1. The difference of pH (ΔpH) between human deoxygenated haemoglobin (Hb) and oxygenated haemoglobin (O2Hb) solutions when equilibrated with physiological pressures of carbon dioxide is (experimentally) much less than previously supposed. 2. This smaller ΔpH is in contradiction to Wyman's (1948) theoretical calculations, wherein no allowance was made for the specific effect of carbamino‐compounds on the amount of base neutralized by haemoglobin. Other previous authorities have also neglected this factor, which when properly allowed for restores the role of carbamino‐compounds in CO2 transport practically to that previously estimated by Ferguson & Roughton (1934a, b). 3. At a given pH and PCO2, more CO2 is bound by Hb solutions than by O2Hb, the difference increasing with pH. This result provides further, and seemingly decisive, evidence that the bound‐CO2 in the blood other than HCO3— (i.e. x‐bound CO2) is oxygen‐linked. 4. According to a modified form of the Henderson—Hasselbalch equation for haemoglobin solutions [Formula: see text]. The value of pK1′ is the same in Hb as in O2Hb solution and from the data in 3 is found to have the value 6·15 at 37° C. 5. The difference between the titration curves of O2Hb and Hb (ΔX̄), at a given pH had been hitherto supposed to be the same in presence of CO2 as in its absence. Our experiments show, however, that ΔX̄ is less in presence of CO2 and at pH > 7·5 may even change sign. This paradoxical effect is also explicable, at any rate semi‐quantitatively, by the effect of carbamino compounds on the buffer power, according to the theory put forward in the paper. 6. The results show that the buffer power (dB/dpH) of haemoglobin solution under physiological conditions is 20‐30% greater than previously estimated, and this also is in line with the new theory. 7. In graphs of total CO2 versus PCO2 in haemoglobin solutions (or blood) it has been customary to suppose that points on straight lines radiating from the origin are points of equal pH. Our data, however, show that the iso‐pH lines drawn through the experimental points in the pH range, 7·2‐7·4 do not, when produced as straight lines, pass through the origin, but intercept the PCO2 axis significantly to the right thereof. 8. Calculations indicate that most of the x‐bound CO2 in haemoglobin solutions at pH 7·2‐7·4 and at 37° C can be accounted for by carbaminobound CO2 without the need of postulating the existence of appreciable amounts of yet other forms, i.e. y‐bound CO2, in this range.
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