Respiratory System MCAT Practice Question
A 45-year-old man with chronic obstructive pulmonary disease presents with a 6-month history of progressive dyspnea. Arterial blood gas analysis shows PaO2 of 55 mmHg and PaCO2 of 50 mmHg. His hemoglobin is 18 g/dL. Laboratory studies reveal elevated 2,3-bisphosphoglycerate (2,3-BPG) levels. A pulse oximetry reading at rest shows 88% oxygen saturation. Compared to a healthy individual at the same PaO2, this patient's hemoglobin releases oxygen more readily to his tissues. Which of the following best explains this physiologic adaptation?
Answer choices
- A2,3-BPG binds to the central cavity between β-globin subunits in deoxygenated hemoglobin, stabilizing the T state and rightward-shifting the oxygen-hemoglobin dissociation curveCorrect answer
- B2,3-BPG competitively inhibits oxygen binding at the heme iron center, reducing overall oxygen carrying capacity
- C2,3-BPG stabilizes the R state conformation of hemoglobin, increasing oxygen affinity and promoting oxygen retention in tissues
- D2,3-BPG disrupts cooperativity between hemoglobin subunits, eliminating the sigmoidal shape of the dissociation curve and causing linear oxygen release
- E2,3-BPG binds to oxygenated hemoglobin and facilitates the R→T transition, enhancing oxygen loading in the lungs
- F2,3-BPG increases hemoglobin's affinity for oxygen by preventing subunit interactions, thereby improving tissue oxygen delivery
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