Respiratory System — MCQs
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What is the normal value of respiratory compliance in ml/cm H2O?
What is the Haldane Effect?
What is the partial pressure for oxygen in the inspired air?
In the relaxation pressure curve, at zero relaxation pressure in chronic smokers:
Which of the following is markedly decreased in restrictive lung disease?
Which of the following statements about breathing is incorrect?
Maximum voluntary ventilation is:
Central chemoreceptors are most sensitive to which of the following changes in blood?
What is the air remaining in the lung after normal expiration?
What does Boyle's Law state?
Respiratory System Indian Medical PG Practice Questions and MCQs
Question 821: What is the normal value of respiratory compliance in ml/cm H2O?
- A. 200 ml/cm H2O (Correct Answer)
- B. 100 ml/cm H2O
- C. 150 ml/cm H2O
- D. 50 ml/cm H2O
Explanation: ***200 ml/cm H2O*** - Normal respiratory system compliance is approximately **200 ml/cm H2O**, indicating a relatively compliant lung and chest wall system. - This value reflects the **change in lung volume per unit change in pressure**, with higher values indicating greater elasticity and ease of inflation (distensibility). *50 ml/cm H2O* - A compliance of **50 ml/cm H2O** is significantly lower than normal, suggesting a **stiff respiratory system**. - This could be indicative of conditions like **pulmonary fibrosis**, **acute respiratory distress syndrome (ARDS)**, or severe asthma, where the lungs are harder to inflate. *100 ml/cm H2O* - A compliance of **100 ml/cm H2O** is typically considered **reduced compliance**, although not as severely as 50 ml/cm H2O. - This value might be seen in moderate lung diseases or conditions causing **reduced chest wall expansion**. *150 ml/cm H2O* - While closer to the normal range, **150 ml/cm H2O** is generally still considered to be on the **lower side of normal or mildly reduced compliance**. - This could indicate early or mild conditions affecting **lung or chest wall mechanics**.
Question 822: What is the Haldane Effect?
- A. O2 delivery by increased CO2
- B. CO2 delivery by increased CO2
- C. CO2 delivery by increased O2 (Correct Answer)
- D. O2 delivery by increased CO
Explanation: ***CO2 delivery by increased O2*** - The **Haldane effect** describes how **oxygenation of hemoglobin** decreases its affinity for **carbon dioxide (CO2)**, leading to the release of CO2 from the blood. - This is crucial in the lungs, where high oxygen levels promote CO2 unloading for exhalation. *O2 delivery by increased CO2* - This describes the **Bohr effect**, where an increase in **carbon dioxide (CO2)** or acidity in the tissues causes hemoglobin to release **oxygen (O2)**. - The Haldane effect is the converse, relating oxygen binding to CO2 release, not the other way around. *CO2 delivery by increased CO2* - This statement is inherently circular and does not describe a physiological effect. - It confuses the mechanism with the substance being transported. *O2 delivery by increased CO* - **Carbon monoxide (CO)** has a much higher affinity for hemoglobin than oxygen, forming **carboxyhemoglobin** and impairing oxygen delivery. - This is related to **carbon monoxide poisoning**, not a physiological regulatory effect like the Haldane or Bohr effects.
Question 823: What is the partial pressure for oxygen in the inspired air?
- A. 158 mm Hg (Correct Answer)
- B. 116 mm Hg
- C. 0.3 mm Hg
- D. 100 mm Hg
Explanation: ***158 mm Hg*** - The partial pressure of oxygen in inspired air (PIO2) is calculated by multiplying the **fraction of inspired oxygen (FiO2)** by the total atmospheric pressure. - At sea level, atmospheric pressure is approximately **760 mm Hg** and FiO2 is 21% (0.21), so 0.21 × 760 mm Hg = **159.6 mm Hg**, which rounds to 158 mm Hg. - This represents **dry atmospheric air** before it enters the respiratory tract. *116 mm Hg* - This value does not correspond to a standard physiological measurement in respiratory physiology. - It is lower than inspired air PO2 but higher than alveolar PO2, making it an intermediate value used as a distractor. - **Humidified tracheal air** has PO2 of approximately 150 mm Hg: (760-47) × 0.21 = 149.7 mm Hg, where 47 mm Hg is water vapor pressure. *0.3 mm Hg* - This value is extremely low and represents the approximate **partial pressure of oxygen in mixed venous blood**, not inspired air. - Such a low value in inspired air would indicate **severe hypoxia** incompatible with life. - This is used as an unrealistic distractor. *100 mm Hg* - This value represents the approximate **partial pressure of oxygen in alveolar air (PAO2) and arterial blood (PaO2)**. - It is lower than inspired air due to humidification, mixing with residual air, and continuous oxygen uptake by blood. - It does not represent the partial pressure of oxygen in the inspired atmospheric air.
Question 824: In the relaxation pressure curve, at zero relaxation pressure in chronic smokers:
- A. Lung volume decreases significantly
- B. Lung volume remains elevated (Correct Answer)
- C. No significant change in lung volume
- D. Lung compliance decreases
Explanation: ***Lung volume remains elevated*** - In chronic smokers, conditions like **emphysema** lead to loss of elastic recoil and **air trapping**. - At zero relaxation pressure (the point where the respiratory system is at its resting equilibrium), the **functional residual capacity (FRC)** is higher due to less elastic recoil, which maintains the lungs at a more inflated state. - The balance between inward lung recoil and outward chest wall recoil shifts, resulting in a new equilibrium at a higher lung volume. *Lung volume decreases significantly* - This would imply increased elastic recoil or significant **airway obstruction** preventing air from entering, which is contrary to the typical pathophysiological changes in chronic smokers (e.g., emphysema). - In emphysema, the **loss of elastic recoil** actually prevents the lungs from deflating efficiently, leading to increased rather than decreased lung volume at rest. *No significant change in lung volume* - Chronic smoking often results in **structural changes** to the lungs, particularly **emphysema**, which significantly alters lung mechanics. - These changes directly impact the **resting lung volume (FRC)** as the balance between elastic recoil and chest wall compliance is disturbed, leading to a noticeable increase. *Lung compliance decreases* - This is incorrect; in emphysema, lung **compliance actually increases** due to destruction of alveolar walls and loss of elastic tissue. - Increased compliance means the lungs are more easily distensible but have reduced elastic recoil, contributing to air trapping and elevated FRC.
Question 825: Which of the following is markedly decreased in restrictive lung disease?
- A. FVC (Correct Answer)
- B. RV
- C. FEV1/FVC
- D. FEV1
Explanation: ***FVC*** - In **restrictive lung disease**, there is a reduction in lung volume due to various causes, leading to a markedly decreased **Forced Vital Capacity (FVC)**. - **FVC** directly measures the total amount of air a person can exhale after a maximal inhalation, which is inherently limited in restrictive conditions. - This is the **hallmark finding** in restrictive lung disease and the most clinically significant decrease. *FEV1* - While **FEV1** (Forced Expiratory Volume in 1 second) is also decreased in restrictive lung disease, its decrease is proportional to the FVC decrease. - A decrease in FEV1 alone is less specific, as it could also indicate obstructive lung disease. - The key is that both FEV1 and FVC decrease together, maintaining a normal or increased ratio. *FEV1/FVC* - The **FEV1/FVC ratio** is typically **normal or even increased** in restrictive lung disease, as both FEV1 and FVC decrease proportionally or FEV1 decreases slightly less. - A decreased FEV1/FVC ratio is characteristic of **obstructive lung disease**, not restrictive. *RV* - **Residual Volume (RV)** is also **decreased** in restrictive lung disease, along with all other lung volumes (TLC, VC, FRC). - However, RV is not measured by standard spirometry and requires body plethysmography or gas dilution techniques. - While RV does decrease, **FVC** is the more clinically significant and readily measurable parameter that is "markedly decreased" and defines restrictive disease on routine pulmonary function testing.
Question 826: Which of the following statements about breathing is incorrect?
- A. Inspiration is an active process
- B. Normal breathing occurs when transpulmonary pressure is 5-8 cm H2O (Correct Answer)
- C. Expiration during quiet breathing is passive
- D. Compliance is influenced by multiple factors including surfactant.
Explanation: ***Normal breathing occurs when transpulmonary pressure is 5-8 cm H2O*** - This statement is **incorrect** because it misrepresents transpulmonary pressure during normal breathing. - Normal **transpulmonary pressure** during quiet breathing typically ranges from approximately **3-6 cm H2O** during inspiration, with an average of about **5 cm H2O** at functional residual capacity. - The range "5-8 cm H2O" is too high for normal quiet breathing. While transpulmonary pressure can reach 8 cm H2O during deeper inspiration, stating this as the range for "normal breathing" is inaccurate. - Transpulmonary pressure is the difference between alveolar pressure and pleural pressure (P_L = P_alv - P_pl), which drives lung inflation. *Expiration during quiet breathing is passive* - During quiet breathing, **expiration is a passive process** driven by the **elastic recoil of the lungs** and chest wall. - No active muscular contraction is required for air to leave the lungs during unforced expiration. *Inspiration is an active process* - **Inspiration is an active process** requiring muscular contraction, primarily of the **diaphragm and external intercostal muscles**. - These muscles contract to increase the thoracic volume, which decreases intrapleural and alveolar pressures, drawing air into the lungs. *Compliance is influenced by multiple factors including surfactant* - **Lung compliance**, a measure of the lung's distensibility, is significantly influenced by **surfactant**. - Surfactant reduces **surface tension** in the alveoli, preventing their collapse and increasing compliance.
Question 827: Maximum voluntary ventilation is:
- A. 25 L/min
- B. 50 L/min
- C. 100 L/min
- D. 150 L/min (Correct Answer)
Explanation: ***150 L/min*** - The **Maximum Voluntary Ventilation (MVV)** represents the largest volume of air that can be breathed in and out using maximal effort over a 10-15 second period. - While it varies among individuals, a typical average value for a healthy adult is approximately **150-170 L/min**. *25 L/min* - This value is significantly lower than the typical MVV; 25 L/min is closer to a normal **resting minute ventilation** (tidal volume multiplied by respiratory rate). - Resting minute ventilation reflects the volume of air exchanged at rest, not the maximum capacity. *50 L/min* - This value is still considerably lower than the average MVV and does not represent the maximum capacity of the respiratory system. - It might be seen in individuals with **severe pulmonary impairment** or at a very high resting metabolic rate. *100 L/min* - While higher than resting values, 100 L/min is generally below the average maximum voluntary ventilation for a healthy adult. - It could represent a MVV in individuals with **mild to moderate respiratory compromise** or less effort during the test.
Question 828: Central chemoreceptors are most sensitive to which of the following changes in blood?
- A. PO2
- B. HCO3-
- C. pH
- D. PCO2 (Correct Answer)
Explanation: ***PCO2*** - Central chemoreceptors, located in the **medulla oblongata**, are exquisitely sensitive to changes in the **partial pressure of carbon dioxide (PCO2)** in the arterial blood. - An increase in blood PCO2 readily crosses the **blood-brain barrier** to the cerebrospinal fluid (CSF), where it is converted to carbonic acid and then to H+ and HCO3-. The resulting **drop in CSF pH** directly stimulates these chemoreceptors, leading to increased ventilation. *PO2* - While **peripheral chemoreceptors** (carotid and aortic bodies) are sensitive to changes in **PO2**, particularly when it drops significantly (below 60 mmHg), central chemoreceptors are not. - The primary role of central chemoreceptors is to monitor and respond to changes in CO2 and pH, rather than oxygen levels. *pH* - Central chemoreceptors are indirectly sensitive to **pH changes** in the cerebrospinal fluid (CSF), which result from blood PCO2 changes. - However, they are not directly or primarily sensitive to changes in **blood pH** because hydrogen ions do not readily cross the blood-brain barrier. *HCO3-* - Bicarbonate ions (**HCO3-**) are important in buffering pH, but central chemoreceptors do not directly sense bicarbonate levels. - Changes in HCO3- indirectly affect pH, and it is the resultant **H+ concentration** in the CSF, derived from CO2, that primarily stimulates central chemoreceptors.
Question 829: What is the air remaining in the lung after normal expiration?
- A. Tidal Volume (TV)
- B. Residual Volume (RV)
- C. Functional Residual Capacity (FRC) (Correct Answer)
- D. Vital Capacity (VC)
Explanation: ***Functional Residual Capacity (FRC)*** - **FRC** represents the volume of air remaining in the lungs after a **normal expiration**. - It is the sum of the **expiratory reserve volume (ERV)** and the **residual volume (RV)**. *Tidal Volume (TV)* - **TV** is the volume of air inspired or expired with a **normal breath**. - It does not represent the total air remaining in the lungs after expiration. *Residual Volume (RV)* - **RV** is the volume of air remaining in the lungs after a **maximal expiration**. - It is a component of FRC but does not fully describe the air remaining after a *normal* expiration. *Vital Capacity (VC)* - **VC** is the maximum volume of air that can be exhaled after a **maximal inspiration**. - It represents the maximum amount of air that can be exchanged with a single breath, not the air remaining after normal expiration.
Question 830: What does Boyle's Law state?
- A. Pressure divided by temperature is constant.
- B. Volume divided by temperature is constant.
- C. PV = constant (Correct Answer)
- D. Pressure multiplied by volume equals the number of moles times the gas constant times temperature.
Explanation: ***PV = constant*** - **Boyle's Law** states that at constant temperature, the pressure and volume of a gas are inversely proportional. - Mathematically expressed as **PV = constant** or **P₁V₁ = P₂V₂** - This means that if the volume of a gas decreases, its pressure increases proportionally, and vice versa. - **Clinically relevant** in understanding lung mechanics during respiration - as thoracic volume increases during inspiration, intrapulmonary pressure decreases, allowing air to flow in. *Pressure divided by temperature is constant.* - This describes **Gay-Lussac's Law** (P/T = constant), which relates pressure and temperature at constant volume. - Shows the direct relationship between pressure and temperature. *Volume divided by temperature is constant.* - This statement describes **Charles's Law** (V/T = constant), which relates the volume and temperature of a gas at constant pressure. - Indicates a direct relationship between volume and temperature. *Pressure multiplied by volume equals the number of moles times the gas constant times temperature.* - This represents the **Ideal Gas Law**: PV = nRT - Combines Boyle's, Charles's, and Avogadro's laws to relate pressure, volume, temperature, and the number of moles of a gas.
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