Question

The bar-headed goose (*Anser indicus*) is renowned for its biannual migration over the Himalayas, a journey that forces the species to fly at altitudes exceeding 8,000 meters. At these elevations, atmospheric pressure is less than half of that at sea level, presenting a severe risk of hypoxia. To combat this, the geese rely on a specialized hemoglobin variant with a high oxygen affinity. This adaptation, which arises from a single amino acid substitution in the alpha-chain of the hemoglobin molecule, allows the birds to extract scarce oxygen from thin mountain air more efficiently than lowland waterfowl. However, this high-affinity hemoglobin binds oxygen so tightly that releasing it to the metabolizing tissues requires a significant drop in blood pH or an increase in body temperature. Consequently, during active flight, the geese do not engage in the typical deep-breathing patterns of other migrating birds; instead, they undergo hyperventilation. The rapid respiration rate lowers the carbon dioxide concentration in their blood, which paradoxically increases blood pH (alkalosis) and shifts the oxygen-dissociation curve, making oxygen release even more difficult. To counteract this potential bottleneck and facilitate oxygen delivery to working flight muscles, the birds rely heavily on localized lactic acid production. The accumulation of lactic acid in active muscle tissues lowers the local pH, which in turn triggers the immediate release of oxygen from the high-affinity hemoglobin directly where it is most needed.

Based on the passage, the immediate release of oxygen from the bar-headed goose's high-affinity hemoglobin to its working flight muscles is directly triggered by which of the following?