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. Th is 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 s ingle subpleural capillary networks of canine lung using in vivo fluor escence videomicroscopy and compare those times with the distribution of plasma transit times in the same capillary networks. On average, pl asma took 1.4 times longer than red blood cells to pass through the ca pillary bed. Decreased transit times with increased cardiac output wer e mitigated by both capillary recruitment and a narrowing of the trans it time distribution. This design feature of the pulmonary capillary b ed kept the shortest times from falling below the theoretical minimum time for complete oxygenation.