Respiratory Mechanics and Gas Exchange Practice Questions
20 free Respiratory Mechanics and Gas Exchange practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
During normal quiet breathing, which pressure gradient primarily drives air into the lungs during inspiration?
- A Atmospheric pressure (Patm) lower than intrapleural pressure (Pip)
- B Intrapleural pressure (Pip) more negative than alveolar pressure (Palv)
- C Alveolar pressure (Palv) higher than atmospheric pressure (Patm)
- D Intrapulmonary pressure equal to atmospheric pressure
Correct answer: Intrapleural pressure (Pip) more negative than alveolar pressure (Palv)
During inspiration, the intrapleural (pleural cavity) pressure becomes more negative, pulling the lungs outward; this lowers alveolar pressure below atmospheric, so air flows in down the pressure gradient.
What is the best definition of “minute ventilation”?
- A Tidal volume multiplied by (tidal volume – dead space)
- B Tidal volume times respiratory rate
- C Alveolar ventilation per breath
- D Total lung capacity divided by respiratory rate
Correct answer: Tidal volume times respiratory rate
Minute ventilation equals tidal volume multiplied by respiratory rate, representing total air moved in and out of the lungs per minute.
Which lung volume component represents the volume of air remaining in the lungs after maximal expiration?
- A Tidal volume (TV)
- B Inspiratory reserve volume (IRV)
- C Expiratory reserve volume (ERV)
- D Residual volume (RV)
Correct answer: Residual volume (RV)
Residual volume is the air that remains in the lungs after a maximal exhalation, preventing alveolar collapse and maintaining some gas in the lungs.
Which statement correctly describes lung (pulmonary) compliance?
- A It is directly proportional to airway resistance
- B It is the volume change per unit pressure change
- C It measures the speed of airflow into the alveoli
- D It increases when surfactant is deficient
Correct answer: It is the volume change per unit pressure change
Lung compliance is defined as the change in lung volume per unit change in transpulmonary pressure (ΔV/ΔP), reflecting how easily the lung can expand.
Which of the following would decrease lung compliance under pathological conditions?
- A Increased surfactant production
- B Loss of alveolar elastic tissue, as in emphysema
- C Thickening of alveolar walls (fibrosis)
- D Increased airway diameter
Correct answer: Thickening of alveolar walls (fibrosis)
Thickening of alveolar walls (as in pulmonary fibrosis) increases stiffness and reduces compliance, making lung expansion more difficult.
What is ‘physiologic dead space’ in the context of ventilation and gas exchange?
- A Air that reaches alveoli and participates in gas exchange
- B Air in conducting airways plus non-perfused alveoli
- C The tidal volume during maximal expiration
- D Air trapped in the alveoli at residual volume
Correct answer: Air in conducting airways plus non-perfused alveoli
Physiologic dead space includes anatomical dead space (conducting airways) plus any alveoli that are ventilated but not perfused; this air does not participate in gas exchange.
How does increasing tidal volume compare to increasing respiratory rate in improving alveolar ventilation in a hypercapnic patient?
- A Increasing tidal volume is more efficient than increasing rate
- B Increasing rate is more efficient than increasing tidal volume
- C Both are equally efficient
- D Neither helps because dead space is fixed
Correct answer: Increasing tidal volume is more efficient than increasing rate
Because alveolar ventilation = respiratory rate × (tidal volume – dead space), increasing tidal volume reduces the proportion lost to dead space and more effectively increases alveolar ventilation than increasing rate.
Which of the following factors does NOT significantly affect the diffusion of O₂ and CO₂ across the alveolar–capillary membrane?
- A Alveolar surface area
- B Thickness of the respiratory membrane
- C Ventilation–perfusion matching
- D Blood flow in systemic arteries
Correct answer: Blood flow in systemic arteries
Gas diffusion depends on alveolar surface area, membrane thickness, and efficient V/Q matching; systemic arterial blood flow doesn’t directly affect diffusion across the alveolar capillaries.
A region of the lung receives ventilation but little blood flow. Which describes this situation and its effect on gas exchange?
- A Shunt; CO₂ builds up and O₂ exchange is enhanced
- B Dead space ventilation; O₂ and CO₂ exchange impaired
- C Perfusion-limited exchange; gas exchange remains normal
- D Diffusion-limited exchange; CO₂ removal increases
Correct answer: Dead space ventilation; O₂ and CO₂ exchange impaired
Ventilation without adequate perfusion defines dead space ventilation, which reduces effective gas exchange (O₂ uptake and CO₂ removal) because blood flow is insufficient.
Which statement best describes the concept of ventilation–perfusion (V/Q) mismatch?
- A Regions where ventilation and perfusion are both zero
- B Uniform ventilation and perfusion throughout the lung
- C Mismatch of ventilation and perfusion in lung regions
- D Equal global reduction in ventilation and perfusion
Correct answer: Mismatch of ventilation and perfusion in lung regions
V/Q mismatch arises when some lung regions receive differing levels of ventilation and perfusion, impairing efficient gas exchange.
Which physiologic mechanism helps redirect blood flow away from poorly ventilated alveoli to well-ventilated areas to optimize gas exchange?
- A Systemic vasodilation
- B Bronchoconstriction
- C Hypoxic pulmonary vasoconstriction
- D Increased surfactant production
Correct answer: Hypoxic pulmonary vasoconstriction
Hypoxic pulmonary vasoconstriction constricts small pulmonary arteries in hypoxic regions, redirecting blood toward better-ventilated alveoli to improve overall V/Q matching.
In a healthy adult at rest, which approximate value of ventilation-perfusion (V/Q) ratio best represents the average for the entire lung?
- A 0.2
- B 0.8
- C 2.0
- D 5.0
Correct answer: 0.8
The average V/Q ratio in a healthy lung at rest is around 0.8, reflecting balance between ventilation and perfusion necessary for efficient gas exchange.
What happens to alveolar oxygen partial pressure (PAO₂) and arterial oxygenation if V/Q mismatch causes negligible ventilation but maintained perfusion in a lung region?
- A PAO₂ rises, arterial oxygenation increases
- B PAO₂ falls toward venous levels, arterial oxygenation decreases
- C PAO₂ remains normal, arterial oxygenation increases
- D PAO₂ falls, but arterial oxygenation unaffected due to compensation
Correct answer: PAO₂ falls toward venous levels, arterial oxygenation decreases
If ventilation drops but perfusion remains, alveolar gas equilibrates with poorly oxygenated blood, causing low PAO₂ and decreased arterial oxygen — similar to a physiological shunt.
Which gas exchange scenario is considered perfusion-limited rather than diffusion-limited under normal conditions?
- A O₂ transfer when alveolar-capillary membrane thickens
- B N₂O uptake under normal membrane conditions
- C CO₂ transfer in pulmonary edema
- D O₂ transfer in interstitial fibrosis
Correct answer: N₂O uptake under normal membrane conditions
Perfusion-limited gases (like N₂O) equilibrate rapidly across the membrane so the limiting factor is capillary blood flow — not the diffusion properties.
Which factor would most directly increase the work of breathing in a patient with lung disease?
- A Increased lung compliance
- B Decreased airway resistance
- C Reduced chest wall compliance
- D Decreased respiratory rate
Correct answer: Reduced chest wall compliance
Decreased chest wall compliance makes it harder to expand the thoracic cavity, increasing the work required for breathing.
Why does surfactant deficiency lead to decreased lung compliance in neonates with respiratory distress syndrome?
- A Because surfactant raises airway resistance to airflow
- B Because surfactant lowers surface tension, easing expansion
- C Because surfactant thickens the alveolar–capillary membrane
- D Because surfactant blocks perfusion of alveolar capillaries
Correct answer: Because surfactant lowers surface tension, easing expansion
Surfactant reduces surface tension at the alveolar air–liquid interface, lowering the pressure needed to expand alveoli; deficiency raises surface tension, increasing the work of expansion and reducing compliance.
Which of the following best describes the effect of increasing physiologic dead space on alveolar ventilation (V_A), assuming constant minute ventilation?
- A Alveolar ventilation increases
- B Alveolar ventilation decreases
- C Alveolar ventilation remains unchanged
- D Minute ventilation becomes zero
Correct answer: Alveolar ventilation decreases
If physiologic dead space increases, more of the tidal volume fails to reach perfused alveoli, so alveolar ventilation decreases even if minute ventilation is unchanged.
Which of the following changes would you expect in a diffusion-limited gas exchange scenario (e.g., thickened alveolar membrane)?
- A Equilibration achieved rapidly despite membrane thickening
- B Blood flow becomes the limiting factor for gas transfer
- C Slower O₂ and CO₂ transfer across the membrane
- D Perfusion increases to overcome diffusion limitation
Correct answer: Slower O₂ and CO₂ transfer across the membrane
Thickening of the alveolar–capillary membrane slows diffusion across it, reducing the rate of O₂ uptake and CO₂ elimination, characteristic of diffusion-limited exchange.
During exercise, which adjustment helps maintain adequate gas exchange despite increased metabolic demand?
- A Decreased tidal volume and decreased respiratory rate
- B Increased dead space fraction
- C Increased tidal volume and alveolar ventilation
- D Decreased perfusion of lung capillaries
Correct answer: Increased tidal volume and alveolar ventilation
Exercise increases metabolic demand; to meet this, tidal volume and alveolar ventilation increase so more O₂ is inhaled and CO₂ removed per minute.
Which feature of the alveolar–capillary membrane optimizes efficient gas exchange under normal conditions?
- A Thick, multi-layered walls and low surface area
- B Thin membrane, large surface area, dense capillaries
- C High airway resistance and low perfusion
- D Minimal capillary network to reduce diffusion distance
Correct answer: Thin membrane, large surface area, dense capillaries
Efficient gas exchange requires a thin respiratory membrane to minimize diffusion distance and a large alveolar surface area with dense capillary network to maximize gas exchange capacity.