39. The Cl content of red blood cells (RBCs) in the venous blood was found to be higher than that in arterial blood in a human subject. Following proposals were made to explain these observations:
    A. The high pCO₂ in venous plasma leads to increased diffusion of CO₂ into RBC and the formation of H₂CO₍₃₎
    B. HCO₃^– content in the RBC of venous blood becomes much greater than that in plasma.
    C. The excess HCO₃^– leaves the RBC of venous blood along with Na⁺ to plasma by a Na⁺ – HCO₃^–symporter,
    D. The increased Na⁺ in the venous plasma is transported to the RBC along with Cl^–

Select the combination with INCORRECT statements from the following options.
(1) A and B                                             (2) B and C
(3) A and D                                            (4) C and D

 

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Introduction

The chloride content of red blood cells (RBCs) in venous blood is significantly higher than that in arterial blood, a phenomenon critical to efficient carbon dioxide transport and acid-base homeostasis. This article explores the physiological basis for this difference, focusing on the role of CO2 metabolism within RBCs, ion exchange mechanisms, and the maintenance of electrical neutrality and osmotic balance.

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The Physiology Behind Increased Chloride in Venous RBCs

Venous blood returning from tissues carries higher concentrations of CO2 produced by metabolism. This elevated CO2 enters RBCs where it undergoes a series of reactions catalyzed by carbonic anhydrase:

CO2+H2O↔H2CO3↔H++HCO3−CO2+H2O↔H2CO3↔H++HCO3−

Key physiological events include:

1. CO2 Hydration and Bicarbonate Production
   CO2 is converted rapidly to bicarbonate (HCO3-) inside RBCs.

2. Bicarbonate Export and Chloride Import (Chloride Shift or Hamburger Effect)
   Since RBC membranes are impermeable to bicarbonate ions, they are exported out of the cell in exchange for chloride ions moving into the RBC via the chloride/bicarbonate exchanger.

3. Effect on Chloride Levels
   This exchange increases chloride concentration inside RBCs in venous blood relative to arterial blood, maintaining electrical neutrality.

4. Role of Sodium
   Unlike bicarbonate-chloride exchange, sodium ions (Na+) are not co-transported with bicarbonate during this process. Sodium levels remain relatively stable and are not part of the main ion exchange driving chloride influx.

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Understanding the Proposed Statements

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Correct Identification of Incorrect Statements

The incorrect statements are:

• B. Incorrect, as intracellular bicarbonate is usually lower in RBCs; excess bicarbonate is exported.

• C and D. Incorrect as bicarbonate transport does not involve sodium co-transport; salt movement centers on chloride/bicarbonate exchange without sodium involvement.

Among options, B and C are paired in one choice, while C and D in another.

Given the question asks to select the combination of incorrect statements from offered options:

(2) B and C

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Physiological Importance of the Chloride Shift

• The chloride shift is vital for the efficient transport of CO2 from tissues to lungs.

• It supports acid-base balance, preventing drastic changes in blood pH.

• Interaction of chloride ions with hemoglobin affects oxygen affinity, linking CO2 and O2 transport dynamics.

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Summary Table

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Conclusion

The statements that are incorrect regarding the chloride content of RBCs in venous blood are:

(2) B and C