Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 511: What is the major constituent of pulmonary surfactant?

A. Dipalmitoyl lecithin (Correct Answer)
B. Dipalmitoyl cephalin
C. Dipalmitoyl serine
D. Dipalmitoyl inositol

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 Dipalmitoyl Lecithin is correct:** Surfactant is composed of approximately 90% lipids and 10% proteins. The most abundant and physiologically significant lipid component is **Dipalmitoylphosphatidylcholine (DPPC)**, also known as **Dipalmitoyl Lecithin**. It accounts for about 60–70% of the total phospholipid content. Its amphipathic nature allows it to align at the surface, effectively neutralizing the cohesive forces of water molecules. **Analysis of Incorrect Options:** * **B, C, and D:** While Phosphatidylinositol, Phosphatidylserine, and Phosphatidylethanolamine (Cephalin) are phospholipids found in cell membranes and in trace amounts within surfactant, they are minor constituents. They do not possess the same potent surface-tension-reducing properties as DPPC. **High-Yield NEET-PG Pearls:** * **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. * **Surfactant Proteins:** SP-A and SP-D are important for innate immunity, while **SP-B and SP-C** are essential for the spreading and stability of the surfactant film. * **Clinical Correlation:** Deficiency of surfactant in premature infants leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease. * **Synthesis:** Surfactant synthesis begins between 24–28 weeks of gestation, reaching adequate levels by 35 weeks. Its production is stimulated by **Glucocorticoids**.

Question 512: The nitrogen washout method is used for estimating which of the following respiratory parameters?

A. Dead space volume
B. Functional residual capacity (Correct Answer)
C. Tidal volume
D. Diffusion capacity

Explanation: **Explanation:** The **Nitrogen Washout Method** (Fowler’s method for anatomic dead space or the open-circuit method for lung volumes) is a classic technique used to measure **Functional Residual Capacity (FRC)**. **Why Option B is Correct:** FRC is the volume of air remaining in the lungs after a normal tidal expiration. Since it cannot be measured by simple spirometry (as the air never leaves the lungs during normal breathing), indirect methods are required. In the Nitrogen Washout method, the patient breathes 100% oxygen. This "washes out" the nitrogen normally present in the lungs (about 78-80%). By measuring the total volume of expired air and the concentration of nitrogen within it, the initial volume of air in the lungs (FRC) can be calculated using the principle of conservation of mass. **Why Other Options are Incorrect:** * **A. Dead space volume:** While "Fowler’s Method" also uses nitrogen washout to measure **Anatomic Dead Space**, in the context of standard lung volume measurement questions, Nitrogen Washout is the primary answer for FRC. * **C. Tidal volume:** This is easily measured using **simple spirometry** and does not require gas dilution techniques. * **D. Diffusion capacity:** This is measured using the **DLCO (Diffusion Capacity of the Lung for Carbon Monoxide)** test, which assesses the integrity of the alveolar-capillary membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Methods to measure FRC:** 1. Helium Dilution (Closed circuit), 2. Nitrogen Washout (Open circuit), 3. Body Plethysmography (Most accurate, measures total thoracic gas volume). * **Body Plethysmography** is superior to gas dilution because it also measures "trapped air" in patients with obstructive diseases (COPD/Asthma). * **Formula:** $FRC = RV + ERV$ (Residual Volume + Expiratory Reserve Volume).

Question 513: What is the respiratory quotient for a carbohydrate meal?

A. 0.7
B. 0.8
C. 0.6
D. 1 (Correct Answer)

Explanation: **Explanation:** The **Respiratory Quotient (RQ)** is the ratio of the volume of carbon dioxide ($CO_2$) produced to the volume of oxygen ($O_2$) consumed per unit of time ($RQ = \frac{CO_2 \text{ produced}}{O_2 \text{ consumed}}$). **Why Option D is Correct:** For carbohydrates, the number of oxygen molecules required for oxidation is exactly equal to the number of carbon dioxide molecules produced. Taking glucose as an example: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$ $RQ = \frac{6CO_2}{6O_2} = \mathbf{1.0}$. Therefore, when an individual consumes a purely carbohydrate meal, the RQ is 1. **Analysis of Incorrect Options:** * **Option A (0.7):** This is the RQ for **Fats** (e.g., Tripalmitin). Fats are "oxygen-poor" molecules and require significantly more external oxygen for complete oxidation. * **Option B (0.8):** This is the approximate RQ for **Proteins**. It is also the average RQ for a **Mixed Diet** (typically cited as 0.8 to 0.82). * **Option C (0.6):** This value is lower than any standard macronutrient. However, an RQ of ~0.7 or slightly lower can be seen during prolonged starvation or untreated diabetes (ketosis). **High-Yield Facts for NEET-PG:** * **Mixed Diet RQ:** 0.8 (Most common exam question). * **Brain RQ:** Always 1.0 (since it relies almost exclusively on glucose). * **RQ > 1.0:** Seen during **Lipogenesis** (conversion of excess carbohydrates to fat, e.g., overfeeding) and during intense exercise (due to buffering of lactic acid by bicarbonate, which releases "excess" $CO_2$). * **RQ < 0.7:** Seen in **Starvation** and **Diabetes Mellitus** (increased fat utilization). * **Measurement:** RQ is measured using a **Douglas bag** or a spirometer (indirect calorimetry).

Question 514: Which of the following pulmonary function changes is NOT typically seen in emphysema?

A. Total Lung Capacity (TLC)
B. Residual Volume (RV)
C. Forced Expiratory Volume in 1 second (FEV1) (Correct Answer)
D. Diffusing capacity of the lung for carbon monoxide (DLCO)

Explanation: ### Explanation The correct answer is **C. Forced Expiratory Volume in 1 second (FEV1)**. The question asks which change is **NOT** typically seen in emphysema. In emphysema, there is a **decrease** in FEV1 due to airway collapse and loss of elastic recoil. Since the other three options (TLC, RV, and DLCO) all undergo characteristic changes (increases or decreases), the phrasing of this specific question implies we are looking for the parameter that does not *increase* or is not *preserved*, or it highlights a misunderstanding of the obstructive pattern. **1. Why FEV1 is the answer:** In emphysema (an obstructive lung disease), the FEV1 is **significantly reduced**. The destruction of alveolar walls leads to a loss of radial traction, causing small airways to collapse during expiration (dynamic compression). Therefore, a "normal" or "increased" FEV1 is never seen. **2. Analysis of Incorrect Options:** * **A. Total Lung Capacity (TLC):** This **increases** in emphysema. Loss of elastic recoil means the chest wall can expand further outward, leading to hyperinflation. * **B. Residual Volume (RV):** This **increases** significantly. Air trapping occurs because airways close prematurely during expiration, leaving more air behind in the lungs. * **C. DLCO:** This **decreases** in emphysema. It is the only obstructive disease where DLCO drops because the destruction of alveolar walls reduces the surface area available for gas exchange. **High-Yield Clinical Pearls for NEET-PG:** * **FEV1/FVC Ratio:** Always **decreased (<0.7)** in obstructive diseases like emphysema. * **Compliance:** Lung compliance is **increased** in emphysema due to the loss of elastic fibers (the lung is "easy to blow up but hard to empty"). * **Pink Puffers:** Classic description of emphysema patients who maintain oxygenation by hyperventilating, despite a low DLCO. * **Centriacinar vs. Panacinar:** Centriacinar is associated with smoking (upper lobes); Panacinar is associated with $\alpha_1$-antitrypsin deficiency (lower lobes).

Question 515: Which factor most affects the oxygen-carrying capacity of blood?

A. Hemoglobin (Correct Answer)
B. pH
C. pCO2
D. Red blood cell count

Explanation: **Explanation:** The **oxygen-carrying capacity** of blood is defined as the maximum amount of oxygen that can be carried by a given volume of blood. This is primarily determined by the concentration of **Hemoglobin (Hb)**. **Why Hemoglobin is correct:** Each gram of pure hemoglobin can bind approximately **1.34 mL of oxygen** (Hüfner's constant). Since about 98-99% of oxygen in the blood is transported bound to hemoglobin (with only 1-2% dissolved in plasma), the total amount of oxygen the blood can hold is directly proportional to the hemoglobin concentration. The formula for Oxygen Content is: $(1.34 \times \text{Hb} \times \text{SaO}_2) + (0.003 \times \text{PaO}_2)$. **Why other options are incorrect:** * **pH and pCO2:** These factors influence the **Oxygen-Hemoglobin Dissociation Curve (OHDC)** and hemoglobin’s *affinity* for oxygen (Bohr Effect). While they determine how easily oxygen is loaded or unloaded, they do not change the total capacity of the blood to carry oxygen. * **Red blood cell count:** While RBCs contain hemoglobin, the count itself is not the direct measure of capacity. For example, in microcytic anemia, the RBC count might be normal or high, but the oxygen-carrying capacity is reduced because the total hemoglobin content is low. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Oxygen Capacity:** In a healthy adult with 15g/dL of Hb, the capacity is approximately **20.1 mL O2/100mL** of blood. * **Anemia vs. CO Poisoning:** In both conditions, the oxygen-carrying capacity is **decreased**, but the PaO2 (dissolved oxygen) remains normal. * **Methemoglobinemia:** Reduces oxygen-carrying capacity because iron is in the ferric state ($Fe^{3+}$), which cannot bind oxygen.

Question 516: Myoglobin binds to how many molecules of O2?

A. 1 (Correct Answer)
B. 2
C. 3
D. 4

Explanation: **Explanation:** The correct answer is **A (1)**. **Underlying Medical Concept:** Myoglobin is a monomeric hemeprotein primarily found in skeletal and cardiac muscle fibers. Unlike Hemoglobin (Hb), which is a tetramer (consisting of four polypeptide chains), Myoglobin consists of a **single polypeptide chain** associated with a **single heme group**. Since one heme group binds to one molecule of oxygen ($O_2$), a single myoglobin molecule can bind to only one $O_2$ molecule. **Analysis of Incorrect Options:** * **Option B & C:** These are incorrect because myoglobin lacks the quaternary structure (multiple subunits) required to bind more than one oxygen molecule. * **Option D (4):** This is a common distractor. **Hemoglobin (HbA)** is a tetramer consisting of four subunits ($2\alpha, 2\beta$), allowing it to bind up to **four** molecules of $O_2$. **High-Yield Facts for NEET-PG:** * **Dissociation Curve:** Due to its single binding site, myoglobin does not show "cooperative binding." Its oxygen dissociation curve is **hyperbolic**, whereas hemoglobin’s curve is **sigmoidal**. * **Affinity:** Myoglobin has a much **higher affinity** for $O_2$ than hemoglobin. It only releases oxygen when the partial pressure ($PO_2$) falls to very low levels (around 5 mmHg), making it an ideal oxygen storage unit for muscle contraction during hypoxia. * **$P_{50}$ Value:** The $P_{50}$ (partial pressure at which 50% is saturated) for myoglobin is approximately **2.75 mmHg**, compared to **26.7 mmHg** for adult hemoglobin. * **Clinical Correlation:** Myoglobinuria (presence of myoglobin in urine) is a hallmark of **Rhabdomyolysis** and can lead to acute kidney injury (AKI).

Question 517: Which of the following is NOT a physiological response to smoking?

A. Decreased HDL
B. Increased hematocrit
C. Increased heart rate and increased catecholamine release
D. Decreased carboxyhemoglobin (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Decreased carboxyhemoglobin**. Smoking actually leads to an **increase** in carboxyhemoglobin (COHb) levels. **1. Why Option D is correct:** Cigarette smoke contains carbon monoxide (CO), which has an affinity for hemoglobin approximately **200–250 times greater** than oxygen. When inhaled, CO binds to hemoglobin to form carboxyhemoglobin. In chronic smokers, COHb levels can reach 5–15%, compared to <1% in non-smokers. This shifts the oxygen-dissociation curve to the **left**, impairing oxygen delivery to tissues. **2. Why the other options are incorrect:** * **A. Decreased HDL:** Smoking alters lipid metabolism, leading to lower levels of High-Density Lipoprotein (HDL—the "good" cholesterol) and increased LDL and triglycerides, contributing to atherosclerosis. * **B. Increased hematocrit:** Due to the chronic functional hypoxia caused by elevated COHb, the kidneys increase **erythropoietin** production. This results in secondary polycythemia (increased RBC count and hematocrit) as a compensatory mechanism to improve oxygen-carrying capacity. * **C. Increased heart rate and catecholamines:** Nicotine stimulates the sympathetic nervous system and the adrenal medulla, leading to the release of epinephrine and norepinephrine. This results in acute increases in heart rate, blood pressure, and myocardial contractility. **High-Yield Clinical Pearls for NEET-PG:** * **Left Shift:** CO poisoning causes a left shift in the $O_2$ dissociation curve (holding onto $O_2$ more tightly). * **Closing Volume:** Smoking increases the "closing volume" of the lungs due to small airway inflammation. * **Ciliary Function:** Smoking paralyzes the mucociliary escalator, leading to the characteristic "smoker’s cough."

Question 518: Which of the following functions is associated with bronchial circulation?

A. Air conditioning of inhaled air (Correct Answer)
B. Drug absorption
C. Gaseous exchange
D. Reserve volume

Explanation: **Explanation:** The lungs have a dual blood supply: the **pulmonary circulation** (for gas exchange) and the **bronchial circulation** (the systemic nutritional supply). **1. Why Option A is Correct:** The bronchial circulation arises from the systemic arteries (aorta) and supplies the conducting airways down to the terminal bronchioles. One of its primary physiological roles is **air conditioning**. As air travels through the conducting zone, the extensive subepithelial capillary network of the bronchial vessels warms and humidifies the inhaled air to body temperature and 100% humidity before it reaches the delicate alveoli. **2. Why Other Options are Incorrect:** * **B. Drug absorption:** While some absorption occurs, it is not a primary physiological *function* of bronchial circulation. Most inhaled drugs are designed for local action or absorbed via the vast surface area of the pulmonary capillaries. * **C. Gaseous exchange:** This is the primary function of the **pulmonary circulation**. The bronchial circulation is "non-respiratory" as it supplies oxygenated blood to the tissues rather than picking up oxygen from the alveoli. * **D. Reserve volume:** This is a static lung volume (ERV) related to pulmonary mechanics and ventilation, not a function of the vascular supply. **Clinical Pearls & High-Yield Facts:** * **Bronchial Shunt:** About 1%–2% of the total cardiac output goes to the bronchial circulation. Most of this blood drains into the pulmonary veins (rather than the right atrium), contributing to the **physiological shunt** (venous admixture). * **Hemoptysis:** In cases of massive hemoptysis (e.g., in Bronchiectasis or TB), the bleeding usually originates from the high-pressure **bronchial arteries**, not the low-pressure pulmonary arteries. * **Nutritional Role:** It supplies the visceral pleura, large blood vessels, and the tracheobronchial tree.

Question 519: Which of the following does not cause hyperventilation?

A. Anxiety
B. High altitude
C. Pulmonary hypertension (Correct Answer)
D. Psychotic illness

Explanation: **Explanation:** The correct answer is **Pulmonary Hypertension**. Hyperventilation is defined as ventilation in excess of metabolic requirements, leading to a decrease in arterial $PCO_2$ (hypocapnia). **Why Pulmonary Hypertension is the correct answer:** Pulmonary hypertension (PH) involves increased resistance in the pulmonary arteries. While severe PH can eventually lead to ventilation-perfusion (V/Q) mismatch and compensatory tachypnea (increased respiratory rate), it does not inherently cause **hyperventilation** (the physiological state of blowing off excess $CO_2$). In many chronic cases, patients maintain normal or even slightly elevated $PCO_2$ due to increased physiological dead space and reduced gas exchange efficiency. It is a hemodynamic/vascular pathology rather than a primary stimulus for over-ventilation. **Analysis of Incorrect Options:** * **Anxiety & Psychotic illness:** These are classic causes of **Psychogenic Hyperventilation**. Emotional stress or psychiatric triggers stimulate the limbic system, which overrides the brainstem's respiratory centers, leading to rapid, deep breathing and subsequent respiratory alkalosis. * **High altitude:** At high altitudes, the low barometric pressure results in a decrease in the partial pressure of inspired oxygen ($PiO_2$). This causes **hypoxemia**, which stimulates peripheral chemoreceptors (carotid bodies), triggering a compensatory increase in ventilation (Hyperventilation) to improve oxygenation. **High-Yield Clinical Pearls for NEET-PG:** * **Hyperventilation vs. Tachypnea:** Tachypnea is simply a fast respiratory rate; Hyperventilation specifically implies a decrease in $PaCO_2$ (<35 mmHg). * **The "Haldane Effect":** In the lungs, oxygenation of hemoglobin promotes the dissociation of $CO_2$ from hemoglobin, facilitating its excretion during hyperventilation. * **Clinical Sign:** Acute hyperventilation can lead to hypocalcemia (due to increased protein binding of calcium in alkalosis), resulting in **carpopedal spasm** (Trousseau’s sign).

Question 520: What is the respiratory quotient on a mixed diet?

A. 0.1
B. 0.33
C. 0.85 (Correct Answer)
D. 0.91

Explanation: **Explanation:** The **Respiratory Quotient (RQ)** is the ratio of the volume of carbon dioxide ($CO_2$) produced to the volume of oxygen ($O_2$) consumed per unit of time ($RQ = CO_2 \text{ produced} / O_2 \text{ consumed}$). It reflects the type of fuel being metabolized by the body. **Why 0.85 is correct:** The RQ varies depending on the substrate being oxidized: * **Carbohydrates:** $RQ = 1.0$ (Equal $O_2$ consumed and $CO_2$ produced). * **Proteins:** $RQ \approx 0.8$. * **Fats:** $RQ \approx 0.7$ (Require more $O_2$ for oxidation relative to $CO_2$ produced). On a **mixed diet** (a typical combination of carbohydrates, proteins, and fats), the average RQ is approximately **0.82 to 0.85**. Therefore, option C is the most accurate representation of a standard physiological state. **Analysis of Incorrect Options:** * **A (0.1) & B (0.33):** These values are physiologically impossible for human metabolism. An RQ below 0.7 is rarely seen except in states of extreme starvation or specific metabolic disorders (like ketogenesis), but even then, it does not drop to 0.1 or 0.33. * **D (0.91):** This value is higher than the average mixed diet. It would indicate a diet very high in carbohydrates or a state of hyperventilation (where $CO_2$ is "blown off" excessively). **High-Yield Clinical Pearls for NEET-PG:** * **Respiratory Exchange Ratio (RER):** While RQ is measured at the cellular level, RER is measured at the mouth using expired gases. In steady-state, $RQ = RER$. * **Hyperventilation:** Increases RER (can go $>1.0$) because $CO_2$ is eliminated faster than it is produced. * **Metabolic Acidosis:** Also increases RER as buffering of $H^+$ by bicarbonate produces extra $CO_2$. * **Brain RQ:** The RQ of the brain is nearly **1.0** because it primarily utilizes glucose.

Practice by Chapter

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Pulmonary Ventilation

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Control of Breathing

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