Samples of unbuffered, whole blood from freshly-caught albacore (Thunnus alalunga Bonnaterre) were equilibrated at 5, 10, 15, 20, 25, 30 and 35 °C and at 0 and 1 % CO2 for construction of oxygen dissociation curves. A strong Bohr effect (−1.17), a negligible Root effect, and a reverse temperature effect (ΔH = +1.72 for 0% CO2 and +0.26 for 1% CO2) characterized these hyperbolic (Hill's n = 1.1) curves. The unusual reverse temperature effect was especially pronounced when blood was quickly warmed or cooled, simulating passage through the heat exchanging, countercurrent vascular rete system of this warm-bodied fish. A diagrammatic model of blood gas dynamics in the rete incorporating these in vitro data illustrates protection of arterial oxygen from premature haemoglobin dissociation and consequent loss to the venous circulation as blood warms in the rete. More conventional temperature effects on the carbon dioxide equilibria of albacore blood lower the Pcoco2 of venous blood being cooled in the rete. This reduces the venous-arterial Pcoco2 gradient, thereby minimizing the diffusion of CO2 to arterial blood with resulting haemoglobin-oxygen dissociation via the strong Bohr effect. The temperature range (10–30 °C) over which the albacore haemoglobin-oxygen binding exhibits the reversed thermal effect closely matches the maximum thermal gradient (ambient water-core body temperature) typically present in this fish, suggesting that its highly specialized haemoglobin-oxygen dissociation characteristics evolved within-and now establishes thermal limits upon-the existing geographic distribution of this species.

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