Distribution of pulmonary capillary red blood cell transit times

In theory, red blood cells can pass through the pulmonary capillaries too rapidly to be completely saturated with oxygen during exercise. This idea has not been directly tested because the transit times of the fastest red blood cells are unknown. We report the first measurements of the entire transit time distribution for red blood cells crossing single subpleural capillary networks of canine lung using in vivo fluorescence videomicroscopy and compare those times with the distribution of plasma transit times in the same capillary networks. On average, plasma took 1.4 times longer than red blood cells to pass through the capillary bed. Decreased transit times with increased cardiac output were mitigated by both capillary recruitment and a narrowing of the transit time distribution. This design feature of the pulmonary capillary bed kept the shortest times from falling below the theoretical minimum time for complete oxygenation.