Oxygen pressure in intra- and extraembryonic blood vessels of early chick embryo.

Oxygen partial pressure (PO2) was measured in vitelline and intraembryonic vessels of 4- and 6-day-old chick embryos in ovo using recessed-tip oxygen microelectrodes. At day 4 mean PO2 values were 6 kPa in the vitelline arteries and 8.5 kPa in the oxygenated blood of the lateral vitelline veins. These values increased significantly by day 6 to 6.5 kPa in the vitelline arteries and 10.8 kPa in the lateral vitelline veins. The PO2 in intraembryonic arteries was found to be close to the PO2 in the vitelline arteries. In intraembryonic veins PO2 values ranged from 2.1 to 4.6 kPa displaying a gradient from cranial to caudal. Our measurements show that the major part of the diffusion resistance between environment and gas exchange vessels is formed by the albumen layer separating the yolk sac from the inner shell membrane, and that the oxygen transport is limited by diffusion. The low PO2 values in the intraembryonic venous vessels suggest that the gas transport system works at the limit of its capacity.