Thermal effects on the blood respiratory properties of southern bluefin tuna, Thunnus maccoyii

Thermal effects on the blood respiratory properties of southern bluefin tuna (Thunnus maccoyii) at 10, 23 and 36 °C, and at 0.5 and 1.5% CO2 were investigated. A reversed temperature effect occurred as the oxygen partial pressure required for 50% haemoglobin saturation (P50) at 0.5% CO2 decreased from 2.9 kPa at 10 °C to 1.7 kPa at 23 °C (apparent heat of oxygenation, ΔH°,  = + 27 kJ mol− 1). However, oxygen binding was essentially independent of temperature at warmer temperatures (P50 = 1.7–2.0 kPa from 23–36 °C at 0.5% CO2; ΔH° = − 6.5 kJ mol− 1). Hill's coefficient (nH) ranged from 1.3 to 1.6, and there was a large effect of temperature on the Bohr factor (ΔlogP50/ΔpH = − 1.6 at 10 °C and − 0.9 at 36 °C). This is the first study of whole blood to demonstrate the thermal dependence of ΔH° itself, whereby the oxygen equilibrium curve is more sensitive to temperature in the lowest thermal range examined. We suggest that the functional basis for these observations lies in the necessity to ensure a sufficient oxygen supply to all tissues, including the heart and liver, without suffering from premature or excessive oxygen unloading around the heat exchanger prior to delivery of oxygen to organs and tissues that lie efferent to the exchanger.