Part 6: Transport of Carbon Dioxide in Blood (25 MCQs)
Part 6: Transport of Carbon Dioxide in Blood (25 MCQs)
Q1. What percentage of CO₂ is transported dissolved in plasma?
A. 3%
B. 7%
C. 23%
D. 70%
Answer: B
Explanation:
- A. Incorrect: Too low.
- B. Correct: About 7% CO₂ is carried dissolved in plasma.
- C. Incorrect: 23% = as carbaminoHb.
- D. Incorrect: 70% = as bicarbonate.
Q2. The majority of CO₂ in blood is transported as:
A. Dissolved in plasma
B. Carbaminohemoglobin
C. Bicarbonate ions
D. Carbon monoxide
Answer: C
Explanation:
- A. Incorrect: Only 7% dissolved.
- B. Incorrect: 23% as carbaminoHb.
- C. Correct: 70% transported as bicarbonate (HCO₃⁻).
- D. Incorrect: CO is toxic, not normal transport form.
Q3. The enzyme that catalyzes conversion of CO₂ into HCO₃⁻ in RBCs is:
A. Carbonic anhydrase
B. Pepsin
C. Amylase
D. Catalase
Answer: A
Explanation:
- A. Correct: Carbonic anhydrase in RBCs converts CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺.
- B. Incorrect: Pepsin digests proteins.
- C. Incorrect: Amylase digests starch.
- D. Incorrect: Catalase breaks H₂O₂.
Q4. Which of the following transports about 23% of CO₂ in blood?
A. Dissolved in plasma
B. Carbaminohemoglobin
C. Bicarbonate ions
D. Fibrinogen
Answer: B
Explanation:
- A. Incorrect: Only 7%.
- B. Correct: 23% transported as carbaminoHb (CO₂ bound to Hb).
- C. Incorrect: 70% as bicarbonate.
- D. Incorrect: Fibrinogen involved in clotting.
Q5. The reversible reaction catalyzed by carbonic anhydrase is:
A. CO₂ + H₂O ↔ H₂CO₃
B. H₂CO₃ → H⁺ + HCO₃⁻
C. Hb + O₂ ↔ HbO₂
D. CO + Hb → CarboxyHb
Answer: A
Explanation:
- A. Correct: Carbonic anhydrase catalyzes CO₂ hydration.
- B. Incorrect: That’s spontaneous dissociation.
- C. Incorrect: Oxygen transport.
- D. Incorrect: CO poisoning.
Q6. The “chloride shift” refers to:
A. Entry of Cl⁻ into RBCs when HCO₃⁻ leaves
B. Exit of Cl⁻ from RBCs when HCO₃⁻ enters
C. Binding of Cl⁻ to Hb
D. Loss of Cl⁻ in plasma
Answer: A
Explanation:
- A. Correct: Cl⁻ enters RBC to balance charge as HCO₃⁻ exits.
- B. Incorrect: Opposite direction.
- C. Incorrect: Cl⁻ doesn’t bind Hb.
- D. Incorrect: Not plasma loss.
Q7. In tissues, the direction of chloride shift is:
A. Cl⁻ enters RBC, HCO₃⁻ leaves
B. Cl⁻ leaves RBC, HCO₃⁻ enters
C. Both Cl⁻ and HCO₃⁻ leave
D. Both Cl⁻ and HCO₃⁻ enter
Answer: A
Explanation:
- A. Correct: At tissues, CO₂ enters RBC, converted to HCO₃⁻, which exits → Cl⁻ enters.
- B. Incorrect: Reverse happens in lungs.
- C. Incorrect: Not both leaving.
- D. Incorrect: Not both entering.
Q8. In lungs, the chloride shift is reversed because:
A. CO₂ diffuses into RBC
B. HCO₃⁻ enters RBC and Cl⁻ exits
C. Carbonic anhydrase is absent
D. Hb affinity for O₂ decreases
Answer: B
Explanation:
- A. Incorrect: In lungs, CO₂ exits RBC.
- B. Correct: HCO₃⁻ re-enters RBC, Cl⁻ exits (reverse shift).
- C. Incorrect: Enzyme is present.
- D. Incorrect: Hb affinity increases, but not the reason.
Q9. Which form of CO₂ transport is directly influenced by Hb binding to O₂ (Haldane effect)?
A. Dissolved CO₂
B. Carbaminohemoglobin
C. Bicarbonate
D. Plasma CO₂
Answer: B
Explanation:
- A. Incorrect: Dissolved form unaffected.
- B. Correct: Hb binding O₂ displaces CO₂ from carbaminoHb (Haldane effect).
- C. Incorrect: Bicarbonate less affected.
- D. Incorrect: Plasma CO₂ unaffected.
Q10. Which of the following is true about Haldane effect?
A. Oxygenated blood can carry more CO₂
B. Deoxygenated blood carries more CO₂
C. CO₂ transport unaffected by O₂ binding
D. Hb cannot carry CO₂
Answer: B
Explanation:
- A. Incorrect: Opposite is true.
- B. Correct: Deoxygenated Hb binds CO₂ better (as carbaminoHb).
- C. Incorrect: O₂ binding influences CO₂ transport.
- D. Incorrect: Hb does carry CO₂.
Q11. The primary buffer system in blood involves:
A. HCl/H⁺
B. Na⁺/K⁺
C. H₂CO₃/HCO₃⁻
D. Hb/Fe²⁺
Answer: C
Explanation:
- A. Incorrect: Not a buffer.
- B. Incorrect: Ions, not buffer pair.
- C. Correct: Carbonic acid/bicarbonate system maintains pH.
- D. Incorrect: Hb buffer is secondary.
Q12. Normal blood pH is maintained at:
A. 6.8
B. 7.0
C. 7.4
D. 8.0
Answer: C
Explanation:
- A. Incorrect: Too acidic.
- B. Incorrect: Neutral, but blood is slightly alkaline.
- C. Correct: Normal pH ≈ 7.4.
- D. Incorrect: Too alkaline.
Q13. During exercise, CO₂ transport in blood increases mainly as:
A. Dissolved form
B. CarbaminoHb
C. Bicarbonate ions
D. Free radicals
Answer: C
Explanation:
- A. Incorrect: Dissolved fraction small.
- B. Incorrect: Increases, but less than bicarbonate.
- C. Correct: Majority CO₂ converted to HCO₃⁻.
- D. Incorrect: Free radicals not transport form.
Q14. Which condition occurs when blood CO₂ rises excessively?
A. Alkalosis
B. Acidosis
C. Hyperthermia
D. Hypoxia
Answer: B
Explanation:
- A. Incorrect: Alkalosis = high pH.
- B. Correct: More CO₂ → more H⁺ → acidosis.
- C. Incorrect: Hyperthermia = high temp.
- D. Incorrect: Hypoxia = low O₂.
Q15. Which condition results from reduced CO₂ in blood due to hyperventilation?
A. Respiratory acidosis
B. Respiratory alkalosis
C. Metabolic acidosis
D. Hypocapnia
Answer: B
Explanation:
- A. Incorrect: More CO₂ = acidosis.
- B. Correct: Excess CO₂ loss → alkalosis.
- C. Incorrect: Metabolic acidosis = due to metabolism.
- D. Incorrect: Hypocapnia is low CO₂, but effect is alkalosis.
Q16. Carbon dioxide binds to hemoglobin at which site?
A. Heme iron
B. Globin’s amino groups
C. Porphyrin ring
D. RBC membrane
Answer: B
Explanation:
- A. Incorrect: Heme iron binds O₂, not CO₂.
- B. Correct: CO₂ binds to N-terminal amino groups of globin.
- C. Incorrect: Not porphyrin.
- D. Incorrect: Not membrane.
Q17. Normal pCO₂ of arterial blood is about:
A. 20 mmHg
B. 30 mmHg
C. 40 mmHg
D. 50 mmHg
Answer: C
Explanation:
- A. Incorrect: Too low.
- B. Incorrect: Still low.
- C. Correct: Arterial pCO₂ ≈ 40 mmHg.
- D. Incorrect: 50 mmHg = venous.
Q18. The bicarbonate ions formed in RBCs move into plasma in exchange for:
A. Na⁺
B. K⁺
C. Cl⁻
D. H⁺
Answer: C
Explanation:
- A. Incorrect: Na⁺ not exchanged.
- B. Incorrect: K⁺ not exchanged.
- C. Correct: Cl⁻ shift maintains electroneutrality.
- D. Incorrect: H⁺ remains bound to Hb.
Q19. In lungs, CO₂ leaves the blood because:
A. Alveolar pCO₂ is higher
B. Blood pCO₂ is higher than alveolar pCO₂
C. O₂ pushes CO₂ out actively
D. Hb releases H⁺
Answer: B
Explanation:
- A. Incorrect: Alveolar pCO₂ is lower.
- B. Correct: Venous blood pCO₂ ≈ 45 mmHg vs alveolar 40 mmHg.
- C. Incorrect: No active pumping.
- D. Incorrect: H⁺ release contributes indirectly.
Q20. Which statement is true regarding carbaminohemoglobin?
A. CO₂ binds at heme site
B. It is unstable and easily reversible
C. It is the major transport form of CO₂
D. It prevents Hb binding O₂
Answer: B
Explanation:
- A. Incorrect: CO₂ binds globin, not heme.
- B. Correct: CarbaminoHb is reversible.
- C. Incorrect: Major form is bicarbonate.
- D. Incorrect: Hb can still bind O₂.
Q21. Which factor enhances CO₂ transport in blood?
A. High pO₂
B. Low pO₂
C. Low pCO₂
D. Low H⁺
Answer: B
Explanation:
- A. Incorrect: High O₂ favors O₂ binding, not CO₂.
- B. Correct: Low O₂ (deoxygenated Hb) binds CO₂ better → Haldane effect.
- C. Incorrect: Low pCO₂ reduces transport.
- D. Incorrect: Low H⁺ reduces bicarbonate.
Q22. The effect of oxygen on CO₂ transport is called:
A. Bohr effect
B. Haldane effect
C. Chloride shift
D. Root effect
Answer: B
Explanation:
- A. Incorrect: Bohr effect = CO₂ effect on O₂ transport.
- B. Correct: Haldane effect = O₂ effect on CO₂ transport.
- C. Incorrect: Chloride shift balances ions.
- D. Incorrect: Root effect seen in fish Hb.
Q23. Which factor increases bicarbonate ion formation in RBCs?
A. High carbonic anhydrase activity
B. Low CO₂ availability
C. High O₂ binding to Hb
D. Low water availability
Answer: A
Explanation:
- A. Correct: Carbonic anhydrase accelerates HCO₃⁻ formation.
- B. Incorrect: Less CO₂ means less bicarbonate.
- C. Incorrect: O₂ binding doesn’t increase bicarbonate.
- D. Incorrect: Water is abundant.
Q24. The major form of CO₂ transported from tissues to lungs is:
A. Dissolved CO₂
B. CarbaminoHb
C. Bicarbonate ions
D. Free CO₂ bubbles
Answer: C
Explanation:
- A. Incorrect: Only 7%.
- B. Incorrect: Only 23%.
- C. Correct: 70% transported as HCO₃⁻.
- D. Incorrect: Free bubbles not present.
Q25. At tissues, the presence of deoxygenated Hb helps CO₂ transport because:
A. Hb directly pumps CO₂
B. Hb buffers H⁺ produced in RBCs
C. Hb converts CO₂ to bicarbonate
D. Hb dissolves CO₂
Answer: B
Explanation:
- A. Incorrect: No pumping.
- B. Correct: Hb binds H⁺, driving bicarbonate formation → helps CO₂ transport.
- C. Incorrect: Enzyme catalyzes conversion, not Hb.
- D. Incorrect: Hb doesn’t dissolve gases.
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