Effects of temperature and PCO₂ and O₂ affinity of pigeon blood: Implications for brain O₂ supply

Bird heads contain paired countercurrent heat exchangers, the ophthalmic retia, which function in brain temperature regulation. Blood, cooled by evaporation from the nasal and buccal mucosa and the ocular surfaces, flows to the venous side of each rete and there gains heat from arterial blood flowing countercurrent to it. The cooled arterial blood then flows to the brain. To ascertain whether characteristics of the blood reaching the cooling surfaces and the retia favor O₂ and CO₂ exchange, as well as heat exchange, we studied blood affinity in relation to temperature (T) and CO₂ tension (Pco₂) in six pigeons (Columba livia). O₂ tension (PO at half-saturation (P₅₀, Torr) was measured at various combinations of T and PCO₂ from 36 to 44°C and 9 to 33 Torr. pH was uncontrolled. O₂ half-saturation of hemoglobin (P₅₀) varied according to P₅₀ = 1.049 + 0.573Pco₂ - 19.444. We propose that shifts in blood O₂ affinity, associated with T and Pco₂ at the mucosa and eyes and in the retia, would enhance the brain O₂ supply by an exchange of O₂ and CO₂ between air and blood at moist cephalic surfaces, thereby augmenting O₂ and reducing CO₂ in the venous return to the retia and diffusion of O₂ from veins to arteries in the retia. This mechanism might have particular importance at high altitude; we calculate that at 7,000 m above sea level both O₂ saturation and PO₂ could double in blood flowing from the retia to the brain.