Fig 1). During inspiration the transpulmonary pressure is increased to an equal extent all over the lung; lower and upper lung regions move along the pressure-volume curve with similar pressure increments. Therefore the change in volume (or ventilation) will be larger in dependent regions of the lungs (M.Ulc-_E_mlU 1991; Nunn 1993).
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Fig. 1 Static pressure-volume curve of the total respiratory system in an upright subject: TLC, total lung capacity. The basal lung units are on a steeper part of the pressure-volume curve than the apical units. Arrows show the changes during quiet tidal breathing.
The pleural pressure gradient is caused by the weight of the lung tissue. In addition, differences in the shape of the thoracic cavity and the lung, and the forces exerted by the abdominal organs on the diaphragm, may also affect the distribution of ventilation. The influence of the latter factor may explain why ventilation may be more uniform from top to bottom in the prone than in the supine position. It is of interest to note that ventilation appears to be evenly distributed in the lungs if gravitational forces are absent, as evidenced by measurements in short periods of weightlessness.
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