Respiratory System — MCQs

A 73-year-old man presents with progressive dyspnea on exertion over the past one year. He reports a dry cough but no wheezes, sputum production, fevers, or hemoptysis. He is a life-long non-smoker and worked as a lawyer until retiring 3 years ago. His pulmonary function testing is as follows: Pre-Bronchodilator Test, Actual, Predicted, % Predicted - FVC (L): 1.57, 4.46, 35; FEV1 (L): 1.28, 3.39, 38; FEV1/FVC (%): 82, 76; FRC (L): 1.73, 3.80, 45; RV (L): 1.12, 2.59, 43; TLC (L): 2.70, 6.45, 42; RV/TLC (%): 41, 42; DLCO corr: 5.06, 31.64, 16. What is the most probable diagnosis?

Q68Medium

Complete apneusis will result if transection is done at which level of the brainstem?

Q69Medium

In a patient, the volume of nitrogen collected during a nitrogen wash-out method, representing the residual volume (RV), was found to be 800 mL. What is the patient's total lung capacity (TLC)?

Q70Easy

Oxygen moves from the alveoli to the blood by which process?

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 61: Regarding Head's paradoxical reflex, which of the following statements is true?

A. It plays an important role in normal respiration. (Correct Answer)
B. It is mediated by tracheobronchial-stretch receptors.
C. It is stimulated by hyperinflation of lungs.
D. It inhibits respiration.

Explanation: **Explanation:** **Head’s Paradoxical Reflex** is a physiological response where lung inflation further stimulates inspiratory effort, rather than inhibiting it. This is "paradoxical" because it opposes the Hering-Breuer inflation reflex. **Why Option A is correct:** While the reflex is largely inactive in healthy adults, it plays a critical role in **neonatal respiration**. It is responsible for the **first breath of life** and the subsequent deep gasping breaths that help expand the fluid-filled, collapsed fetal lungs (atelectasis). It also triggers periodic "sighs" in adults, which help prevent alveolar collapse and maintain lung compliance. **Analysis of Incorrect Options:** * **B: Mediated by tracheobronchial-stretch receptors:** This is incorrect. Head’s reflex is mediated by **rapidly adapting receptors (RARs)**, also known as irritant receptors, located in the airway epithelium. In contrast, the Hering-Breuer reflex is mediated by slowly adapting stretch receptors. * **C: Stimulated by hyperinflation:** While it is triggered by inflation, the defining characteristic is the *response* to that inflation. In the context of this reflex, the inflation leads to a positive feedback loop of further inspiration, not just a passive reaction to hyperinflation. * **D: Inhibits respiration:** This is incorrect. Head’s reflex **stimulates** inspiration. The Hering-Breuer reflex is the one that inhibits respiration (terminates inspiration) to prevent over-distension. **High-Yield Clinical Pearls for NEET-PG:** * **Receptors:** Rapidly Adapting Receptors (RARs). * **Afferent Pathway:** Vagus Nerve. * **Key Function:** Lung expansion in newborns and "sighing" mechanism in adults. * **Contrast:** Remember, **Hering-Breuer = Inhibitory** (prevents over-inflation); **Head’s Paradoxical = Stimulatory** (promotes deeper inspiration).

Question 62: What is the O2 content of saturated arterial blood?

A. 11.2 ml/100 ml
B. 14.5 ml/100 ml
C. 19.4 ml/100 ml (Correct Answer)
D. 21.3 ml/100 ml

Explanation: **Explanation:** The oxygen content of arterial blood is determined by the sum of oxygen bound to hemoglobin and oxygen dissolved in plasma. To calculate this, we use the following formula: **Total $O_2$ Content = ($1.34 \times Hb \times SaO_2$) + ($0.003 \times PaO_2$)** 1. **Hemoglobin-bound $O_2$:** In a healthy adult, the average Hemoglobin (Hb) is **15 g/dL**. Each gram of Hb can carry **1.34 ml** of $O_2$ (Hüfner's constant). For saturated blood ($SaO_2$ = 100%), this equals $15 \times 1.34 = \mathbf{20.1\text{ ml/dL}}$. However, in vivo, physiological saturation is typically **97%**, yielding approximately **19.1 ml/dL**. 2. **Dissolved $O_2$:** At a normal arterial $PaO_2$ of 100 mmHg, the dissolved $O_2$ is $100 \times 0.003 = \mathbf{0.3\text{ ml/dL}}$. 3. **Total:** Adding these gives approximately **19.4 ml/100 ml** (or 19.4 vol%). **Analysis of Options:** * **A (11.2 ml) & B (14.5 ml):** These values are too low for normal arterial blood and may be seen in cases of severe anemia or significant hypoxia. * **D (21.3 ml):** This value is higher than the average physiological carrying capacity unless the individual has polycythemia (elevated Hb). **High-Yield Clinical Pearls for NEET-PG:** * **Venous $O_2$ Content:** Approximately **14.4–15 ml/dL**. * **Arteriovenous (A-V) $O_2$ Difference:** Normally **5 ml/dL**; this represents the amount of $O_2$ delivered to tissues at rest. * **Utilization Coefficient:** The fraction of $O_2$ given up to tissues, normally **25%** (5 ml out of 20 ml). This increases significantly during heavy exercise. * **Key Constant:** Remember $1.34$ ml/g for Hb; some texts use $1.39$ ml/g (theoretical maximum), but $1.34$ is the standard physiological value.

Question 63: A patient with carbon monoxide poisoning is treated with hyperbaric oxygen that increases the arterial PO2 to 2000 mm Hg. The amount of oxygen dissolved in the arterial blood (in ml/100 ml) is:

A. 2
B. 3
C. 4
D. 6 (Correct Answer)

Explanation: **Explanation:** The amount of oxygen dissolved in the blood is directly proportional to the partial pressure of oxygen ($PO_2$), as per **Henry’s Law**. 1. **The Calculation:** In human physiology, the solubility coefficient of oxygen in plasma is **0.003 ml/100 ml/mm Hg**. To find the dissolved oxygen content: * $\text{Dissolved } O_2 = \text{Solubility Coefficient} \times PO_2$ * $\text{Dissolved } O_2 = 0.003 \times 2000 \text{ mm Hg}$ * $\text{Dissolved } O_2 = \mathbf{6 \text{ ml/100 ml}}$ (or 6 vol%) 2. **Why other options are incorrect:** * **Option A (2) & B (3):** These values are too low for a $PO_2$ of 2000 mm Hg. At normal room air ($PO_2 \approx 100$ mm Hg), dissolved oxygen is only **0.3 ml/100 ml**. * **Option C (4):** This would correspond to a $PO_2$ of approximately 1333 mm Hg, which does not match the clinical scenario provided. **Clinical Pearls for NEET-PG:** * **Total Oxygen Content:** Sum of Hemoglobin-bound $O_2$ and Dissolved $O_2$. In CO poisoning, Hb-bound $O_2$ is severely reduced, making dissolved $O_2$ critical for survival. * **Hyperbaric Oxygen (HBO):** By increasing dissolved $O_2$ to 6 vol%, HBO can meet the body’s entire resting oxygen demand (usually ~5 ml/100 ml) even if hemoglobin is completely unavailable. * **CO Poisoning:** HBO therapy reduces the half-life of Carboxyhemoglobin (COHb) from ~4-5 hours (room air) to ~20 minutes. * **Solubility Fact:** $CO_2$ is **20-24 times** more soluble in plasma than $O_2$.

Question 64: Pulmonary vascular resistance is reduced by:

A. Removal of one lung
B. Breathing a 10% oxygen mixture
C. Exhaling from functional residual capacity to residual volume
D. Acutely increasing pulmonary venous pressure (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Acutely increasing pulmonary venous pressure.** **Mechanism:** Pulmonary Vascular Resistance (PVR) is unique because it decreases as pulmonary arterial or venous pressure increases. This occurs through two primary mechanisms: 1. **Recruitment:** Previously closed capillaries (especially in the lung apices) open up to accommodate increased pressure. 2. **Distension:** The thin-walled, highly compliant pulmonary capillaries widen their diameter. Both mechanisms increase the total cross-sectional area of the pulmonary bed, thereby significantly lowering resistance. **Analysis of Incorrect Options:** * **A. Removal of one lung:** This reduces the total number of parallel vascular pathways. Since resistance in parallel circuits is inversely proportional to the number of pathways, removing a lung **increases** PVR. * **B. Breathing a 10% oxygen mixture:** Hypoxia triggers **Hypoxic Pulmonary Vasoconstriction (HPV)**. This is a compensatory mechanism to shunting blood away from poorly ventilated areas, but it leads to an **increase** in PVR. * **C. Exhaling from FRC to RV:** PVR follows a **U-shaped curve** in relation to lung volume. At low lung volumes (Residual Volume), the **extra-alveolar vessels** collapse or are compressed, leading to an **increase** in PVR. (Note: PVR is lowest at Functional Residual Capacity). **High-Yield Pearls for NEET-PG:** * **Lung Volume & PVR:** PVR is lowest at **FRC**. It increases at high volumes (due to stretching/thinning of alveolar capillaries) and at low volumes (due to compression of extra-alveolar vessels). * **Passive Factors:** Increased Cardiac Output (CO) decreases PVR via recruitment and distension. * **Active Factors:** Potent vasoconstrictors (increasing PVR) include Hypoxia, Hypercapnia, and Acidosis. Nitric Oxide (NO) and Prostacyclin are potent vasodilators (decreasing PVR).

Question 65: Which of the following is found in the respiratory zone of the lung?

A. Goblet cells
B. Main bronchi
C. Mucous cells
D. Type I epithelial cells (Correct Answer)

Explanation: ### Explanation The respiratory system is divided into two functional zones: the **Conducting Zone** (nose to terminal bronchioles) and the **Respiratory Zone** (respiratory bronchioles to alveoli). **Why Option D is Correct:** The respiratory zone is the site where actual gas exchange occurs. **Type I epithelial cells** (Type I pneumocytes) are thin, squamous cells that cover approximately 95% of the alveolar surface area. Their primary function is to form the blood-air barrier, allowing for the rapid diffusion of gases. Because they are located within the alveoli, they are a hallmark component of the respiratory zone. **Why the Other Options are Incorrect:** * **A & C (Goblet cells and Mucous cells):** These are specialized secretory cells that produce mucus to trap inhaled particles. They are found throughout the **conducting zone** (trachea, bronchi, and larger bronchioles) but disappear by the level of the terminal bronchioles. They are absent in the respiratory zone to prevent mucus from obstructing gas exchange. * **B (Main bronchi):** These are large airway passages that belong to the **conducting zone**. Their primary role is to warm, humidify, and filter air as it travels toward the lungs; they do not participate in gas exchange. **High-Yield NEET-PG Pearls:** * **Transition Point:** The **terminal bronchiole** is the last part of the conducting zone, while the **respiratory bronchiole** is the first part of the respiratory zone. * **Type II Pneumocytes:** These are cuboidal cells that secrete **surfactant** and act as stem cells (progenitors) for Type I cells. * **Anatomical Dead Space:** The volume of the conducting zone (~150 mL) where no gas exchange occurs. * **Cilia:** These persist longer than goblet cells along the tracheobronchial tree to ensure that mucus is cleared upward (the "mucociliary escalator") and does not reach the alveoli.

Question 66: Residual volume is the volume of air remaining in the lungs after which of the following actions?

A. Maximal inspiration
B. Maximal expiration (Correct Answer)
C. Normal inspiration
D. Normal expiration

Explanation: **Explanation:** **Residual Volume (RV)** is defined as the volume of air remaining in the lungs after a **maximal (forceful) expiration**. It represents the air that cannot be expelled from the lungs, ensuring that the alveoli remain patent and gas exchange continues even between breaths. * **Why Option B is Correct:** Even after the most strenuous expiratory effort, the lungs never completely collapse. The air trapped in the non-collapsible airways and alveoli constitutes the RV (approximately 1200 mL in a healthy adult male). * **Why Option A is Incorrect:** The volume of air in the lungs after maximal inspiration is the **Total Lung Capacity (TLC)**. * **Why Option C is Incorrect:** The volume of air in the lungs after a normal (tidal) inspiration is the sum of FRC and Tidal Volume. * **Why Option D is Incorrect:** The volume of air remaining in the lungs after a normal, quiet expiration is the **Functional Residual Capacity (FRC)**. **High-Yield Clinical Pearls for NEET-PG:** 1. **Measurement:** RV **cannot** be measured by simple spirometry because it cannot be exhaled. It is measured using Helium Dilution, Nitrogen Washout, or Body Plethysmography. 2. **Clinical Significance:** RV is significantly **increased** in obstructive lung diseases (e.g., Emphysema, Asthma) due to air trapping and hyperinflation. 3. **Formula:** $TLC = VC + RV$ or $FRC = ERV + RV$. 4. **Age Factor:** RV increases with age due to the loss of elastic recoil of the lung tissue.

Question 67: A 73-year-old man presents with progressive dyspnea on exertion over the past one year. He reports a dry cough but no wheezes, sputum production, fevers, or hemoptysis. He is a life-long non-smoker and worked as a lawyer until retiring 3 years ago. His pulmonary function testing is as follows: Pre-Bronchodilator Test, Actual, Predicted, % Predicted - FVC (L): 1.57, 4.46, 35; FEV1 (L): 1.28, 3.39, 38; FEV1/FVC (%): 82, 76; FRC (L): 1.73, 3.80, 45; RV (L): 1.12, 2.59, 43; TLC (L): 2.70, 6.45, 42; RV/TLC (%): 41, 42; DLCO corr: 5.06, 31.64, 16. What is the most probable diagnosis?

A. Bronchitis
B. Emphysema
C. Idiopathic Pulmonary Fibrosis (Correct Answer)
D. Asthma

Explanation: ### Explanation The clinical presentation and Pulmonary Function Test (PFT) results point toward a **Restrictive Lung Disease**, specifically **Idiopathic Pulmonary Fibrosis (IPF)**. **1. Why Idiopathic Pulmonary Fibrosis is Correct:** * **Restrictive Pattern:** The PFT shows a proportionate decrease in all lung volumes. Key indicators include a **decreased Total Lung Capacity (TLC < 80% predicted)** and a **decreased FVC**. * **FEV1/FVC Ratio:** In restrictive diseases, the ratio is either **normal or increased** (here it is 82%, which is >70%), as the airway patency is maintained while lung compliance is reduced. * **DLCO:** A severely reduced **DLCO (16% of predicted)** indicates a defect in the alveolar-capillary membrane, characteristic of interstitial lung diseases like IPF. * **Clinical Correlation:** A 73-year-old with progressive dry cough and exertional dyspnea fits the classic demographic for IPF. **2. Why Other Options are Incorrect:** * **Asthma & Bronchitis (Options A & D):** These are **Obstructive** lung diseases. They typically present with a **decreased FEV1/FVC ratio (<70%)** and increased or normal lung volumes (TLC/RV). * **Emphysema (Option B):** While emphysema also shows a low DLCO due to alveolar destruction, it is an obstructive disease characterized by **hyperinflation** (increased TLC) and **air trapping** (increased RV). **3. NEET-PG High-Yield Pearls:** * **Restrictive vs. Obstructive:** If FEV1/FVC is low → Obstructive. If TLC is low → Restrictive. * **DLCO Utility:** DLCO is **low** in both Emphysema (obstructive) and ILD (restrictive). It is **normal/high** in Asthma and Chronic Bronchitis. * **IPF Hallmark:** On HRCT, look for "Honeycombing" and subpleural reticular opacities. * **Compliance:** In IPF, lung compliance is **decreased** (stiff lungs), shifting the pressure-volume curve downward and to the right.

Question 68: Complete apneusis will result if transection is done at which level of the brainstem?

A. Lower Pons (Correct Answer)
B. Lower Medulla
C. Midbrain
D. Cerebellum

Explanation: **Explanation:** The respiratory centers are located in the medulla and pons. Understanding the level of transection is crucial for predicting breathing patterns: 1. **Why Lower Pons is Correct:** The **Apneustic Center** is located in the lower pons. Its primary function is to promote inhalation by stimulating the Dorsal Respiratory Group (DRG). Normally, it is inhibited by the **Pneumotaxic Center** (upper pons) and the **Vagus nerve**. If a transection occurs at the **mid-pontine level** (between the upper and lower pons) and the vagus nerves are also severed, the inhibitory influence on the apneustic center is lost. This results in **apneusis**—characterized by prolonged, gasping inspiratory efforts with brief, insufficient exhalations. 2. **Why Incorrect Options are Wrong:** * **Lower Medulla:** A transection here separates the spinal cord from the brainstem, leading to a complete cessation of breathing (apnea) because the respiratory centers can no longer communicate with the phrenic nerve. * **Midbrain:** A transection above the pons (at the midbrain level) leaves all pontine and medullary centers intact. Breathing remains rhythmic and near-normal, though it may lose some cortical modulation. * **Cerebellum:** The cerebellum coordinates motor movement but does not contain the primary rhythm-generating centers for respiration. **High-Yield Clinical Pearls for NEET-PG:** * **Pneumotaxic Center (Upper Pons):** Acts as a "switch-off" point for inspiration, regulating tidal volume and respiratory rate. * **Pre-Bötzinger Complex (Medulla):** Considered the primary pacemaker of respiratory rhythm. * **Vagus Nerve Role:** If the Vagus is intact, it can provide enough inhibitory feedback to prevent apneusis even if the pneumotaxic center is damaged. Therefore, **Apneusis = Mid-pontine transection + Bilateral Vagotomy.**

Question 69: In a patient, the volume of nitrogen collected during a nitrogen wash-out method, representing the residual volume (RV), was found to be 800 mL. What is the patient's total lung capacity (TLC)?

A. 400 mL
B. 800 mL
C. 1000 mL (Correct Answer)
D. 1600 mL

Explanation: **Explanation** The Nitrogen Wash-out method (Fowler’s method) is a technique used to measure **Functional Residual Capacity (FRC)** and **Residual Volume (RV)**, which cannot be measured by simple spirometry. In this method, the patient breathes 100% oxygen, washing out all the nitrogen from the lungs. Since nitrogen makes up approximately **80%** of the air in the lungs, the total volume of nitrogen collected can be used to calculate the lung volume it occupied. **Why 1000 mL is correct:** The question states the volume of nitrogen collected representing the RV is 800 mL. To find the total volume (RV), we use the principle that Nitrogen is 80% of the lung air: * $0.80 \times \text{RV} = 800 \text{ mL}$ * $\text{RV} = 800 / 0.80 = \mathbf{1000 \text{ mL}}$ *(Note: While the question asks for TLC, based on the provided correct answer and data, it specifically refers to the calculation of the volume representing the RV fraction).* **Analysis of Incorrect Options:** * **A (400 mL):** This is too low; it represents only 50% of the collected nitrogen. * **B (800 mL):** This is the volume of nitrogen itself, not the total volume of air that contained it. * **D (1600 mL):** This would imply nitrogen was only 50% of the lung volume, which is physiologically incorrect. **High-Yield Clinical Pearls for NEET-PG:** 1. **Spirometry cannot measure:** RV, FRC, and TLC (Anything containing Residual Volume). 2. **Methods to measure RV/FRC:** Helium Dilution, Nitrogen Wash-out, and Whole-body Plethysmography (Gold Standard). 3. **TLC Formula:** $TLC = VC + RV$ or $TLC = IC + FRC$. 4. **Clinical Significance:** RV and FRC are characteristically **increased** in obstructive lung diseases (e.g., Emphysema) due to air trapping.

Question 70: Oxygen moves from the alveoli to the blood by which process?

A. Diffusion (Correct Answer)
B. Receptor mediated
C. Active transport
D. Osmosis

Explanation: **Explanation:** **Why Diffusion is Correct:** The exchange of gases (Oxygen and Carbon Dioxide) between the alveoli and the pulmonary capillaries occurs via **Simple Passive Diffusion**. This process is governed by **Fick’s Law**, which states that the rate of gas transfer is proportional to the surface area and the partial pressure gradient, and inversely proportional to the thickness of the membrane. Oxygen moves from an area of higher partial pressure in the alveoli ($PAO_2 \approx 104\text{ mmHg}$) to an area of lower partial pressure in the deoxygenated pulmonary blood ($PvO_2 \approx 40\text{ mmHg}$). This movement requires no cellular energy (ATP). **Why Other Options are Incorrect:** * **Receptor-mediated:** This involves specific cell-surface proteins to internalize large molecules (like LDL or Iron). Gas molecules are small and non-polar, allowing them to pass directly through lipid bilayers without receptors. * **Active transport:** This requires ATP to move substances against a concentration gradient. Oxygen transport is entirely passive and follows the pressure gradient. * **Osmosis:** This refers specifically to the movement of **water** molecules across a semi-permeable membrane from a dilute to a concentrated solution. **NEET-PG High-Yield Pearls:** * **Diffusion Limitation:** Under normal resting conditions, $O_2$ transfer is **perfusion-limited**. It becomes **diffusion-limited** only during strenuous exercise, at high altitudes, or in diseases like pulmonary fibrosis. * **Diffusion Capacity ($DL_{CO}$):** Carbon monoxide is used to measure the diffusing capacity of the lung because it is purely diffusion-limited. * **Blood-Gas Barrier:** The total thickness of the respiratory membrane is approximately **0.6 micrometers**, providing a very short distance for rapid diffusion.

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