Part 4: Exchange of Gases (25 MCQs)

Part 4: Exchange of Gases (25 MCQs)

Q1. The primary site of gaseous exchange in humans is:

A. Trachea
B. Bronchi
C. Alveoli
D. Pleura

Answer: C

Explanation:

• A. Incorrect: Trachea is a conducting passage.
• B. Incorrect: Bronchi only conduct air.
• C. Correct: Alveoli with thin walls and rich blood supply allow gas exchange.
• D. Incorrect: Pleura is protective covering.

Q2. The diffusion of gases across the alveolar membrane occurs due to:

A. Active transport
B. Osmosis
C. Simple diffusion along pressure gradients
D. Facilitated diffusion

Answer: C

Explanation:

• A. Incorrect: No ATP required.
• B. Incorrect: Osmosis applies to water, not gases.
• C. Correct: Gases diffuse passively down partial pressure gradients.
• D. Incorrect: No carrier proteins are involved.

Q3. The thickness of the respiratory membrane is about:

A. 50 µm
B. 5 µm
C. 0.5 µm
D. 5 mm

Answer: C

Explanation:

• A. Incorrect: Too thick for diffusion.
• B. Incorrect: Still too thick.
• C. Correct: Respiratory membrane is ~0.5 µm thin.
• D. Incorrect: Way too thick.

Q4. Which law explains diffusion of gases based on their partial pressure?

A. Dalton’s law
B. Boyle’s law
C. Henry’s law
D. Laplace’s law

Answer: A

Explanation:

• A. Correct: Dalton’s law states each gas exerts pressure proportional to its concentration.
• B. Incorrect: Boyle’s law relates volume and pressure.
• C. Incorrect: Henry’s law = gas solubility.
• D. Incorrect: Laplace’s law relates to surface tension.

Q5. The partial pressure of oxygen (pO₂) in alveoli is approximately:

A. 40 mmHg
B. 60 mmHg
C. 104 mmHg
D. 159 mmHg

Answer: C

Explanation:

• A. Incorrect: 40 mmHg is venous blood pO₂.
• B. Incorrect: Too low for alveoli.
• C. Correct: Alveolar pO₂ ≈ 104 mmHg.
• D. Incorrect: 159 mmHg = atmospheric pO₂ at sea level.

Q6. The partial pressure of CO₂ in alveoli is about:

A. 0.3 mmHg
B. 40 mmHg
C. 95 mmHg
D. 100 mmHg

Answer: B

Explanation:

• A. Incorrect: Too low.
• B. Correct: Alveolar pCO₂ ≈ 40 mmHg.
• C. Incorrect: 95 mmHg = venous blood pO₂.
• D. Incorrect: 100 mmHg = arterial blood pO₂.

Q7. The primary driving force for diffusion of oxygen across alveoli is:

A. Concentration gradient
B. Partial pressure gradient
C. Active pumping by ATP
D. Temperature difference

Answer: B

Explanation:

• A. Incorrect: Concentration is linked but measured as partial pressure.
• B. Correct: Oxygen diffuses from high alveolar pO₂ to low venous pO₂.
• C. Incorrect: Active transport not required.
• D. Incorrect: Temperature does not drive exchange.

Q8. The partial pressure of oxygen in deoxygenated venous blood is approximately:

A. 104 mmHg
B. 95 mmHg
C. 40 mmHg
D. 75 mmHg

Answer: C

Explanation:

• A. Incorrect: 104 mmHg = alveolar pO₂.
• B. Incorrect: 95 mmHg = arterial blood.
• C. Correct: Venous blood pO₂ ≈ 40 mmHg.
• D. Incorrect: Not standard venous value.

Q9. The exchange of gases at tissues is called:

A. Internal respiration
B. External respiration
C. Cellular respiration
D. Pulmonary ventilation

Answer: A

Explanation:

• A. Correct: Internal respiration = exchange between systemic blood and tissues.
• B. Incorrect: External respiration = alveoli ↔ blood.
• C. Incorrect: Cellular respiration = mitochondria.
• D. Incorrect: Ventilation = air movement.

Q10. Oxygen diffuses from alveoli into blood because:

A. Alveolar pO₂ is higher than venous pO₂
B. Blood temperature is higher than alveoli
C. Alveolar pCO₂ is higher
D. Alveoli actively pump O₂

Answer: A

Explanation:

• A. Correct: pO₂ gradient drives diffusion (104 → 40 mmHg).
• B. Incorrect: Temperature irrelevant.
• C. Incorrect: pCO₂ does not drive O₂ diffusion.
• D. Incorrect: No active pumping.

Q11. Carbon dioxide diffuses out of blood into alveoli because:

A. Alveolar pCO₂ is higher
B. Blood pCO₂ is higher than alveolar pCO₂
C. Alveoli secrete CO₂
D. Active transport occurs

Answer: B

Explanation:

• A. Incorrect: Alveolar pCO₂ is lower (40 mmHg).
• B. Correct: Venous blood pCO₂ (≈45 mmHg) > alveoli (≈40 mmHg).
• C. Incorrect: Alveoli don’t secrete CO₂.
• D. Incorrect: Passive diffusion only.

Q12. Which gas diffuses more rapidly across respiratory membrane?

A. Oxygen
B. Nitrogen
C. Carbon dioxide
D. Carbon monoxide

Answer: C

Explanation:

• A. Incorrect: O₂ is slower compared to CO₂.
• B. Incorrect: Nitrogen is poorly soluble.
• C. Correct: CO₂ diffuses ~20 times faster due to higher solubility.
• D. Incorrect: CO binds Hb strongly, but diffusion is slower.

Q13. Which law explains the solubility of gases in blood plasma?

A. Boyle’s law
B. Charles’ law
C. Dalton’s law
D. Henry’s law

Answer: D

Explanation:

• A. Incorrect: Pressure-volume law.
• B. Incorrect: Temp-volume relation.
• C. Incorrect: Dalton’s = partial pressures.
• D. Correct: Henry’s law = gas solubility in liquid proportional to partial pressure.

Q14. Which factor reduces diffusion of gases across alveoli?

A. Increased surface area
B. Thin membrane
C. Edema (fluid accumulation)
D. High partial pressure gradient

Answer: C

Explanation:

• A. Incorrect: Surface area increases diffusion.
• B. Incorrect: Thin membrane improves diffusion.
• C. Correct: Edema thickens membrane, reducing diffusion.
• D. Incorrect: Gradient increases diffusion.

Q15. What is the partial pressure of oxygen in arterial blood?

A. 40 mmHg
B. 95 mmHg
C. 104 mmHg
D. 159 mmHg

Answer: B

Explanation:

• A. Incorrect: Venous value.
• B. Correct: Arterial blood pO₂ ≈ 95 mmHg.
• C. Incorrect: Alveolar value.
• D. Incorrect: Atmospheric value.

Q16. The partial pressure of CO₂ in venous blood is:

A. 40 mmHg
B. 45 mmHg
C. 104 mmHg
D. 159 mmHg

Answer: B

Explanation:

• A. Incorrect: 40 mmHg = alveolar pCO₂.
• B. Correct: Venous pCO₂ ≈ 45 mmHg.
• C. Incorrect: Not CO₂ value.
• D. Incorrect: Atmospheric pO₂, not CO₂.

Q17. Which factor does NOT influence rate of gas diffusion across alveolar membrane?

A. Membrane thickness
B. Surface area
C. Solubility of gas
D. Color of blood

Answer: D

Explanation:

• A. Correct factor: Thickness affects diffusion.
• B. Correct factor: Surface area important.
• C. Correct factor: Solubility critical.
• D. Correct answer: Blood color is irrelevant.

Q18. Which condition decreases alveolar pO₂?

A. Hyperventilation
B. High altitude
C. Exercise
D. Both B and C

Answer: D

Explanation:

• A. Incorrect: Hyperventilation increases alveolar pO₂.
• B. Correct: High altitude decreases atmospheric pO₂.
• C. Correct: Exercise increases O₂ demand, lowering alveolar pO₂.
• D. Correct answer: Both B and C.

Q19. At high altitude, the major problem is:

A. High alveolar pO₂
B. Low alveolar pO₂
C. High alveolar pCO₂
D. High intrapulmonary pressure

Answer: B

Explanation:

• A. Incorrect: Not higher, but lower.
• B. Correct: Reduced barometric pressure → low alveolar pO₂.
• C. Incorrect: pCO₂ decreases due to hyperventilation.
• D. Incorrect: Intrapulmonary pressure normal.

Q20. The difference in pO₂ between alveolar air and venous blood is approximately:

A. 20 mmHg
B. 40 mmHg
C. 64 mmHg
D. 104 mmHg

Answer: C

Explanation:

• A. Incorrect: Too low.
• B. Incorrect: Value corresponds to venous blood pO₂.
• C. Correct: Alveoli (104) – venous (40) ≈ 64 mmHg.
• D. Incorrect: Not correct difference.

Q21. What is the respiratory quotient (RQ) when carbohydrate is the substrate?

A. 0.7
B. 0.8
C. 1.0
D. 1.2

Answer: C

Explanation:

• A. Incorrect: RQ for fats.
• B. Incorrect: RQ for proteins.
• C. Correct: RQ for carbohydrates = 1.0.
• D. Incorrect: Not physiological.

Q22. The driving force for CO₂ diffusion out of blood is:

A. Solubility of CO₂ in water
B. Venous pCO₂ higher than alveolar pCO₂
C. Alveolar pCO₂ higher than venous pCO₂
D. ATP-dependent pumping

Answer: B

Explanation:

• A. Incorrect: Solubility aids but doesn’t drive diffusion.
• B. Correct: Venous pCO₂ (45) > alveolar (40).
• C. Incorrect: Wrong gradient.
• D. Incorrect: Passive, not ATP-driven.

Q23. Oxygen transport from lungs to tissues is facilitated because:

A. Tissue pO₂ is higher than arterial blood pO₂
B. Tissue pO₂ is lower than arterial blood pO₂
C. Tissue pCO₂ is lower than alveoli
D. Arterial pO₂ is lower than venous

Answer: B

Explanation:

• A. Incorrect: Tissue pO₂ is lower.
• B. Correct: O₂ diffuses from blood (95 mmHg) to tissues (40 mmHg).
• C. Incorrect: Tissues have higher pCO₂.
• D. Incorrect: Arterial is higher than venous.

Q24. The respiratory membrane includes all EXCEPT:

A. Alveolar epithelium
B. Capillary endothelium
C. Fused basement membrane
D. Pleural epithelium

Answer: D

Explanation:

• A. Correct component.
• B. Correct component.
• C. Correct component.
• D. Correct answer: Pleura is not part of respiratory membrane.

Q25. In systemic tissues, oxygen diffuses from blood into tissues because:

A. Tissue pO₂ is lower
B. Tissue pO₂ is higher
C. Blood pO₂ is lower
D. Both blood and tissue pO₂ are equal

Answer: A

Explanation:

• A. Correct: Tissue pO₂ (≈40 mmHg) is lower than arterial blood (≈95 mmHg).
• B. Incorrect: Tissues are not higher.
• C. Incorrect: Blood has higher O₂.
• D. Incorrect: Never equal.

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