Blood pH, hemoglobin function and oxygen transport throughout the dive cycle of emperor penguins

Optimal blood O2 transport during a dive and during the apnea-to-eupnea transition at the surface is essential as cardiovascular adjustments and the onset of hyperventilation shift an animal from a state of hypoxia during late ascent to one of O2 replenishment and metabolic restoration at the surface. Because pH affects the O2 affinity of hemoglobin (Hb), we examined the relationship of blood pH to carbon dioxide and lactate in emperor penguins (Aptenodytes forsteri), and determined that pH 7.3 was an appropriate lower bound for blood pH during the dive and surface intervals. Hb saturation calculated from PO2 profiles at pH 7.5 to 7.3 revealed that (a) arterial oxygenation remained high regardless of pH until late ascent; (b) the blood O2 store depletion rate increased by 27% with an end-dive pH of 7.3 versus 7.4, resulting in a 5% increase in the total body O2 store depletion rate; the blood O2 store was not depleted even at dive durations 2 times the aerobic dive limit; and (c) restoration of blood O2 in the surface period was faster at higher pH, but occurred within 3 min regardless of pH. We conclude that (i) despite elevated blood O2 store depletion rates at a final pH of 7.3, the change in total body O2 store depletion rate was small, and (ii) blood pH and the high affinity and Bohr effect of emperor penguin Hb were most critical during late ascent and the apnea-to-eupnea transition, the most physiologically challenging and dynamic phases of a dive.