Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 811: What is the total surface area of the respiratory membrane in a healthy adult human?

A. 30 m2
B. 50 m2
C. 75 m2 (Correct Answer)
D. 100 m2

Explanation: ***75 m²*** - The **total surface area** of the respiratory membrane in a healthy adult human is approximately **70-80 m²**, with 75 m² being the most accurate estimate among the given options. - This large surface area is primarily attributed to the presence of approximately **300-500 million alveoli**, which are crucial for efficient gas exchange. - Modern measurements using **stereological techniques** have refined earlier estimates and established this range as the current standard. *100 m²* - This value represents an **older estimate** that has been revised downward with more accurate measurement techniques. - While historically cited in older textbooks, current physiological data supports a **smaller surface area** of approximately 70-80 m². *30 m²* - This value is significantly **underestimated** for the total respiratory membrane surface area. - Such a small surface area would result in highly **inefficient gas exchange**, leading to severe respiratory compromise and inability to meet metabolic demands. *50 m²* - While larger than 30 m², this is still an **underestimation** of the full respiratory membrane surface area. - It does not adequately account for the extensive and intricate branching of the **respiratory bronchioles** and the vast number of alveolar sacs.

Question 812: In zero gravity, the V/Q ratio is?

A. 0.8
B. 1 (Correct Answer)
C. 2
D. 3

Explanation: ***Correct: 1*** - In **zero gravity**, the normal physiological effects of gravity on both ventilation and perfusion are eliminated, leading to a more uniform distribution. - Without gravity, blood flow and gas distribution become more even throughout the lungs, resulting in a V/Q ratio that approaches **unity (1)** across all lung regions. - This represents the ideal physiological state where ventilation perfectly matches perfusion. *Incorrect: 0.8* - A V/Q ratio of **0.8** represents the **average normal V/Q ratio** in an upright individual on Earth, where gravity creates disparities in ventilation and perfusion. - This value is an average, with regional variations (apex ~3.3, base ~0.6) in the lungs; it does not reflect the uniform conditions of zero gravity. *Incorrect: 2* - A V/Q ratio of **2** would indicate a significant **ventilation-perfusion mismatch** where ventilation greatly exceeds perfusion. - This scenario suggests substantial **dead space ventilation**, which is not the expected outcome in a zero-gravity environment where distribution is balanced. *Incorrect: 3* - A V/Q ratio of **3** represents an even more extreme case of **ventilation exceeding perfusion**, indicating severe physiologic dead space. - Such a high V/Q ratio would signify a major functional impairment, which is contrary to the more ideal and uniform distribution expected in zero gravity.

Question 813: Which of the following statements about lung compliance is false?

A. Decreased in emphysema (Correct Answer)
B. Total compliance is 0.2 L/cm H2O
C. A measure of lung distensibility
D. Change in volume per unit change in pressure

Explanation: ***Decreased in emphysema*** - This statement is **false** because **emphysema** is characterized by the destruction of elastic fibers in the lung parenchyma, which paradoxically leads to an **increase** in lung compliance. - The loss of elastic recoil makes the lungs more distensible and easier to inflate, but also impairs their ability to passively exhale. *Total compliance is 0.2 L/cm H2O* - This value represents the **normal total lung compliance** in a healthy adult (0.17 to 0.25 L/cm H2O), including both lung and chest wall compliance. - Lung compliance alone is typically around 0.2 L/cm H2O for healthy lungs. *A measure of lung distensibility* - **Compliance** is intrinsically defined as a measure of how easily the lungs or chest wall can be stretched or distended. - High compliance means the lungs are easy to inflate, while low compliance means they are stiff and difficult to inflate. *Change in volume per unit change in pressure* - This is the explicit **formula and definition of compliance** (C = ΔV/ΔP). - It quantifies the change in lung volume in response to a given change in transpulmonary pressure.

Question 814: Damage to pneumotaxic center along with vagus nerve causes which type of respiration?

A. Cheyne-Stokes breathing
B. Deep and slow breathing
C. Shallow and rapid breathing
D. Apneustic breathing (Correct Answer)

Explanation: ***Apneustic breathing*** - Damage to the **pneumotaxic center** prevents the normal inhibition of inspiration, leading to **prolonged inspiratory gasps**. - **Vagal nerve damage** further removes the inhibitory feedback from the lungs, exacerbating the inspiratory "holds" characteristic of apneustic breathing. *Cheyne-Stokes breathing* - This pattern is characterized by a **crescendo-decrescendo pattern** of breathing, interspersed with periods of **apnea**. - It is often associated with conditions like **heart failure**, stroke, or severe neurological damage, not specifically the pneumotaxic center and vagus nerve. *Deep and slow breathing* - This pattern can be seen in conditions like **Kussmaul breathing** (due to metabolic acidosis) or as a compensatory mechanism. - It does not directly result from the combined damage of the **pneumotaxic center** and the **vagus nerve**. *Shallow and rapid breathing* - This pattern is commonly seen in restrictive lung diseases, anxiety, or pain, where tidal volume is decreased and respiratory rate increased. - It does not reflect the **prolonged inspiration** that would result from a compromised pneumotaxic center and vagal input.

Question 815: Which of the following parameters indicates the elimination of CO2 from the lungs?

A. pH
B. PaCO2 (Correct Answer)
C. PaO2
D. HCO3 level

Explanation: ***PaCO2*** - **Partial pressure of carbon dioxide in arterial blood (PaCO2)** directly reflects the efficiency of **alveolar ventilation**, which is the process of eliminating CO2 from the lungs. - When CO2 elimination is adequate, PaCO2 remains within the normal range (35-45 mmHg); higher or lower values indicate ventilatory impairment or hyperventilation, respectively. *PaO2* - **PaO2** measures the partial pressure of **oxygen in arterial blood** and indicates oxygenation, not the efficiency of carbon dioxide elimination. - While CO2 elimination and oxygenation are interdependent, **PaO2** primarily reflects how well oxygen is being transported from the lungs to the blood. *pH* - **pH** indicates the **acidity or alkalinity of the blood**, which is influenced by both respiratory (CO2) and metabolic (bicarbonate) components. - Although CO2 elimination affects pH through the carbonic acid-bicarbonate buffer system, pH itself is an overall measure of acid-base balance, not a direct indicator of CO2 elimination. *HCO3 level* - **Bicarbonate (HCO3-)** is a **metabolic component** of the acid-base balance, primarily regulated by the kidneys. - While it helps buffer CO2-induced acid changes, HCO3 level alone does not directly reflect the efficiency of CO2 elimination from the lungs.

Question 816: In patients with emphysematous bullae, total lung volume is best determined by?

A. Spirometry
B. Any of the above
C. Helium dilution method
D. Plethysmography (Correct Answer)

Explanation: ***Plethysmography*** - This method accurately measures **total lung capacity (TLC)**, functional residual capacity (FRC), and residual volume (RV) by determining the **volume of gas in the thorax**. - It is particularly useful in conditions like **emphysema** with air trapping and bullae, as it accounts for **non-communicating air spaces** that other methods miss. *Spirometry* - Spirometry measures volumes of air that can be **exhaled or inhaled forcibly**, such as FVC and FEV1. - It cannot measure residual volume (RV) or total lung capacity (TLC) directly, especially in cases of **air trapping** where trapped air cannot be exhaled. *Helium dilution method* - The helium dilution method measures **communicating lung volumes**, like functional residual capacity (FRC), by assessing the dilution of a known concentration of helium after rebreathing. - In conditions with **emphysematous bullae** and air trapping, it **underestimates total lung volume** because it cannot measure air in non-communicating or poorly communicating spaces. *Any of the above* - Only plethysmography can accurately measure total lung volume in the presence of **emphysematous bullae** due to its ability to measure both communicating and non-communicating air spaces. - Spirometry and helium dilution methods would provide **inaccurate or incomplete measurements** in this clinical scenario.

Question 817: What physiological mechanism is responsible for the increase in the duration of expiration?

A. J-reflex
B. Head's paradoxical reflex
C. Proprioceptors
D. Hering-Breuer reflex (Correct Answer)

Explanation: ***Hering-Breuer reflex*** - The **Hering-Breuer reflex** is initiated by **stretch receptors in the bronchi and bronchioles** which are activated during lung inflation. - This reflex **inhibits inspiration** and **prolongs expiration**, preventing overinflation of the lungs. *J-reflex* - The **J-reflex** is stimulated by **juxtacapillary (J) receptors** in the alveolar walls, usually in response to pulmonary edema or congestion. - It typically causes **rapid, shallow breathing** and **bronchoconstriction**, not prolonged expiration. *Head's paradoxical reflex* - **Head's paradoxical reflex** (also known as the **inflation reflex** in newborns) involves an inspiratory effort triggered by lung inflation, often overcoming the Hering-Breuer reflex in specific conditions. - It tends to **increase respiratory rate** and depth, not prolong expiration. *Proprioceptors* - **Proprioceptors** are sensory receptors in muscles, tendons, and joints that provide information about body position and movement. - While they can influence respiration during exercise, they are not primarily responsible for directly **increasing the duration of expiration** as a reflex mechanism against overinflation.

Question 818: What is the respiratory quotient?

A. CO2 released to O2 consumed (Correct Answer)
B. CO2 consumed to O2 released
C. O2 released to CO2 consumed
D. O2 consumed to CO2 released

Explanation: **CO2 released to O2 consumed** - The **respiratory quotient (RQ)** is a ratio used in metabolism to describe the proportion of **carbon dioxide (CO2) produced** by the body relative to the **oxygen (O2) consumed**. - It is calculated as the **volume of CO2 released** divided by the **volume of O2 consumed** over a specific period. - RQ = VCO2/VO2, where VCO2 is CO2 production and VO2 is O2 consumption. *CO2 consumed to O2 released* - This option is incorrect as it reverses the correct order and refers to **CO2 consumption and O2 release**, which are not the standard components of the RQ calculation. - The body primarily **releases CO2** and **consumes O2** during cellular respiration. *O2 released to CO2 consumed* - This option is also incorrect because it inverts both the gases and the direction of their metabolic flow (release vs. consumption). - Metabolic processes involve **O2 consumption** and **CO2 release**, not the other way around. *O2 consumed to CO2 released* - This option incorrectly reverses the numerator and denominator in the RQ formula. - The standard definition places **CO2 production** in the numerator and **O2 consumption** in the denominator.

Question 819: What is the normal transpulmonary pressure during quiet breathing?

A. 0 to + 1 cm H2O
B. 0 to -1 cm H2O
C. +5 to +8 cm H2O (Correct Answer)
D. - 8 to - 5 cm H2O

Explanation: ***+5 to +8 cm H2O*** - Transpulmonary pressure (P_tp) is the **difference between alveolar pressure and pleural pressure** (P_alv - P_pl). - During quiet breathing at **functional residual capacity (FRC)**, alveolar pressure is **0 cm H2O** (atmospheric) while pleural pressure is approximately **-5 cm H2O**, giving P_tp = **+5 cm H2O**. - At end-inspiration during quiet breathing, pleural pressure becomes more negative (**-8 cm H2O**) while alveolar pressure remains near atmospheric, resulting in P_tp ≈ **+8 cm H2O**. - This positive transpulmonary pressure gradient is essential to **keep the lungs inflated** against elastic recoil and prevent **atelectasis**. *0 to +1 cm H2O* - This pressure is far too low to maintain lung inflation against elastic recoil forces. - Normal transpulmonary pressure must be several cm H2O positive to counterbalance the lung's tendency to collapse. - This value would result in **near-complete lung collapse**. *0 to -1 cm H2O* - A negative or zero transpulmonary pressure would mean pleural pressure equals or exceeds alveolar pressure. - This condition would cause **immediate lung collapse (pneumothorax)** as there would be no pressure gradient to keep the lungs expanded. *-8 to -5 cm H2O* - This range represents **pleural pressure**, not transpulmonary pressure. - Pleural pressure is indeed -5 to -8 cm H2O during quiet breathing, but transpulmonary pressure is calculated as the difference between alveolar and pleural pressures. - Confusing pleural pressure with transpulmonary pressure is a common error.

Question 820: Vital capacity is measured by:

A. Plethysmography
B. Nitrogen washout technique
C. Spirometer (Correct Answer)
D. Gas-dilution method

Explanation: ***Spirometer*** - A **spirometer** is a device used to measure lung volumes and capacities, including **vital capacity**. - It measures the volume of air inspired and expired by evaluating mechanical changes in the volume of air in the lungs. *Plethysmography* - **Plethysmography** is primarily used to measure **residual volume** and **total lung capacity**, not vital capacity directly. - This method measures changes in body volume to infer changes in lung volume. *Gas-dilution method* - The **gas-dilution method**, typically using helium, is used to measure the **functional residual capacity (FRC)** and subsequently calculate residual volume and total lung capacity. - It involves rebreathing a known concentration of gas to determine the volume of gas already in the lungs. *Nitrogen washout technique* - The **nitrogen washout technique** is also used to measure **functional residual capacity (FRC)** and detect uneven ventilation. - It involves breathing 100% oxygen to wash out all nitrogen from the lungs, allowing for calculation of lung volumes.

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