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1. The carbamate (HbCO₂) concentration in oxygenated and deoxygenated human adult and foetal red blood cells was estimated at a constant pressure of carbon dioxide (P_CO2 = 40 mm Hg) and various pH values of the serum. The Donnan ratio for chloride and bicarbonate ions was used to calculate the bicarbonate concentration in the red cells. With this figure the carbamate concentration was calculated as follows:

[HbCO₂] = [Total CO₂] - [HCO^-₃] - [dissolved CO₂].

2. At a given pH value in the red cell deoxygenated foetal red cells contain more HbCO₂ than deoxygenated adult ones. Upon oxygenation (at constant pH) HbCO₂ drops in both types of erythrocytes to lower values than in deoxygenated cells. The fraction of `oxylabile carbamate' (-HbCO₂/HbO₂) at a red cell pH of 7·2 and a P_CO2 of 40 mm Hg is 0·117 in foetal and 0·081 in adult erythrocytes.

3. From the fraction of moles carbamate formed per Hb monomer (moles CO₂/mole Hb_i) K'_c and K'_z, the apparent carbamate equilibrium constants were calculated which can be used to estimate the carbamate concentration in normal adult and foetal blood.

4. The first apparent dissociation constant of carbonic acid is significantly higher in oxygenated (-log₁₀K'₁ = pK'₁ = 6·10) than in deoxygenated (pK'₁ = 6·12) adult red cells, whereas in foetal red cells the difference is smaller and statistically not significant.

5. For a given set of physiological conditions in arterial and mixed venous blood in respect to oxygen saturation, P_CO2 and pH, the fractional contribution of carbamino compounds of haemoglobin to the amount of carbon dioxide which is exchanged during the respiratory cycle was computed on the basis of the present results and found to be 10·5% in adult and 19% in foetal blood.

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