Blood Composition, Gas Transport, Immune Function (3B) Practice Test

In a controlled treadmill study, arterial blood samples were collected from participants at rest and after 6 minutes of high-intensity exercise. Despite a similar arterial $P_{O_2}$ across conditions, the measured hemoglobin $P_{50}$ (the $P_{O_2}$ at 50% saturation) increased after exercise. Which scenario most likely explains the observed change in oxygen affinity based on hemoglobin physiology?

In a crossover study of healthy adults performing cycle exercise at 70% of maximal workload, arterial blood was sampled at rest and after 8 minutes of steady-state exercise. Investigators reported a rightward shift of the hemoglobin–oxygen dissociation curve during exercise without a change in hemoglobin concentration. Selected arterial values are shown.

Which scenario most likely explains the observed decrease in hemoglobin oxygen affinity during exercise?

Table: Arterial blood measurements

• Rest: pH 7.40, $P_{CO_2}$ 40 mmHg, Lactate 1.0 mM
• Exercise: pH 7.32, $P_{CO_2}$ 48 mmHg, Lactate 4.2 mM
• Hemoglobin: 15.0 g/dL (both conditions)
• Temperature (core): 37.0°C (rest) vs 38.2°C (exercise)