Part 2: Mechanism of Breathing (25 MCQs)

Q1. During normal inspiration, which of the following occurs?

A. Diaphragm relaxes, thoracic volume decreases
B. Diaphragm contracts, thoracic volume increases
C. External intercostals relax, rib cage moves down
D. Internal intercostals contract, thoracic volume decreases

Answer: B

Explanation:

A. Incorrect: Relaxed diaphragm = expiration.
B. Correct: Diaphragm contraction → dome flattens → thoracic cavity expands.
C. Incorrect: External intercostals contract during inspiration, lifting ribs up.
D. Incorrect: Internal intercostals aid forced expiration, not inspiration.

Q2. Which pressure drives air into the lungs during inspiration?

A. Atmospheric pressure higher than intrapulmonary pressure
B. Intrapulmonary pressure higher than atmospheric pressure
C. Intrapleural pressure equal to atmospheric pressure
D. Intrapulmonary pressure equal to atmospheric pressure

Answer: A

Explanation:

A. Correct: Inspiration occurs when intrapulmonary pressure falls below atmospheric.
B. Incorrect: Air moves out if intrapulmonary > atmospheric.
C. Incorrect: Intrapleural pressure is always negative, not equal.
D. Incorrect: No airflow if equal.

Q3. Normal expiration is a:

A. Active process using diaphragm
B. Passive process due to elastic recoil
C. Active process using abdominal muscles
D. Voluntary action always

Answer: B

Explanation:

A. Incorrect: Inspiration is active, expiration normally passive.
B. Correct: Normal expiration is passive due to recoil of lungs and thoracic cage.
C. Incorrect: Abdominal muscles act in forced expiration.
D. Incorrect: Breathing is involuntary.

Q4. Which muscles contract during forced expiration?

A. External intercostals
B. Internal intercostals and abdominal muscles
C. Diaphragm only
D. Pectoral muscles

Answer: B

Explanation:

A. Incorrect: External intercostals act in inspiration.
B. Correct: Internal intercostals pull ribs down; abdominals push diaphragm up.
C. Incorrect: Diaphragm contracts in inspiration.
D. Incorrect: Pectorals not primary expiratory muscles.

Q5. Intrapleural pressure during normal breathing is:

A. Always positive
B. Always negative
C. Equal to atmospheric pressure
D. Zero

Answer: B

Explanation:

A. Incorrect: Positive pressure collapses lungs.
B. Correct: Intrapleural pressure is always negative to keep lungs inflated.
C. Incorrect: Never equal to atmospheric.
D. Incorrect: Not zero.

Q6. The dome-shaped muscular sheet separating thoracic and abdominal cavities is:

A. Intercostal muscle
B. Diaphragm
C. Pleura
D. Sternum

Answer: B

Explanation:

A. Incorrect: Intercostals are between ribs.
B. Correct: Diaphragm contracts during inspiration.
C. Incorrect: Pleura is lung covering.
D. Incorrect: Sternum is breastbone.

Q7. What happens to intrapulmonary pressure during inspiration?

A. Rises above atmospheric pressure
B. Falls below atmospheric pressure
C. Remains constant
D. Becomes zero

Answer: B

Explanation:

A. Incorrect: Air would leave lungs.
B. Correct: Intrapulmonary pressure falls (by ~2 mmHg) → air enters.
C. Incorrect: Not constant.
D. Incorrect: Not zero.

Q8. Which statement is correct regarding expiration?

A. Intrapulmonary pressure decreases below atmospheric
B. Thoracic volume increases
C. Air moves out as intrapulmonary pressure rises
D. Diaphragm contracts actively

Answer: C

Explanation:

A. Incorrect: Pressure rises, not falls.
B. Incorrect: Thoracic volume decreases.
C. Correct: Expiration occurs as intrapulmonary pressure > atmospheric.
D. Incorrect: Diaphragm relaxes.

Q9. Which phase of breathing is active in normal quiet breathing?

A. Inspiration
B. Expiration
C. Both active
D. Both passive

Answer: A

Explanation:

A. Correct: Inspiration requires muscle contraction.
B. Incorrect: Expiration is passive.
C. Incorrect: Not both.
D. Incorrect: Not both passive.

Q10. Which of the following defines compliance of lungs?

A. Elastic recoil of lungs
B. Ease with which lungs can be expanded
C. Ability to carry oxygen
D. Ability to resist collapse

Answer: B

Explanation:

A. Incorrect: Recoil is opposite of compliance.
B. Correct: Compliance = distensibility of lungs.
C. Incorrect: That is oxygen capacity.
D. Incorrect: Collapse resistance depends on surfactant.

Q11. Surfactant in alveoli helps in:

A. Increasing surface tension
B. Decreasing surface tension
C. Increasing compliance
D. Both B and C

Answer: D

Explanation:

A. Incorrect: Increased tension collapses alveoli.
B. Correct: Surfactant reduces tension.
C. Correct: Reduced tension increases compliance.
D. Correct answer: Both B and C are true.

Q12. Which law explains the movement of air due to pressure gradient during breathing?

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

Answer: A

Explanation:

A. Correct: Boyle’s law → pressure inversely proportional to volume.
B. Incorrect: Charles’ law relates temp & volume.
C. Incorrect: Dalton’s law → partial pressures of gases.
D. Incorrect: Henry’s law → gas solubility in liquids.

Q13. During forced inspiration, which muscles additionally contract?

A. Sternocleidomastoid and scalenes
B. Internal intercostals
C. Abdominal muscles
D. Pectoralis major

Answer: A

Explanation:

A. Correct: Accessory muscles (sternocleidomastoid, scalenes) assist in forced inspiration.
B. Incorrect: Internal intercostals act in expiration.
C. Incorrect: Abdominals act in forced expiration.
D. Incorrect: Pectorals are not main inspiratory muscles.

Q14. Which event occurs when diaphragm relaxes?

A. Thoracic volume increases
B. Lungs expand
C. Thoracic volume decreases
D. Inspiration occurs

Answer: C

Explanation:

A. Incorrect: Relaxation decreases volume.
B. Incorrect: Lungs shrink, not expand.
C. Correct: Relaxation pushes diaphragm up, reduces thoracic volume.
D. Incorrect: Expiration occurs.

Q15. The pleural fluid present between pleural membranes helps in:

A. Gas exchange
B. Preventing lung collapse and friction
C. Digestion of pathogens
D. Air storage

Answer: B

Explanation:

A. Incorrect: Gas exchange occurs in alveoli.
B. Correct: Pleural fluid lubricates and prevents collapse.
C. Incorrect: It is not immune fluid.
D. Incorrect: Air is not stored there.

Q16. Intrapulmonary pressure is also called:

A. Alveolar pressure
B. Atmospheric pressure
C. Intrapleural pressure
D. Transpulmonary pressure

Answer: A

Explanation:

A. Correct: Pressure within alveoli = intrapulmonary.
B. Incorrect: Atmospheric = outside.
C. Incorrect: Intrapleural = pleural cavity.
D. Incorrect: Transpulmonary = difference between alveolar and pleural pressure.

Q17. Which statement about normal expiration is correct?

A. Requires contraction of diaphragm
B. Occurs due to elastic recoil
C. Requires contraction of intercostal muscles
D. Is an active process

Answer: B

Explanation:

A. Incorrect: Diaphragm relaxes.
B. Correct: Expiration is passive due to recoil.
C. Incorrect: No muscle contraction needed.
D. Incorrect: Not active normally.

Q18. Which muscles are primarily involved in quiet expiration?

A. External intercostals
B. Internal intercostals
C. Abdominal muscles
D. None

Answer: D

Explanation:

A. Incorrect: External intercostals act in inspiration.
B. Incorrect: Internal intercostals act in forced expiration.
C. Incorrect: Abdominals act in forced expiration.
D. Correct: Normal expiration is passive, no muscle action.

Q19. What is the normal respiratory rate in a healthy adult human?

A. 6–8 breaths/min
B. 12–16 breaths/min
C. 20–24 breaths/min
D. 30–40 breaths/min

Answer: B

Explanation:

A. Incorrect: Too low.
B. Correct: Normal is ~12–16/min.
C. Incorrect: Slightly high (child).
D. Incorrect: Very high (infant).

Q20. Which factor prevents alveolar collapse at the end of expiration?

A. Pleural pressure
B. Surfactant
C. Alveolar macrophages
D. Pulmonary veins

Answer: B

Explanation:

A. Incorrect: Pleural pressure is negative, but surfactant is main.
B. Correct: Surfactant lowers surface tension, prevents collapse.
C. Incorrect: Macrophages do immunity.
D. Incorrect: Pulmonary veins transport oxygenated blood.

Q21. Which law explains diffusion of gases based on their partial pressure?

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

Answer: B

Explanation:

A. Incorrect: Boyle’s law = pressure-volume.
B. Correct: Dalton’s law = partial pressures govern diffusion.
C. Incorrect: Henry’s law = solubility in liquids.
D. Incorrect: Laplace law = surface tension in alveoli.

Q22. Which condition will make breathing more difficult?

A. Increased compliance
B. Presence of surfactant
C. Decreased compliance
D. Increased alveolar surface area

Answer: C

Explanation:

A. Incorrect: Easier expansion.
B. Incorrect: Surfactant aids breathing.
C. Correct: Low compliance = stiff lungs, harder breathing.
D. Incorrect: More surface area improves exchange.

Q23. The term “Eupnea” refers to:

A. Difficult breathing
B. Normal quiet breathing
C. Rapid breathing
D. Absence of breathing

Answer: B

Explanation:

A. Incorrect: Difficult = dyspnea.
B. Correct: Eupnea = normal quiet breathing.
C. Incorrect: Rapid = tachypnea.
D. Incorrect: Absence = apnea.

Q24. Which pressure is always less than atmospheric pressure during normal breathing?

A. Intrapulmonary pressure
B. Intrapleural pressure
C. Transpulmonary pressure
D. Alveolar pressure

Answer: B

Explanation:

A. Incorrect: Intrapulmonary = atmospheric at end of breathing cycle.
B. Correct: Intrapleural is always negative.
C. Incorrect: Transpulmonary is difference.
D. Incorrect: Alveolar equals atmospheric at rest.

Q25. Which factor primarily determines airflow into lungs?

A. Blood pressure
B. Difference between atmospheric and intrapulmonary pressure
C. Difference between arterial and venous pressure
D. Temperature

Answer: B

Explanation:

A. Incorrect: Blood pressure irrelevant.
B. Correct: Air flows due to pressure gradient between atmosphere and alveoli.
C. Incorrect: That relates to blood flow.
D. Incorrect: Temperature is not primary factor.

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Part 2: Mechanism of Breathing (25 MCQs)