Pinshow B, Bernstein M H, Arad Z
Am J Physiol. 1985 Dec;249(6 Pt 2):R758-64. doi: 10.1152/ajpregu.1985.249.6.R758.
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 O2 affinity in relation to temperature (T) and CO2 tension (PCO2) in six pigeons (Columba livia). O2 tension (PO2) at half-saturation (P50, Torr) was measured at various combinations of T and PCO2 from 36 to 44 degrees C and 9 to 33 Torr. pH was uncontrolled. O2 half-saturation of hemoglobin (P50) varied according to P50 = 1.049T + 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.
鸟类头部含有成对的逆流热交换器——眼视网膜，其作用是调节脑部温度。血液通过鼻腔、颊黏膜和眼表面的蒸发而冷却，流向每个视网膜的静脉侧，并在那里从与其逆流的动脉血中获取热量。然后，冷却后的动脉血流向脑部。为了确定到达冷却表面和视网膜的血液特性是否有利于氧气和二氧化碳的交换以及热交换，我们研究了6只家鸽（Columba livia）血液的氧亲和力与温度（T）和二氧化碳分压（PCO₂）的关系。在36至44摄氏度和9至33托的T和PCO₂的各种组合下，测量了半饱和时的氧分压（PO₂，托）。pH未作控制。血红蛋白的氧半饱和度（P50）根据公式P50 = 1.049T + 0.573PCO₂ - 19.444变化。我们提出，在黏膜、眼睛和视网膜处，与T和PCO₂相关的血液氧亲和力变化，将通过在湿润的头部表面空气与血液之间进行氧气和二氧化碳的交换来增强脑部的氧气供应，从而增加视网膜静脉回流中的氧气并减少二氧化碳，并促进氧气在视网膜中从静脉扩散到动脉。这种机制在高海拔地区可能尤为重要；我们计算得出，在海拔7000米处，从视网膜流向脑部的血液中的氧饱和度和PO₂可能会增加一倍。
