Capillary transit times and kinetics of oxygenation in the primary respiratory organ of early chick embryo.

In early avian embryos respiratory gas exchange between environment and blood takes place via the blood vessels of the yolk sac. To investigate the principles governing oxygen transfer we determined the RBC velocity, the length of the a-v channels, and the RBC transit time in the vitelline capillary network of 4-day-old chick embryos. Measurements were carried out using a digital video analyzing system for on-line velocity measurements (modified dual video window technique) and morphometric determinations. Velocity measurements were performed on fluorescent (FITC)-labeled erythrocytes. All measured parameters exhibited a large scatter. The mean RBC velocity was 193 +/- 117 (+/- SD) microns/sec and the mean length of the a-v channels was 434 +/- 291 microns. The transit time ranged between 0.1 and 17 sec (median value 2.5 sec). Using previously determined oxygen hemoglobin dissociation curves and blood oxygen partial pressure (Meuer and Baumann, 1988, Respir. Physiol 71, 331-342; Meuer et al., 1989, J. Dev. Physiol. 11, 354-359) the kinetics of the oxygen uptake by the blood was calculated. This gave an estimated transit time required for 99% oxygen saturation of 1.5 sec. The data show that in more than a third of all a-v channels the transit time is shorter than required for complete blood oxygenation. This is consistent with recent determinations of the blood oxygen saturation of mixed arterialized blood (89% at Day 4; Meuer and Baumann, 1988).