Oxygen transfer during exercise-induced hypocapnia

A panting animal, the sheep, develops hyperthermia (40-41°C) , hypocapnia (10-20 Torr), and lactacidosis (3-5 mM) during exercise at 50% of maximum oxygen consumption. Experiments were performed to determine if the lactacidosis was caused by impaired oxygen transfer caused by a hypocapnic increase in oxygen affinity of hemoglobin. A flow probe (Transonic Systems, Inc., Ithaca, NY) was placed on the external iliac artery and blood samples were taken from the carotid artery and iliac vein. Net oxygen and lactate flux were calculated for one hindlimb during exercise. Oxygen delivery to the limb increased due to a rise in both hemoglobin concentration (7 to 11 g/dl) and blood flow (0.5 to 2.5 1/min). Oxygen extraction remained constant during exercise and the arteriovenous difference in lactate was zero. It was concluded that 1) there was no impediment to oxygen transfer associated with exercise-induced hypocapnia and that any increase in oxygen affinity of hemoglobin was offset by tissue hyperthermia and 2) the increase in plasma lactate concentration was due to relative hypoxia of tissues other than the muscles associated with exercise.