A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
An investigator is studying the resting rate of oxygen consumption in the lower limbs of individuals with peripheral vascular disease. The rate of blood flow in a study subject's femoral vessels is measured using Doppler ultrasonography, and blood samples from the femoral vein and femoral artery are obtained. The blood samples are irradiated and centrifuged, after which the erythrocyte fractions from each sample are hemolyzed using 10% saline. Compared to the femoral vein, which of the following findings would be expected in the hemolysate from the femoral artery?
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A
Lower chloride concentrationCorrect. Arterial RBCs have lower chloride than venous RBCs because the chloride shift exports Cl– from RBCs in the pulmonary capillaries as bicarbonate re-enters and is converted back to CO2.
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B
Higher ADP/ATP ratioIncorrect. ADP/ATP ratio is largely driven by metabolic activity; arterial blood does not have a higher ADP/ATP ratio in RBCs than venous blood.
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C
Higher carbaminohemoglobin concentrationIncorrect. Carbaminohemoglobin (CO2 bound to hemoglobin amine groups) is HIGHER in venous blood, where CO2 is being transported, not in arterial blood.
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D
Lower potassium concentrationIncorrect. Potassium concentration in RBCs does not differ significantly between arterial and venous samples in normal physiology.
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E
Lower 2,3-bisphosphoglycerate concentrationIncorrect. 2,3-BPG concentration in erythrocytes reflects chronic adaptations (altitude, anemia) and does not differ meaningfully between paired arterial and venous samples from the same vascular bed.
↑ Tap an answer to reveal the reasoning
Answer: A. The chloride shift (Hamburger phenomenon) is fundamental to how blood carries CO2. In peripheral tissues, CO2 enters red cells and is converted to bicarbonate by carbonic anhydrase. Bicarbonate exits the RBC in exchange for chloride entering — so venous blood RBCs have higher intracellular chloride than arterial blood RBCs. Conversely, in the lungs, the chloride shift reverses: bicarbonate re-enters the RBC, is reconverted to CO2 for exhalation, and chloride exits. Therefore, arterial RBCs have LOWER chloride than venous RBCs.
When hemolysate from arterial (femoral artery) blood is compared to venous (femoral vein) blood, the arterial sample will show lower chloride concentration inside the cells. This reflects the reverse-direction chloride flux that occurs as blood passes through pulmonary capillaries.
Clinical pearl: the chloride shift is the elegant mechanism by which RBCs maintain electroneutrality during bicarbonate transport — and explains why arterial RBCs have different ionic composition than venous RBCs.
**Why each option:**
**A.** Arterial RBCs have lower chloride than venous RBCs because the chloride shift exports Cl– from RBCs in the pulmonary capillaries as bicarbonate re-enters and is converted back to CO2.
**B.** ADP/ATP ratio is largely driven by metabolic activity; arterial blood does not have a higher ADP/ATP ratio in RBCs than venous blood.
**C.** Carbaminohemoglobin (CO2 bound to hemoglobin amine groups) is HIGHER in venous blood, where CO2 is being transported, not in arterial blood.
**D.** Potassium concentration in RBCs does not differ significantly between arterial and venous samples in normal physiology.