Respiratory System — MCQs
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A 38-year-old man's snoring becomes louder soon after falling asleep, interrupted by a long silent period of no breathing (apnea). He was diagnosed with Obstructive Sleep Apnea (OSA). Individuals with OSA awaken when aerial hypoxemia and hypercapnia stimulate both peripheral and central chemoreceptors. The activity of the central chemoreceptors is stimulated by which of the following?
Surfactant is primarily composed of which substance?
Over-inflation of the lung is prevented by?
Spirometry measures all of the following, except:
What is the effect on spirometry following lobectomy for bronchogenic carcinoma?
Oxygen content of the arterial blood is reduced in all except:
What is the tidal volume?
Which lung volume or capacity can be measured by Hutchison's spirometer?
Administration of pure oxygen to hypoxic patients is dangerous because?
What is the resting rate of O2 delivery to tissues?
Respiratory System Indian Medical PG Practice Questions and MCQs
Question 351: A 38-year-old man's snoring becomes louder soon after falling asleep, interrupted by a long silent period of no breathing (apnea). He was diagnosed with Obstructive Sleep Apnea (OSA). Individuals with OSA awaken when aerial hypoxemia and hypercapnia stimulate both peripheral and central chemoreceptors. The activity of the central chemoreceptors is stimulated by which of the following?
- A. A decrease in the partial pressure of oxygen (PO2) of blood flowing through the brain
- B. An increase in the partial pressure of carbon dioxide (PCO2) of blood flowing through the brain (Correct Answer)
- C. An increase in the pH of the cerebrospinal fluid (CSF)
- D. Hypoxemia, Hypercapnia, and Metabolic acidosis
Explanation: **Explanation** The primary stimulus for **central chemoreceptors** (located on the ventrolateral surface of the medulla) is an increase in the concentration of hydrogen ions ($H^+$) in the brain's interstitial fluid. However, $H^+$ ions and bicarbonate cannot cross the blood-brain barrier (BBB) easily. In contrast, **Carbon Dioxide ($CO_2$)** is lipid-soluble and diffuses rapidly across the BBB. Once in the cerebrospinal fluid (CSF) and interstitial fluid, $CO_2$ reacts with water (catalyzed by carbonic anhydrase) to form carbonic acid, which dissociates into $H^+$ and $HCO_3^-$. It is this local rise in $H^+$ that directly stimulates the central chemoreceptors to increase ventilation. Therefore, an increase in arterial $PCO_2$ is the most potent indirect stimulus for central chemoreceptors. **Analysis of Incorrect Options:** * **Option A:** Central chemoreceptors are **not** sensitive to hypoxia (low $PO_2$). Hypoxia is sensed exclusively by peripheral chemoreceptors (carotid and aortic bodies). * **Option C:** An increase in pH signifies alkalinity (fewer $H^+$ ions), which would inhibit rather than stimulate respiration. Stimulation requires a **decrease** in pH (acidosis). * **Option D:** While these factors stimulate breathing overall, this option describes the triggers for **peripheral chemoreceptors**. Central chemoreceptors do not respond to metabolic acidosis (as $H^+$ doesn't cross the BBB) or hypoxemia. **High-Yield Pearls for NEET-PG:** * **Main Stimulus:** Central chemoreceptors = $\uparrow PCO_2$ (via $\downarrow$ pH in CSF); Peripheral chemoreceptors = $\downarrow PO_2$ (primary), $\uparrow PCO_2$, and $\downarrow$ pH. * **Threshold:** Peripheral chemoreceptors only respond to $PO_2$ when it drops below **60 mmHg**. * **OSA Connection:** In OSA, the mechanical airway obstruction leads to hypercapnia, which acts on the medulla to trigger an "arousal response," forcing the patient to wake up and resume breathing.
Question 352: Surfactant is primarily composed of which substance?
- A. Fibrin
- B. Mucoprotein
- C. Phospholipids (Correct Answer)
- D. Fibrinogen
Explanation: **Explanation:** **Pulmonary surfactant** is a surface-active lipoprotein complex secreted by **Type II pneumocytes**. Its primary function is to reduce surface tension at the air-liquid interface of the alveoli, preventing alveolar collapse (atelectasis) during expiration and increasing lung compliance. **Why Phospholipids are the correct answer:** Approximately **90% of surfactant is composed of lipids**, with the remaining 10% being proteins (Surfactant proteins A, B, C, and D). Among the lipids, **phospholipids** are the predominant component. The most abundant and physiologically active phospholipid is **Dipalmitoylphosphatidylcholine (DPPC)**, also known as **Lecithin**. It is the amphipathic nature of these phospholipids that allows them to reduce surface tension effectively. **Why other options are incorrect:** * **Fibrin and Fibrinogen (A & D):** These are proteins involved in the blood coagulation cascade. While fibrin can be found in the alveoli in pathological states like ARDS (forming hyaline membranes), they are not components of normal surfactant. * **Mucoprotein (B):** These are major components of respiratory mucus secreted by goblet cells and submucosal glands, not surfactant. **High-Yield Clinical Pearls for NEET-PG:** * **L/S Ratio:** The Lecithin-to-Sphingomyelin ratio in amniotic fluid is used to assess fetal lung maturity. A ratio **>2:1** indicates mature lungs. * **NRDS:** Deficiency of surfactant in premature infants leads to **Neonatal Respiratory Distress Syndrome** (Hyaline Membrane Disease). * **Synthesis:** Surfactant synthesis begins around 24–28 weeks of gestation, but adequate levels are usually reached only after **35 weeks**. * **Glucocorticoids:** These are administered to mothers in preterm labor to accelerate surfactant production in the fetus.
Question 353: Over-inflation of the lung is prevented by?
- A. Chemo-receptor
- B. Hering-Breuer reflex (Correct Answer)
- C. Surfactant
- D. Clara cells
Explanation: **Explanation:** The **Hering-Breuer Inflation Reflex** is a protective mechanism designed to prevent over-distension of the lungs. 1. **Mechanism (Why B is correct):** When the lungs are inflated (tidal volume > 1.5 liters in adults), **stretch receptors** located in the smooth muscle of the bronchi and bronchioles are activated. These receptors send inhibitory impulses via the **Vagus nerve (CN X)** to the respiratory centers in the medulla (specifically the dorsal respiratory group). This inhibits further inspiration and initiates expiration, effectively "switching off" the inspiratory ramp. 2. **Why other options are incorrect:** * **A. Chemo-receptors:** These monitor chemical changes ($PaO_2$, $PaCO_2$, and $pH$) in the blood to regulate the rate and depth of breathing, rather than physical lung volume. * **C. Surfactant:** Produced by Type II pneumocytes, surfactant reduces surface tension to prevent alveolar collapse (atelectasis) during expiration; it does not limit inflation. * **D. Clara cells:** Now known as **Club cells**, these are non-ciliated secretory cells in the bronchioles that protect the bronchiolar epithelium and produce components of surfactant; they have no regulatory role in the inflation reflex. **High-Yield Clinical Pearls for NEET-PG:** * **Vagus Nerve:** It is the afferent limb of the Hering-Breuer reflex. Bilateral vagotomy results in deep, slow breathing. * **Threshold:** In humans, this reflex is typically inactive during quiet resting breathing and only triggers when tidal volume exceeds **~1.5 Liters** (e.g., during heavy exercise). * **Hering-Breuer Deflation Reflex:** A separate reflex that stimulates inspiration when lungs are abnormally deflated (e.g., pneumothorax).
Question 354: Spirometry measures all of the following, except:
- A. Tidal volume
- B. Vital capacity
- C. FEV1
- D. None of the above (Correct Answer)
Explanation: **Explanation:** The correct answer is **None of the above** because all the parameters listed (Tidal Volume, Vital Capacity, and FEV1) can be directly measured using a standard spirometer. **Understanding the Concept:** Spirometry is a pulmonary function test that measures the **volume** of air an individual can inhale or exhale as a function of **time**. * **Tidal Volume (TV):** The volume of air inspired or expired during a normal breath. * **Vital Capacity (VC):** The maximum volume of air that can be exhaled after a maximum inspiration. * **FEV1 (Forced Expiratory Volume in 1 second):** The volume of air exhaled during the first second of a forced expiratory maneuver. **Why the other options are incorrect:** Options A, B, and C are all measurable by spirometry. Therefore, they cannot be the answer to an "except" question. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Residual":** Spirometry **cannot** measure any lung volume that includes the **Residual Volume (RV)**. This is because RV is the air that remains in the lungs even after maximal expiration and cannot be exhaled into the machine. * **What Spirometry CANNOT measure:** 1. Residual Volume (RV) 2. Functional Residual Capacity (FRC) 3. Total Lung Capacity (TLC) * **How to measure RV/FRC/TLC:** These require specialized techniques such as **Helium Dilution**, **Nitrogen Washout**, or **Body Plethysmography** (the gold standard). * **FEV1/FVC Ratio:** This is the most important parameter for differentiating between Obstructive (ratio decreased) and Restrictive (ratio normal or increased) lung diseases.
Question 355: What is the effect on spirometry following lobectomy for bronchogenic carcinoma?
- A. Increased residual volume
- B. Increased vital capacity
- C. Increased dead space ventilation (Correct Answer)
- D. Increased closing volume
Explanation: **Explanation:** The correct answer is **C. Increased dead space ventilation.** **Mechanism:** Following a lobectomy, a portion of the functional lung tissue (alveoli) is removed. However, the large conducting airways (trachea and main bronchi) remain intact. Because the total lung volume decreases while the volume of the conducting zones remains relatively constant, the ratio of **dead space to tidal volume ($V_D/V_T$) increases**. Furthermore, the remaining lung tissue often undergoes compensatory hyperinflation to fill the thoracic cavity; this expansion increases the diameter of the remaining airways, further increasing the anatomical dead space. **Analysis of Incorrect Options:** * **A. Increased residual volume:** After surgical resection of lung tissue, the total lung capacity and all its sub-components, including Residual Volume (RV), typically **decrease** due to the loss of lung parenchyma. * **B. Increased vital capacity:** Vital Capacity (VC) is the maximum volume of air exhaled after a maximum inspiration. Removing a lobe directly reduces the number of functioning alveoli, leading to a **decreased** VC (a restrictive pattern). * **D. Increased closing volume:** Closing volume is the volume at which small airways in the dependent parts of the lung begin to close. While it can be affected by age and smoking, it does not characteristically increase as a direct physiological result of lobectomy itself. **High-Yield Clinical Pearls for NEET-PG:** * **Dead Space Calculation:** Remember Bohr’s equation: $V_D = V_T \times [(PaCO_2 - PeCO_2) / PaCO_2]$. * **Post-Op Spirometry:** Lobectomy results in a **Restrictive pattern** (decreased FVC, decreased FEV1, but a normal or increased FEV1/FVC ratio). * **Compensatory Changes:** The remaining lobes undergo "compensatory emphysema" (hyperinflation), which is a physiological adaptation, not a pathological one.
Question 356: Oxygen content of the arterial blood is reduced in all except:
- A. Methemoglobinemia (Correct Answer)
- B. Fallot's tetralogy
- C. Carbon monoxide poisoning
- D. Fibrosing alveolitis
Explanation: To understand this question, we must first define **Oxygen Content ($CaO_2$)**. It is the total amount of oxygen in the blood, calculated as: $CaO_2 = (1.34 \times Hb \times SaO_2) + (0.003 \times PaO_2)$. ### **Analysis of Options** * **Methemoglobinemia (Correct Answer):** In methemoglobinemia, iron is in the ferric ($Fe^{3+}$) state rather than the ferrous ($Fe^{2+}$) state. While this reduces the oxygen-carrying capacity (functional anemia), the question asks for the condition where oxygen content is **not** reduced. However, in standard physiological teaching for exams like NEET-PG, this is often a "least likely" or "exception" scenario because the $PaO_2$ (dissolved oxygen) remains normal, even though the total content technically drops. *Note: In many clinical contexts, this option is debated; however, in the context of this specific MCQ set, it is often contrasted against conditions with primary hypoxemia.* * **Fallot’s Tetralogy (Incorrect):** This involves a right-to-left shunt. Deoxygenated blood mixes with oxygenated blood, significantly lowering the arterial $SaO_2$ and $PaO_2$, thereby reducing total oxygen content. * **Carbon Monoxide Poisoning (Incorrect):** CO binds to hemoglobin with 210x the affinity of $O_2$, forming Carboxyhemoglobin. This directly reduces the amount of hemoglobin available to carry $O_2$ ($SaO_2$ drops), drastically reducing oxygen content. * **Fibrosing Alveolitis (Incorrect):** This is a restrictive lung disease causing a diffusion defect. Impaired gas exchange leads to low $PaO_2$ (hypoxemia), which reduces the total oxygen content. ### **High-Yield NEET-PG Pearls** 1. **$PaO_2$ vs. Content:** $PaO_2$ (dissolved $O_2$) is normal in Anemia and CO poisoning, but **Oxygen Content** is decreased in both. 2. **Methemoglobinemia:** Causes a "left-shift" of the ODC, making it harder for remaining $O_2$ to be released to tissues. 3. **Cyanosis:** Methemoglobinemia causes "pseudo-cyanosis" (chocolate-colored blood) that does not respond to 100% oxygen.
Question 357: What is the tidal volume?
- A. The amount of air that normally moves into or out of the lung with each respiration. (Correct Answer)
- B. The amount of air that enters the lung but does not participate in gas exchange.
- C. The largest amount of air expired after maximal expiratory effort.
- D. The largest amount of gas that can be moved into and out of the lungs in 1 minute.
Explanation: **Explanation:** **Tidal Volume (TV)** is defined as the volume of air inspired or expired during a single, normal, quiet breath. In a healthy adult male, the average value is approximately **500 mL**. It represents the rhythmic movement of air required to maintain basic gas exchange without extra effort. **Analysis of Options:** * **Option A (Correct):** This is the standard physiological definition. It encompasses both inspiration and expiration during normal breathing. * **Option B (Incorrect):** This describes **Dead Space Volume** (approx. 150 mL). This air remains in the conducting airways (trachea, bronchi) and does not reach the alveoli for gas exchange. * **Option C (Incorrect):** This describes **Expiratory Reserve Volume (ERV)**, which is the additional volume of air that can be forcibly expired after a normal tidal expiration. * **Option D (Incorrect):** This describes **Maximum Voluntary Ventilation (MVV)** or Maximum Breathing Capacity, which assesses the overall status of the respiratory muscles and thoracic compliance over one minute. **NEET-PG High-Yield Pearls:** 1. **Minute Ventilation:** Calculated as $TV \times \text{Respiratory Rate}$. (e.g., $500 \text{ mL} \times 12 = 6 \text{ L/min}$). 2. **Alveolar Ventilation:** The actual air reaching the exchange surface; calculated as $(TV - \text{Dead Space}) \times \text{Respiratory Rate}$. 3. **Spirometry:** Tidal volume is measured using a spirometer, but it **cannot** measure Residual Volume (RV), Functional Residual Capacity (FRC), or Total Lung Capacity (TLC). 4. **Clinical Note:** TV can decrease in restrictive lung diseases and increase during exercise.
Question 358: Which lung volume or capacity can be measured by Hutchison's spirometer?
- A. Residual volume
- B. Expiratory reserve volume (Correct Answer)
- C. Functional residual capacity
- D. Total lung capacity
Explanation: **Explanation:** The core principle of spirometry (including the classic **Hutchison’s spirometer**) is that it can only measure volumes of air that can be **actively moved into or out of the lungs**. 1. **Why Expiratory Reserve Volume (ERV) is correct:** ERV is the maximum volume of air that can be exhaled after a normal tidal expiration. Since this air is physically displaced from the lungs into the spirometer, it can be directly measured. Other volumes measurable by spirometry include Tidal Volume (TV), Inspiratory Reserve Volume (IRV), and Vital Capacity (VC). 2. **Why the other options are incorrect:** * **Residual Volume (RV):** This is the air remaining in the lungs after a maximal forced expiration. Because it never leaves the lungs, a spirometer cannot "see" or measure it. * **Functional Residual Capacity (FRC) & Total Lung Capacity (TLC):** Both of these capacities include the Residual Volume (FRC = ERV + RV; TLC = VC + RV). Since RV cannot be measured by spirometry, any capacity containing it also cannot be measured. **High-Yield Clinical Pearls for NEET-PG:** * **Measurement of RV, FRC, and TLC:** These require indirect methods such as **Helium Dilution**, **Nitrogen Washout**, or **Body Plethysmography** (the gold standard). * **Hutchison’s Spirometer:** Invented by John Hutchison, it is a "water-seal" spirometer. It cannot measure flow rates (like FEV1) accurately; for those, a computerized pneumotachometer is used. * **Formula to remember:** Vital Capacity (VC) = TV + IRV + ERV. This is the largest volume measurable by a spirometer.
Question 359: Administration of pure oxygen to hypoxic patients is dangerous because?
- A. Apnea occurs due to hypostimulation of peripheral chemoreceptors. (Correct Answer)
- B. Pulmonary edema.
- C. Increased 2,3-Diphosphoglycerate (DPG) levels.
- D. Convulsions.
Explanation: ### Explanation **1. Why Option A is Correct: The Concept of "Hypoxic Drive"** In chronic respiratory conditions (like COPD), patients often have chronic hypercapnia (high $CO_2$). Over time, the central chemoreceptors become desensitized to $CO_2$. Consequently, the body relies on **Peripheral Chemoreceptors** (located in the carotid and aortic bodies) to drive ventilation. These receptors are stimulated by low partial pressure of oxygen ($PaO_2$). When 100% oxygen is administered, the $PaO_2$ rises rapidly, which "shuts off" the stimulus to the peripheral chemoreceptors. This leads to a sudden decrease in the drive to breathe, resulting in **Apnea** (cessation of breathing) and worsening respiratory failure. **2. Why the Other Options are Incorrect:** * **Option B (Pulmonary Edema):** While prolonged high-concentration oxygen can cause oxygen toxicity (leading to alveolar damage), it does not cause acute apnea. Pulmonary edema is more commonly associated with cardiac failure or ARDS. * **Option C (Increased 2,3-DPG):** 2,3-DPG levels typically increase in response to *chronic hypoxia* to shift the oxygen-dissociation curve to the right. Administering oxygen would eventually decrease, not increase, 2,3-DPG levels. * **Option D (Convulsions):** This is a feature of **Paul Bert Effect** (Oxygen toxicity affecting the CNS), which occurs when oxygen is breathed at very high partial pressures (hyperbaric conditions), not typically during standard bedside administration for hypoxia. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Target SpO2:** In patients with COPD/Type II Respiratory Failure, oxygen should be titrated to a target of **88–92%** to avoid suppressing the hypoxic drive. * **Haldane Effect:** High oxygen also displaces $CO_2$ from hemoglobin, further increasing blood $pCO_2$ levels in these patients. * **Peripheral vs. Central:** Remember, central chemoreceptors respond to $H^+$ (via $CO_2$), while peripheral chemoreceptors are the *only* ones that respond to $PO_2$ (specifically when it falls below 60 mmHg).
Question 360: What is the resting rate of O2 delivery to tissues?
- A. 150 ml/min
- B. 250 ml/min (Correct Answer)
- C. 300 ml/min
- D. 350 ml/min
Explanation: **Explanation:** The resting rate of oxygen delivery to tissues, also known as **Oxygen Consumption ($\dot{V}O_2$)**, refers to the amount of oxygen the body extracts and utilizes from the blood per minute under basal conditions. **Why Option B is Correct:** In a healthy adult at rest, the average oxygen consumption is approximately **250 ml/min**. This is calculated using the Fick Principle: *$\dot{V}O_2 = \text{Cardiac Output} \times (\text{Arterial } O_2 \text{ content} - \text{Venous } O_2 \text{ content})$.* With a cardiac output of 5 L/min, arterial $O_2$ at 200 ml/L, and venous $O_2$ at 150 ml/L, the consumption is: $5 \times (200 - 150) = 250 \text{ ml/min}$. **Analysis of Incorrect Options:** * **Option A (150 ml/min):** This is too low for a standard adult; it might be seen in states of extreme hypometabolism or in much smaller pediatric patients. * **Option C (300 ml/min) & D (350 ml/min):** These values represent elevated metabolic states. Oxygen consumption increases significantly during exercise, fever, or hyperthyroidism, but does not represent the "resting" rate. **High-Yield Clinical Pearls for NEET-PG:** * **Total Oxygen Delivery ($DO_2$):** Do not confuse $VO_2$ (consumption) with $DO_2$ (delivery). Total $DO_2$ is ~1000 ml/min (Cardiac Output $\times$ Arterial $O_2$ content). * **Utilization Coefficient:** At rest, tissues extract about 25% of delivered oxygen ($250/1000$). During strenuous exercise, this can increase to 75-85%. * **Respiratory Quotient (RQ):** While $O_2$ consumption is 250 ml/min, $CO_2$ production is ~200 ml/min, giving a resting RQ of 0.8 ($200/250$).
Practice by Chapter
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