[Contact time of red blood cells with alveolar air]

The contact time between red blood cells and alveolar air is one of the most important factors affecting red blood cell oxygenation in the lungs. Many methods have been presented for contact time measurement, however, no work, evaluating it by tracing red blood cell movement through the pulmonary capillary network has been reported. In this study, the contact time was measured by tracing red blood cells, stained with fluorescein isothiocyanate (FITC-RBC), with a fluorescent microscope in living rat lung. It was assumed that the transit time of red blood cells through the pulmonary capillary network was equivalent to the contact time. The flow patterns of red blood cells were complicated, namely, FITC-RBC discharged from one arteriole flowed into several venules through many routes in the pulmonary capillary network, and those ejected from several arterioles pored into one venule. Furthermore, the FITC-RBC flow rate changed many times in the pulmonary capillary network. The contact time varied from 0 to 1.2 seconds. The measurement was performed in each classified route by the straight distance between arteriole and venule, in order to simplify the various capillary networks. The results revealed that the contact time was 0.3, 0.4, 0.5, 0.7, 0.7 and 0.7 seconds on the average respectively in the routes of 101--150, 151--200, 201--250, 251--300 and 301--350 microns straight distance. The frequencies of the above distance routes were 5, 8, 15, 23, 26 and 21%, respectively. As a result, the total lung contact time calculated using these frequencies was 0.7 +/- 0.2 seconds (mean +/- standard deviation) on average.