Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 31: Compared to the base, the apex of the upright human lung has:

A. A higher PO2 (Correct Answer)
B. A higher ventilation
C. A lower pH in end-capillary blood
D. A higher blood flow

Explanation: 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).

Question 32: Volume of air taken in and given out during normal respiration is referred to as:

A. Inspiratory Reserve Volume (IRV)
B. Tidal Volume (TV) (Correct Answer)
C. Expiratory Reserve Volume (ERV)
D. Vital Capacity (VC)

Explanation: **Explanation:** The correct answer is **Tidal Volume (TV)**. **1. Why Tidal Volume is correct:** Tidal Volume is defined as the volume of air inspired or expired during a single, normal, quiet respiratory cycle. In a healthy adult male, the average value is approximately **500 mL**. It represents the rhythmic "ebb and flow" of breathing at rest, similar to the tides of the ocean. **2. Why other options are incorrect:** * **Inspiratory Reserve Volume (IRV):** This is the maximum extra volume of air that can be inspired *over and above* the normal tidal volume (approx. 2500–3000 mL). It is used during deep or forced inspiration. * **Expiratory Reserve Volume (ERV):** This is the maximum extra volume of air that can be expired by forceful expiration *after* the end of a normal tidal expiration (approx. 1000–1100 mL). * **Vital Capacity (VC):** This is a "capacity" (sum of two or more volumes). It is the maximum amount of air a person can expel from the lungs after first filling the lungs to their maximum extent ($VC = IRV + TV + ERV$). **3. NEET-PG High-Yield Pearls:** * **Minute Ventilation:** Calculated as $TV \times \text{Respiratory Rate}$. (e.g., $500 \text{ mL} \times 12 \text{ bpm} = 6 \text{ L/min}$). * **Alveolar Ventilation:** This is more clinically significant than Minute Ventilation as it accounts for **Anatomic Dead Space** (approx. 150 mL). Formula: $(TV - \text{Dead Space}) \times \text{Respiratory Rate}$. * **Measurement:** All lung volumes and capacities can be measured by **Spirometry**, *except* for Residual Volume (RV), Functional Residual Capacity (FRC), and Total Lung Capacity (TLC). These require helium dilution or body plethysmography.

Question 33: Hypoxic pulmonary vasoconstriction:

A. Depends more on the PO2 of mixed venous blood than alveolar gas
B. Is released in the transition from placental to air respiration (Correct Answer)
C. Involves CO2 uptake in vascular smooth muscle
D. Shunts blood flow from well-ventilated regions of diseased lungs

Explanation: ### Explanation **Hypoxic Pulmonary Vasoconstriction (HPV)** is a unique physiological mechanism where pulmonary arterioles constrict in response to low alveolar oxygen (hypoxia). Unlike systemic circulation (where hypoxia causes vasodilation), the lung redirects blood flow away from poorly ventilated areas to well-ventilated areas to optimize ventilation-perfusion (V/Q) matching. #### Why Option B is Correct: In the fetus, the lungs are non-functional and filled with fluid, resulting in low alveolar $PO_2$ and high pulmonary vascular resistance (PVR) due to intense HPV. At birth, the first breath replaces fluid with air, rapidly increasing alveolar $PO_2$. This **releases (reverses) the vasoconstriction**, causing a dramatic drop in PVR and a 10-fold increase in pulmonary blood flow, facilitating the transition to air respiration. #### Analysis of Incorrect Options: * **Option A:** HPV depends primarily on **alveolar gas $PO_2$** rather than mixed venous $PO_2$. The sensor is located in the pulmonary precapillary vessels which are sensitive to the oxygen tension in the adjacent alveoli. * **Option C:** HPV is triggered by **low oxygen levels**, not $CO_2$ uptake. While $CO_2$ and pH can modulate pulmonary tone, the primary stimulus for this specific reflex is hypoxia. * **Option D:** HPV shunts blood **away** from poorly ventilated regions **toward** well-ventilated regions. This minimizes "wasted" perfusion and reduces the physiological shunt. #### High-Yield Facts for NEET-PG: * **Mechanism:** Hypoxia inhibits voltage-gated $K^+$ channels in pulmonary artery smooth muscle cells, leading to depolarization and $Ca^{2+}$ influx, causing contraction. * **Clinical Significance:** Generalized HPV (e.g., at high altitudes) leads to **Pulmonary Hypertension** and can cause High-Altitude Pulmonary Edema (HAPE). * **Drug Interaction:** Most inhaled anesthetics can inhibit HPV, potentially worsening V/Q mismatch during surgery.

Question 34: A shift to the right of the oxygen dissociation curve is caused by all of the following, EXCEPT:

A. Fall in pH
B. Rise in temperature
C. Decrease in 2,3-bisphosphoglycerate (2,3-BPG) (Correct Answer)
D. None of the above

Explanation: **Explanation:** The Oxygen Dissociation Curve (ODC) represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **"Shift to the Right"** indicates a decreased affinity of hemoglobin for oxygen, meaning oxygen is more easily released to the tissues. **Why Option C is the Correct Answer:** **2,3-Bisphosphoglycerate (2,3-BPG)** is a byproduct of glycolysis in RBCs that binds to the beta chains of deoxyhemoglobin, stabilizing the "T" (Tense) state and promoting oxygen release. Therefore, an **increase** in 2,3-BPG shifts the curve to the right. Conversely, a **decrease** in 2,3-BPG (as seen in stored blood) increases hemoglobin's affinity for oxygen, shifting the curve to the **left**. Since the question asks for the exception, a decrease in 2,3-BPG is the correct choice. **Analysis of Incorrect Options:** * **Option A (Fall in pH):** A decrease in pH (acidosis) or an increase in $PCO_2$ reduces hemoglobin's affinity for oxygen. This is known as the **Bohr Effect**, which shifts the curve to the **right**. * **Option B (Rise in temperature):** Increased metabolic activity (e.g., exercise or fever) raises local temperature, which denatures the bond between oxygen and hemoglobin, shifting the curve to the **right** to facilitate unloading. **High-Yield NEET-PG Pearls:** * **CADET, face Right!:** A useful mnemonic for factors shifting the curve to the **Right**: **C**O2 increase, **A**cidosis, **D**PG (2,3-BPG) increase, **E**xercise, **T**emperature increase. * **Fetal Hemoglobin (HbF):** Shifts the curve to the **Left** because it lacks beta chains and does not bind 2,3-BPG effectively, allowing the fetus to pull oxygen from maternal blood. * **Carbon Monoxide (CO):** Shifts the curve to the **Left** and changes it from sigmoidal to hyperbolic, preventing oxygen release at tissues.

Question 35: Which of the following is true about the hemoglobin-oxygen dissociation curve?

A. Acidosis shifts the oxygen dissociation curve to the right. (Correct Answer)
B. Increased carbon dioxide shifts the curve to the left.
C. Hypoxia shifts the curve to the left.
D. 2,3-diphosphoglycerate has no effect on the curve.

Explanation: The hemoglobin-oxygen dissociation curve (ODC) is a sigmoid-shaped graph representing the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. **Explanation of the Correct Option:** **Option A** is correct because **Acidosis** (increased $H^+$ concentration) decreases hemoglobin's affinity for oxygen, causing it to release $O_2$ more readily to the tissues. This phenomenon is known as the **Bohr Effect**. On the graph, a decrease in affinity is represented by a **shift to the right**, meaning a higher $PO_2$ is required to achieve the same level of saturation. **Analysis of Incorrect Options:** * **Option B:** Increased $CO_2$ (Hypercapnia) actually shifts the curve to the **right**, not the left. Like $H^+$, $CO_2$ stabilizes the "Tense" (T) state of hemoglobin, promoting oxygen unloading. * **Option C:** Chronic hypoxia (such as at high altitudes) leads to an increase in **2,3-BPG** production, which shifts the curve to the **right** to facilitate oxygen delivery to tissues. * **Option D:** **2,3-DPG (or 2,3-BPG)** is a critical regulator; it binds to the beta chains of hemoglobin and shifts the curve to the **right**. **High-Yield NEET-PG Pearls:** * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis, **D**PG (2,3-BPG) increase, **E**xercise, and **T**emperature increase. * **Left Shift:** Occurs in Fetal Hemoglobin (HbF), CO poisoning, Methemoglobinemia, and Hypothermia. * **P50 Value:** The $PO_2$ at which hemoglobin is 50% saturated. A right shift **increases** the P50 (Normal P50 $\approx$ 26.6 mmHg).

Question 36: Which compound shifts the oxygen dissociation curve to the right?

A. Phosphoglycerate
B. 2,3 DPG (Correct Answer)
C. 1,3 DPG
D. Glyceraldehyde

Explanation: **Explanation:** The oxygen dissociation curve (ODC) represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin ($Hb$). A **shift to the right** indicates a decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to the tissues. **Why 2,3-DPG is correct:** **2,3-Diphosphoglycerate (2,3-DPG)**, also known as 2,3-BPG, is a byproduct of the Rappaport-Luebering shunt in glycolysis within erythrocytes. It binds to the beta chains of deoxyhemoglobin, stabilizing the **T (Tense) state**. This reduces hemoglobin's affinity for oxygen, shifting the curve to the right and promoting oxygen release at the tissue level. Levels of 2,3-DPG increase during chronic hypoxia, high-altitude adaptation, and anemia. **Analysis of Incorrect Options:** * **A & C (Phosphoglycerate / 1,3-DPG):** These are intermediate metabolites in the Embden-Meyerhof pathway (glycolysis). While 1,3-DPG is the precursor to 2,3-DPG, these compounds themselves do not bind to hemoglobin or significantly influence its oxygen affinity. * **D (Glyceraldehyde):** This is a simple sugar (triose) involved in carbohydrate metabolism but has no physiological role in modulating the oxygen-hemoglobin bond. **High-Yield Clinical Pearls for NEET-PG:** * **Factors shifting the curve to the RIGHT (CADET, face Right!):** **C**O2 increase, **A**cidosis ($H^+$), **D**PG (2,3-DPG) increase, **E**xercise, and **T**emperature increase. * **Fetal Hemoglobin (HbF):** Shifts the curve to the **left** because it has a lower affinity for 2,3-DPG (due to gamma chains instead of beta chains), allowing the fetus to pull oxygen from maternal blood. * **Stored Blood:** 2,3-DPG levels decrease in stored blood, shifting the curve to the **left** and potentially impairing oxygen delivery upon massive transfusion.

Question 37: Which of the following conditions is NOT associated with a decrease in Residual Volume?

A. Emphysema (Correct Answer)
B. Bacterial pneumonia
C. Interstitial lung disease
D. Lung abscess

Explanation: ### 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.

Question 38: What is true about Maximum Mid-expiratory Flow Rate (MMEFR)?

A. 150 liters per minute
B. Measured with maximum voluntary effort
C. Maximum breathing capacity per minute
D. All of the above (Correct Answer)

Explanation: **Explanation:** **Maximum Mid-Expiratory Flow Rate (MMEFR)**, also known as **FEF 25–75%**, is a sensitive index of airway obstruction, particularly in the smaller, peripheral airways. 1. **Why Option D is correct:** * **Value (Option A):** The normal average value for a healthy adult is approximately **150–300 Liters per minute**. While it varies by age and gender, 150 L/min is considered the standard baseline for clinical assessment. * **Effort (Option B):** MMEFR is derived from the Forced Vital Capacity (FVC) maneuver. It requires the patient to inhale to Total Lung Capacity (TLC) and then exhale with **maximum voluntary effort** into a spirometer. * **Definition (Option C):** It represents the average flow rate during the middle half (25% to 75%) of a forced expiration. Conceptually, it measures the "maximum breathing capacity" specifically during this mid-expiratory phase, reflecting the patency of small airways. 2. **Clinical Significance:** * Unlike FEV1, which is effort-dependent and reflects large airway function, MMEFR is **effort-independent** in its later stages and is the most sensitive indicator for **early obstructive lung disease** (e.g., early stages of COPD or asthma) where small airways are affected first. **High-Yield NEET-PG Pearls:** * **Small Airway Disease:** MMEFR is the best test to detect "silent zone" (small airway) involvement. * **Effort Independence:** The middle and terminal parts of the expiratory flow are independent of the effort once the maximum flow has been reached. * **Normal FEV1/FVC:** In early obstructive disease, the FEV1/FVC ratio may be normal, but the MMEFR will be significantly reduced.

Question 39: Medullary chemoreceptors are primarily sensitive to which of the following?

A. H+ concentration in the cerebrospinal fluid (CSF) (Correct Answer)
B. CO2 concentration in the cerebrospinal fluid (CSF)
C. H+ concentration in the blood
D. CO2 concentration in the blood

Explanation: **Explanation:** The **central (medullary) chemoreceptors** are located on the ventrolateral surface of the medulla oblongata. They are the primary regulators of the respiratory drive in response to metabolic changes. **1. Why Option A is correct:** The blood-brain barrier (BBB) is highly permeable to dissolved gases like $CO_2$ but impermeable to ions like $H^+$ and $HCO_3^-$. When arterial $PCO_2$ rises, $CO_2$ diffuses across the BBB into the cerebrospinal fluid (CSF). In the CSF, $CO_2$ reacts with water to form carbonic acid, which dissociates into $H^+$ and $HCO_3^-$. Because the CSF has very little protein buffering capacity, the $H^+$ concentration rises rapidly. It is this **$H^+$ concentration in the CSF** that directly stimulates the medullary chemoreceptors to increase ventilation. **2. Why other options are incorrect:** * **Option B:** While $CO_2$ is the trigger that crosses the BBB, it must be converted to $H^+$ to stimulate the receptors. $CO_2$ itself is not the direct stimulant. * **Option C & D:** Medullary chemoreceptors are "shielded" from systemic $H^+$ and $CO_2$ by the BBB. Changes in blood $H^+$ (e.g., metabolic acidosis) primarily stimulate **peripheral chemoreceptors** (carotid and aortic bodies), not the central ones. **Clinical Pearls & High-Yield Facts:** * **Primary Stimulus:** $CO_2$ is the most potent stimulus for respiration, acting via central chemoreceptors. * **Peripheral vs. Central:** Central chemoreceptors account for ~70-80% of the ventilatory response to $CO_2$, while peripheral chemoreceptors account for the remaining 20-30%. * **Hypoxia:** Central chemoreceptors are **not** stimulated by hypoxia; in fact, prolonged severe hypoxia can depress the central respiratory centers. Hypoxia is sensed exclusively by peripheral chemoreceptors. * **Adaptation:** In chronic hypercapnia (e.g., COPD), the CSF $HCO_3^-$ levels increase to buffer the $H^+$, leading to a "resetting" of the central chemoreceptors.

Question 40: What is the normal partial pressure of carbon dioxide (pCO2) in exhaled air?

A. 36 mm Hg
B. 27 mm Hg (Correct Answer)
C. 40 mm Hg
D. 17 mm Hg

Explanation: 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).

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