The adult lung has over 300,000,000 alveoli and the distribution of ventilation is not uniform. Airway closure occurs in dependent lung at the diaphragm if the subject is erect or at the back if supine. In subjects with small airways disease such as from asthma or smoking, the airway closure is more prominent and occurs earlier in life. Figure 3 shows a scintigram taken by the Massachusetts General Hospital positron camera of the distribution of l3N tracer gas during inhalation in a 29-year old man (14). In this case inhalation of the tracer occurred from near residual volume as is necessary at that age to collapse airways. The image in Figure 3A (left) is taken at residual volume after equilibration with air and the tracer l3N which is almost insoluble. The image in Figure 3A (right) is of a bolus of 250 ml of 13N labeled tracer inhaled from residual volume to total lung capacity. The tracer goes mainly to the apices of the lung. The image in Figure 3B (left) was made after the subject rebreathed the highly soluble gas nitrous oxide and then breath held at residual volume with an open glottis while the nitrous oxide streamed from the reservoir into the bloodstream. The image (Fig. 3B, left) was taken 8 seconds after injection of a bolus of 13N in air into the mouth and showed the trachea and central airways. The image in Figure 3B (right) was 24 seconds later and showed a distribution of the gas similar to that seen with the inhaled bolus in the lower left. Waiting longer got no more tracer into the bases. Thus, the airways to the base were clearly closed, not allowing the inhaled gas to reach them even though a perfusion scan at residual volume showed persistent blood flow to the bases. This non-smoking young man had to exhale considerably to show airway closure. However, by age 44 even subjects with a normal forced expiratory volume in 1 second (FEV1) show airway collapse during tidal volume breathing while supine. Normals by age 65 show airway closure during tidal breathing even seated (18). These areas of low ventilation have the potential to decrease blood oxygenation considerably and it is in these areas that HPV could play a major role in reducing lung blood flow.
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