Problem 4 - Entrance Test

A competitive swimmer performs a prolonged underwater dive, holding their breath. Which of the following changes would most likely occur in their blood to facilitate oxygen delivery to tissues, and how would it affect the oxygen-hemoglobin dissociation curve?

A. Decreased blood pH, shifting the curve to the left.B. Increased blood PCO2, shifting the curve to the right.C. Decreased body temperature, shifting the curve to the right.D. Increased 2,3-BPG levels, shifting the curve to the left.

Correct: B

During a prolonged breath hold and intense physical activity (like swimming), the swimmer's body will accumulate CO2 and lactic acid, leading to several physiological changes to optimize oxygen delivery: 1. Increased PCO2: As CO2 builds up in the blood, PCO2 increases. 2. Decreased pH (Increased Acidity): Increased PCO2 leads to the formation of carbonic acid (H2CO3), which dissociates into H+ and HCO3-, decreasing blood pH. Lactic acid from anaerobic metabolism also contributes to this pH drop. 3. Increased Temperature: Muscle activity generates heat, leading to a slight increase in body temperature. 4. Increased 2,3-BPG: While 2,3-BPG levels do increase in conditions of chronic hypoxia, for an acute event like a single breath hold, its effect might be less immediate compared to pH, PCO2, and temperature. The Bohr effect states that a decrease in pH (or an increase in H+ concentration) and an increase in PCO2 shifts the oxygen-hemoglobin dissociation curve to the right. A rightward shift signifies a decreased affinity of hemoglobin for oxygen, meaning hemoglobin releases oxygen more readily to the tissues, which is crucial for exercising muscles. An increase in temperature also shifts the curve to the right. Let's analyze the options: A. Decreased blood pH (correct direction) but 'shifting the curve to the left' (incorrect effect). Decreased pH shifts it to the right. B. Increased blood PCO2 (correct direction) and 'shifting the curve to the right' (correct effect). This accurately describes the physiological adaptation for enhanced oxygen delivery to tissues under these conditions. C. Decreased body temperature would shift the curve to the left, which would decrease oxygen release to tissues. Body temperature would likely increase during intense exercise. D. Increased 2,3-BPG levels do shift the curve to the right (correct effect), but the statement 'shifting the curve to the left' is incorrect. Also, the immediate impact of PCO2 and pH is more dominant in acute situations. Therefore, increased blood PCO2 causing a rightward shift is the most accurate and immediate compensatory mechanism listed.