Measurement of pH in blood vessels and interstitium of 4 and 6 days-old chick embryos.

The pH in intra- and extraembryonic blood vessels and in the embryonic interstitium has been measured in 4 and 6 days-old chick embryos using pH microelectrodes. The mean pH of the lateral vitelline vein was 8.0 at day 4 and 7.89 at day 6 and that of the vitelline artery 7.8 and 7.66, respectively. The pH of the blood in the embryonic heart was the same at day 4 and 6 (7.68 and 7.67). The lowest blood pH was recorded in the jugular vein (7.42 at day 6). The pH values measured in the interstitium show less variability. However a clear cranio-caudal gradient was apparent at day 4 where pH fell from 7.68 to 7.47. At day 6 the pH values of the interstitium ranged between 7.61 and 7.52 showing no preferential distribution. Using previously determined values for blood PO2 and the oxygen haemoglobin equilibrium curves, we calculated the oxygen saturation of the blood at the measured pH. In the vitelline vein the O2 saturation is 89% at day 4 and more than 95% at day 6, whereas that of the vitelline artery is 37% and 25%, respectively. When ambient PCO2 is raised to about 8 torr the pH falls to 7.81 (7.68) in the vitelline vein at day 4 (6) and the oxygen saturation decreases to 64% and 79%, respectively. Thus the high pH values found in the vitelline vein seem to be necessary for adequate oxygen uptake of the embryo.