AJP - Regulatory, Integrative and Comparative Physiology, Vol 249, Issue 6 758-R764, Copyright © 1985 by American Physiological Society
Effects of temperature and PCO2 on O2 affinity of pigeon blood: implications for brain O2 supply
B. Pinshow, M. H. Bernstein and Z. Arad
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.
