1. Respiratory properties of blood of Polypterus senegalus were studied in vitro at 30 °C.

2. The following values were found in species which did not suffer from an apparent anaemia: erythrocyte count 0.7-1.15 x 106/mm3, haemoglobin concentration 10-14 g/100 ml, haematocrit 33-43 vol. %, mean erythrocyte volume 300-480,µm3, and mean erythrocyte size 18 x 10 µm. Subtracting the calculated volume of the nucleus, the ‘true’ mean corpuscular haemoglobin concentration in the erythrocyte cytoplasm was found to be 34 g/100 ml, which corresponds to the haemoglobin concentration in the non-nucleated mammalian erythrocytes.

3. Transport of oxygen: The oxygen affinity of haemoglobin was found to be moderate, P50 being 23.5 mmHg at Pco2, 6 mmHg and pH 7.7. The O2 dissociation curve had a typical sigmoid shape. The oxygen capacity of the investigated samples was 13.5-18.5 ml/100 ml, showing a binding capacity of approximately 1.34 ml O2 per 1 g Hb at pH > 7.3. A slight Root effect was observed at pH lower than 7.2. The coefficients of the Bohr effect (Δ log P 50/Δ pH) and of the temperature shift of the oxygen dissociation curve (Δ log P 50/ΔT) were -0.43 and 0.022, respectively.

4. Transport of carbon dioxide: the CO2 combining power in oxygenated blood was 12.5 mM/l at PCOCO2 30 mmHg and pH 7.3 and the CO2 dissociation curve showed the typical steep initial portion. The buffering capacity of blood (CHCO3-/Δ pH) was moderate, the coefficient being -15.4. The Haldane effect was also found to be moderate, CCO2/O2 cap. being -0.10.

The dissociation constant of plasma carbonic acid (pK') was assessed to be 0.03 higher than that of human plasma at corresponding pH and temperature 30 °C.

5. The position of the absorption maxima of Polypterus oxyhaemoglobin and cyanmethaemoglobin solutions was the same as that of human haemoglobin solutions.

6. The results are compared with the respiratory properties of lungfish and human blood.

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