Oxygen hemoglobin release

The structure of glycerate-2,3-bisphosphate, an allosteric effector for hemoglobin oxygen release. 

Tohgi H, Sasaki K, Chiba K and Nozaki Y (1990). Effect of vinpocetine on oxygen release of hemoglobin and erythrocyte organic polyphosphate concentrations in patients with vascular dementia of the Binswanger type. Arzneimittelforschung, 40, 640-643. 

Kawai N, Ohkawa H, Maejima H, et al. Oxygen releasing from cellular hemoglobin. Artif Cells Blood Substit Immobil Biotechnol 1998 26 507. 

The ATP can then power another contraction. Eventually, the amount of ATP available approaches a level too low to be bound by myosin in the muscle, even though it is by no means exhausted. The protons (acid) from metabolism cause hemoglobin to release its oxygen more readily, promoting a switch to aerobic metabolism. Lactate and protons from glycolysis may also lead to fatigue and an inability to sustain the level of speed that was possible earlier. In most humans, this seems to occur after a run of about 400 meters, which is why running quarters is one of the most unpleasant exercises for any athlete, no matter how well conditioned. 

Which one of the following conditions causes hemoglobin to release oxygen more readily ... 

The primary function of the erythrocyte is to carry oxygen from the lungs to the peripheral tissues. Its optimal design enables efficient oxygen transport via the hemoglobin molecule. The general metabolic state of the patient and local factors control oxygen release. 

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