Respiratory System Practice Questions

Q: The compliance curve of the lung is shown below. Curve A signifies which of the following?

Emphysema. ***Emphysema*** - **Increased compliance** due to **loss of elastic recoil** from destruction of **alveolar walls** and **elastic fibers**. - The **pressure-volume curve shifts upward and to the left**, requiring less pressure to achieve the same lung volume expansion. *Pulmonary fibrosis* - Shows **decreased compliance** due to **increased stiffness** from **collagen deposition** and **fibrotic tissue**. - The **pressure-volume curve shifts downward and to the right**, requiring greater pressure for lung expansion. *Atelectasis* - Results in **decreased compliance** due to **collapsed alveoli** and **reduced functional lung volume**. - Creates a **flattened pressure-volume curve** with poor lung expandability and increased work of breathing. *ARDS* - Characterized by **severely decreased compliance** from **pulmonary edema**, **inflammation**, and **surfactant dysfunction**. - The **pressure-volume curve shows marked reduction** in lung expandability with high opening pressures required.

Q: In the work of breathing, what percentage fraction does tissue resistance contribute?

7%. **Explanation:** The "Work of Breathing" (WOB) refers to the energy expended by the respiratory muscles to overcome the mechanical impedances of the respiratory system. This resistance is broadly divided into two categories: **Elastic Resistance** (65%) and **Non-elastic/Viscous Resistance** (35%). The **Non-elastic resistance** is further subdivided: 1. **Airway Resistance (80% of non-elastic):** Caused by friction between the air molecules and the walls of the tracheobronchial tree. 2. **Tissue Resistance (20% of non-elastic):** Also known as viscous resistance, this is caused by the friction between the lungs and the chest wall, and the sliding of the pleural surfaces. **Why 7% is correct:** Mathematically, tissue resistance accounts for approximately 20% of the non-elastic resistance. Since non-elastic resistance is 35% of the total work, the calculation is: **20% of 35% = 7%**. Therefore, tissue resistance contributes roughly 7% to the total work of breathing in a healthy individual. **Analysis of Incorrect Options:** * **B (14%):** This value is too high for tissue resistance in normal physiology; it may be seen in specific restrictive pathologies but is not the standard physiological value. * **C (28%):** This represents the majority of the non-elastic resistance (Airway Resistance), which is roughly 28% of the total work (80% of 35%). * **D (65%):** This represents the **Elastic Resistance** (Compliance work), which is the energy required to expand the elastic tissues of the lungs and chest wall. **High-Yield Facts for NEET-PG:** * **Most of the work of breathing** (65%) is spent overcoming elastic recoil. * **Airway resistance** is highest in the **medium-sized bronchi** (generations 2-5) and lowest in the terminal bronchioles due to the large total cross-sectional area. * In diseases like **Emphysema**, work increases due to loss of elastic recoil, while in **Asthma**, work increases due to high airway resistance.

Q: What is the normal partial pressure of oxygen (PaO2) in a healthy adult?

80 mm Hg. **Explanation:** The partial pressure of arterial oxygen (**PaO2**) represents the amount of oxygen dissolved in the arterial blood. In a healthy adult breathing room air (21% O2) at sea level, the normal range is typically **80 to 100 mm Hg**. This value is determined by the alveolar oxygen tension (PAO2) and the efficiency of gas exchange across the alveolar-capillary membrane. **Why Option C is correct:** 80 mm Hg falls within the standard physiological range for a healthy adult. While the theoretical maximum at sea level is closer to 100 mm Hg, factors like the physiological shunt (bronchial and thebesian veins) slightly lower the arterial tension. **Analysis of Incorrect Options:** * **A (45 mm Hg):** This is the normal partial pressure of arterial **carbon dioxide (PaCO2)** or the partial pressure of oxygen in **mixed venous blood (PvO2)**. * **B (110 mm Hg):** This value is higher than what is achievable breathing room air at sea level. It would only be seen with supplemental oxygen therapy. * **D (60 mm Hg):** This is the threshold for **hypoxemia** and respiratory failure. At this point, the oxygen-hemoglobin dissociation curve enters its steep phase, leading to a rapid drop in SaO2. **High-Yield Clinical Pearls for NEET-PG:** * **Alveolar Gas Equation:** $PAO_2 = FiO_2(P_{atm} - P_{H2O}) - (PaCO_2 / R)$. * **A-a Gradient:** The difference between alveolar (A) and arterial (a) oxygen. Normal is **5–15 mm Hg**; an increase indicates intrinsic lung disease or a V/Q mismatch. * **Hypoxemia vs. Hypoxia:** Hypoxemia is low PaO2 in blood; Hypoxia is low oxygen delivery to tissues. * **P50 Value:** The PaO2 at which hemoglobin is 50% saturated is **26.7 mm Hg**.

Q: Compared to the base, the apex of the upright human lung has:

A higher PO2. In the upright lung, both ventilation (V) and perfusion (Q) decrease from the base to the apex due to gravity. However, **perfusion decreases much more steeply than ventilation**. This results in a **higher Ventilation-Perfusion (V/Q) ratio at the apex** (~3.3) compared to the base (~0.6). **Why Option A is Correct:** Because the apex is "over-ventilated" relative to its blood flow (high V/Q), less oxygen is extracted from the alveoli per unit of ventilation. This leads to a **higher alveolar and end-capillary PO2** (approx. 130 mmHg at the apex vs. 89 mmHg at the base). **Why Other Options are Incorrect:** * **B & D:** Both ventilation and blood flow are **lowest at the apex** and highest at the base. Gravity pulls blood and lung tissue downward; the base is more compliant and better perfused. * **C:** Due to the high V/Q ratio at the apex, more CO2 is washed out, leading to a **lower PCO2**. A lower PCO2 results in a **higher (more alkaline) pH** in the end-capillary blood, not lower. **High-Yield Clinical Pearls for NEET-PG:** * **West Zones:** The apex represents Zone 1 (PA > Pa > Pv), though in healthy individuals, it is usually a functional Zone 2. * **Tuberculosis:** *Mycobacterium tuberculosis* has a predilection for the lung apices because the **higher PO2** provides a favorable environment for this obligate aerobe. * **V/Q Summary:** At the apex, V/Q is high (↑PO2, ↓PCO2, ↑pH). At the base, V/Q is low (↓PO2, ↑PCO2, ↓pH).

Q: Which of the following conditions is NOT associated with a decrease in Residual Volume?

Emphysema. ### Explanation **Residual Volume (RV)** is the volume of air remaining in the lungs after a maximal forceful expiration. Changes in RV are primarily determined by the lung's elastic recoil and airway patency. #### 1. Why Emphysema is the Correct Answer In **Emphysema** (a type of Chronic Obstructive Pulmonary Disease), there is a destruction of alveolar walls and loss of elastic recoil. This leads to two major consequences: * **Air Trapping:** The loss of radial traction causes small airways to collapse during expiration. * **Hyperinflation:** The lungs become overly compliant and cannot effectively expel air. Consequently, **Residual Volume (RV), Functional Residual Capacity (FRC), and Total Lung Capacity (TLC) are all increased** in emphysema, not decreased. #### 2. Analysis of Incorrect Options (Conditions with Decreased RV) Options B, C, and D represent **Restrictive Lung Patterns** or space-occupying lesions that reduce the available lung volume: * **Interstitial Lung Disease (ILD):** Increased elastic recoil (stiff lungs) pulls the airways open but limits expansion, leading to a global decrease in all lung volumes, including RV. * **Bacterial Pneumonia & Lung Abscess:** These are "filling disorders" where inflammatory exudate, pus, or consolidation replaces air spaces. This physical displacement of air-containing tissue results in a decrease in RV. #### 3. NEET-PG High-Yield Pearls * **Obstructive Diseases (Asthma, COPD):** RV ↑, FRC ↑, TLC ↑, FEV1/FVC ratio ↓. * **Restrictive Diseases (Fibrosis, Kyphoscoliosis):** RV ↓, FRC ↓, TLC ↓, FEV1/FVC ratio is Normal or ↑. * **RV Measurement:** RV cannot be measured by simple spirometry; it requires **Helium Dilution, Nitrogen Washout, or Body Plethysmography.** * **Aging:** RV naturally increases with age due to the loss of elastic recoil, even in healthy individuals.

Q: What is the normal partial pressure of carbon dioxide (pCO2) in exhaled air?

27 mm Hg. The partial pressure of carbon dioxide in exhaled air ($P_E CO_2$) is approximately **27 mm Hg**. This value is a result of the mixing of alveolar air with air from the anatomical dead space. ### **Detailed Explanation** 1. **The Concept of Mixing:** During expiration, the first portion of air expelled comes from the **anatomical dead space** (trachea and bronchi), where $PCO_2$ is nearly **0 mm Hg** (similar to atmospheric air). The latter portion is **alveolar air**, which has a $PCO_2$ of **40 mm Hg**. 2. **The Resultant Value:** The total exhaled air (mixed expired air) is a combination of these two. Since dead space accounts for roughly one-third of a normal tidal volume, the $PCO_2$ is diluted from 40 mm Hg down to approximately **27–28 mm Hg**. ### **Analysis of Options** * **A. 36 mm Hg:** This is too high for mixed expired air; it is closer to the $PCO_2$ of end-tidal air (which represents pure alveolar air). * **B. 27 mm Hg (Correct):** This represents the average $PCO_2$ in a full breath of expired air after dilution by dead space. * **C. 40 mm Hg:** This is the $PCO_2$ of **alveolar air** and **systemic arterial blood ($PaCO_2$)**. * **D. 17 mm Hg:** This value is too low and would only be seen in states of extreme hyperventilation or increased dead space ventilation. ### **NEET-PG High-Yield Pearls** * **Bohr Equation:** Uses the difference between $PaCO_2$ (40) and $P_E CO_2$ (27) to calculate the **Physiological Dead Space**. * **End-Tidal $CO_2$ ($EtCO_2$):** In clinical anesthesia, $EtCO_2$ monitors the very last portion of expired air. It is usually **35–40 mm Hg**, reflecting alveolar $CO_2$ levels. * **Inspired Air $PCO_2$:** Is negligible (~0.3 mm Hg).

Question 1

The compliance curve of the lung is shown below. Curve A signifies which of the following?

Accepted Answer

Emphysema

Answer

Pulmonary fibrosis

Answer

Atelectasis

Answer

ARDS

Question 2

In the work of breathing, what percentage fraction does tissue resistance contribute?

Accepted Answer

7%

Answer

14%

Answer

28%

Answer

65%

Question 3

What is the normal partial pressure of oxygen (PaO2) in a healthy adult?

Accepted Answer

80 mm Hg

Answer

45 mm Hg

Answer

110 mm Hg

Answer

60 mm Hg

Question 4

What is the sensitivity of chemoreceptors in COPD?

Accepted Answer

Increased to PCO2

Answer

Decreased to H+

Answer

Increased to H+

Answer

Increased to PO2

Question 5

Vital capacity is the sum of which of the following?

Accepted Answer

Inspiratory reserve volume, Tidal volume, and Expiratory reserve volume

Answer

Tidal volume, Inspiratory reserve volume, and Residual volume

Answer

Expiratory reserve volume, Inspiratory reserve volume, and Residual volume

Answer

Residual volume, Inspiratory volume, and Expiratory reserve volume

Question 6

Compared to the base, the apex of the upright human lung has:

Accepted Answer

A higher PO2

Answer

A higher ventilation

Answer

A lower pH in end-capillary blood

Answer

A higher blood flow

Question 7

Hypoxic pulmonary vasoconstriction:

Accepted Answer

Is released in the transition from placental to air respiration

Answer

Depends more on the PO2 of mixed venous blood than alveolar gas

Answer

Involves CO2 uptake in vascular smooth muscle

Answer

Shunts blood flow from well-ventilated regions of diseased lungs

Question 8

Which of the following conditions is NOT associated with a decrease in Residual Volume?

Accepted Answer

Emphysema

Answer

Bacterial pneumonia

Answer

Interstitial lung disease

Answer

Lung abscess

Question 9

Medullary chemoreceptors are primarily sensitive to which of the following?

Accepted Answer

H+ concentration in the cerebrospinal fluid (CSF)

Answer

CO2 concentration in the cerebrospinal fluid (CSF)

Answer

H+ concentration in the blood

Answer

CO2 concentration in the blood

Question 10

What is the normal partial pressure of carbon dioxide (pCO2) in exhaled air?

Accepted Answer

27 mm Hg

Answer

36 mm Hg

Answer

40 mm Hg

Answer

17 mm Hg

Respiratory System — MCQs

Respiratory System — MCQs

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