PO2, PCO2 and pH levels in the umbilical and uterine blood of the mare and ewe.

1. Foetal and maternal blood gas tensions and pH levels have been investigated in the mare and the ewe during late pregnancy under a number of experimental conditions.2. Observations were made on anaesthetized animals with the foetus in utero. Simultaneous blood samples were withdrawn from a maternal artery and uterine vein, and from the two umbilical vessels, through catheters inserted at the beginning of each experiment.3. At normal maternal arterial P(O2) (80-110 mm Hg) the umbilical venous P(O2) of the foal was very high (49 mm Hg) and the P(O2) in the umbilical artery (33 mm Hg) was similar to that in the umbilical vein of the lamb (34 mm Hg).4. The P(O2) difference between the umbilical and uterine venous blood was 17 mm Hg in the ewe but only 4 mm Hg in the mare. The corresponding P(CO2) gradients were about one quarter of those for oxygen in both species.5. When the maternal arterial P(O2) was raised above 100 mm Hg both uterine venous and umbilical P(O2) increased in the mare, and the gradient between the uterine vein and umbilical vein was reversed, whereas little change occurred in the corresponding vessels of the ewe.6. Alterations in maternal arterial P(CO2) were associated with concomitant changes in the blood in the other three vessels but in both species the P(CO2) difference between uterine and umbilical venous blood appeared to remain constant.7. Foetal blood pH levels followed those of the mother if the changes in the maternal blood were of respiratory origin. The effects of prolonged changes in maternal pH on foetal levels were investigated in the ewe. Foetal pH remained unchanged during maternal alkalaemia (pH 7.7) induced by Na(2)CO(3) infusions, but increased during a similar rise in maternal pH induced by hyperventilation.8. The present observations have been compared with findings on the conscious animal and possible explanations for the differences between the two species are discussed.