The oxygen and carbon monoxide capacities of fetal and adult blood.

1. The oxygen capacities of foetal and adult blood have been compared by direct measurement of their oxygen and carbon monoxide combining power. Corrections have been applied to the capacities obtained from oxygen content determinations for the presence of carboxyhaemoglobin and methaemoglobin.2. Mean values for the capacities of foetal blood derived from oxygen and carbon monoxide content were 1.312 ml./g (S.E. +/- 0.007) and 1.331 (S.E. +/- 0.002) respectively, while those for adult blood were 1.306 ml./g (S.E. +/- 0.006) and 1.331 (S.E. +/- 0.0005) respectively.3. For neither carbon monoxide nor oxygen was there a significant difference (P > 0.05) of capacity between adult and foetal blood. There was, however, a significant difference (P < 0.05) on comparing the two methods of measurement on each form of blood and values from both methods were significantly different (P < 0.001) from the theoretical value of 1.39 ml. oxygen/g haemoglobin derived from the molecular weight of haemoglobin.4. These results suggest that the International Cyanmethaemoglobin Standard gives values of haemoglobin concentration which are approximately 6% higher than levels indicated by oxygen combining power.