Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 191: Which of the following conditions increases the diffusion capacity of the lungs?

A. Congestive heart failure (Correct Answer)
B. Pulmonary embolism
C. Chronic lung disease
D. Anemia

Explanation: **Explanation:** The **Diffusing Capacity of the Lung (DLCO)** measures the ability of the lungs to transfer gas from inhaled air to the red blood cells in pulmonary capillaries. It is determined by the surface area of the blood-gas barrier, the thickness of the membrane, and the volume of blood in the pulmonary capillaries. **Why Option A is correct:** In **Congestive Heart Failure (CHF)**, specifically early-stage or left-sided heart failure, there is a backup of blood into the pulmonary circulation. This leads to **pulmonary capillary engorgement** (increased pulmonary capillary blood volume). Since more red blood cells are available in the capillaries to bind with gas molecules, the diffusing capacity initially increases. **Why the other options are incorrect:** * **B. Pulmonary Embolism:** This causes an obstruction in the pulmonary arteries, decreasing the blood flow to the alveoli (increasing dead space). Less blood in the capillaries leads to a **decrease** in DLCO. * **C. Chronic Lung Disease:** Conditions like emphysema decrease the surface area for exchange, while interstitial lung diseases (fibrosis) increase the thickness of the blood-gas barrier. Both result in a **decrease** in DLCO. * **D. Anemia:** DLCO is highly dependent on hemoglobin levels. In anemia, there is a reduced amount of hemoglobin available to bind to the test gas (Carbon Monoxide), leading to a **decrease** in measured DLCO. **High-Yield Clinical Pearls for NEET-PG:** * **DLCO Increases in:** Polycythemia, Exercise, Supine position (due to increased venous return), and Left-to-Right shunts. * **DLCO Decreases in:** Emphysema (only COPD type with low DLCO), Anemia, Pulmonary Fibrosis, and Pulmonary Embolism. * **Gold Standard Gas:** Carbon Monoxide (CO) is used to measure diffusion because it is diffusion-limited and has a very high affinity for hemoglobin.

Question 192: What is the respiratory quotient of the brain?

A. 0.6-0.7
B. 0.7-0.8
C. 0.8-0.9
D. 0.9-1.0 (Correct Answer)

Explanation: **Explanation** The Respiratory Quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed ($RQ = CO_2 \text{ produced} / O_2 \text{ consumed}$). It reflects the type of fuel being metabolized by a specific organ or the body as a whole. **Why Option D is Correct:** The brain primarily utilizes **glucose** as its sole energy source under normal physiological conditions. The stoichiometric oxidation of carbohydrates (glucose) follows the equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$. Since the amount of $CO_2$ produced equals the $O_2$ consumed, the **RQ for carbohydrates is 1.0**. In vivo, the brain's RQ is measured between **0.97 and 0.99**, making **0.9–1.0** the most accurate range. **Analysis of Incorrect Options:** * **Option A (0.6–0.7):** This range is characteristic of **pure lipid (fat) metabolism**. Fats are oxygen-poor molecules, requiring more external oxygen for oxidation. * **Option B (0.7–0.8):** This range typically represents **protein metabolism** (approx. 0.8) or a state of prolonged starvation where the body shifts to ketone bodies and fats. * **Option C (0.8–0.9):** This represents a **mixed diet** (average human RQ is ~0.82). While the whole body operates in this range, the brain remains specifically dependent on glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Starvation Exception:** During prolonged starvation, the brain adapts to use **ketone bodies** (acetoacetate and $\beta$-hydroxybutyrate), which would slightly lower the brain's RQ. * **RQ Values to Remember:** * Carbohydrates: 1.0 * Proteins: 0.8 * Lipids: 0.7 * Mixed Diet: 0.82 * **Overfeeding/Lipogenesis:** An RQ **> 1.0** suggests net fat synthesis (lipogenesis), often seen in patients being overfed via TPN (Total Parenteral Nutrition).

Question 193: Which of the following is true about lung compliance?

A. It is the ratio of change in volume to change in pressure.
B. It has distinct inspiratory and expiratory components.
C. Hysteresis is a characteristic of lung compliance.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** **Lung compliance** is a measure of the lungs' ability to stretch and expand. It is defined as the change in lung volume per unit change in transpulmonary pressure ($C = \Delta V / \Delta P$). This makes **Option A** correct. The relationship between pressure and volume is not linear and differs during inflation and deflation. This phenomenon is known as **Hysteresis** (**Option C**). Hysteresis occurs primarily due to the presence of **surfactant**, which reduces surface tension more effectively during deflation than inflation, and the recruitment/derecruitment of alveoli. Because of this, the pressure-volume curve has **distinct inspiratory and expiratory components** (**Option B**), where the expiratory limb shows higher compliance (greater volume for a given pressure) than the inspiratory limb. Since all statements are accurate, **Option D** is the correct answer. **Why other options are considered part of the correct whole:** * **Option A:** Defines the mathematical property of compliance. * **Option B & C:** Describe the physiological behavior of the lung tissue and surface tension forces during a respiratory cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Increased Compliance:** Seen in **Emphysema** (due to loss of elastic fibers) and with aging. * **Decreased Compliance:** Seen in **Pulmonary Fibrosis**, Pulmonary Edema, and Deficiency of Surfactant (NRDS). * **Total Compliance:** The respiratory system's compliance depends on both the **lung** and the **chest wall**. They are arranged in series, so: $1/C_{total} = 1/C_{lung} + 1/C_{chest\ wall}$. * **Specific Compliance:** Compliance divided by Functional Residual Capacity (FRC); it is used to compare lungs of different sizes.

Question 194: Alveolar-arterial oxygen gradient is increased in all except?

A. Diffusion defects
B. Right to left shunt
C. Chronic bronchitis (Correct Answer)
D. Interstitial lung disease

Explanation: The **Alveolar-arterial (A-a) gradient** is a measure of the difference between the oxygen concentration in the alveoli and the arterial blood. It is a crucial tool for differentiating causes of hypoxemia. ### **Why Chronic Bronchitis is the Correct Answer** In **Chronic Bronchitis** (a type of COPD), the primary mechanism of hypoxemia is **Alveolar Hypoventilation**. When ventilation decreases, the partial pressure of oxygen in the alveoli ($PAO_2$) drops, and the arterial oxygen ($PaO_2$) drops proportionally. Because both values decrease together, the **A-a gradient remains normal**. *Note: Other conditions with a normal A-a gradient include high altitude and opioid overdose.* ### **Analysis of Incorrect Options (Increased A-a Gradient)** * **Diffusion Defects (A) & Interstitial Lung Disease (D):** In ILD, the alveolar-capillary membrane is thickened. Oxygen cannot easily cross into the blood even if the alveoli are well-ventilated, leading to a wide gap between $PAO_2$ and $PaO_2$. * **Right to Left Shunt (B):** Deoxygenated blood bypasses ventilated alveoli and mixes with oxygenated blood. This significantly lowers $PaO_2$ while $PAO_2$ remains normal, increasing the gradient. ### **High-Yield NEET-PG Pearls** 1. **Formula:** $A-a\text{ Gradient} = PAO_2 - PaO_2$. * Normal value: $< 15\text{ mmHg}$ (increases with age: $(\text{Age}/4) + 4$). 2. **Normal A-a Gradient Hypoxemia:** Only two causes—**Hypoventilation** and **Low $FiO_2$** (High altitude). 3. **Increased A-a Gradient Hypoxemia:** V/Q mismatch (most common), Shunt, and Diffusion limitation. 4. **Clinical Tip:** If hypoxemia does **not** correct with 100% oxygen, think **Shunt**. If it does correct, think V/Q mismatch.

Question 195: Which of the following does NOT affect the chemical regulation of respiration?

A. pH
B. pCO2
C. pO2
D. Blood Pressure (Correct Answer)

Explanation: ### Explanation The chemical regulation of respiration is primarily governed by **chemoreceptors** that monitor the chemical composition of arterial blood and cerebrospinal fluid (CSF). **Why Blood Pressure is the Correct Answer:** Blood pressure is a **hemodynamic parameter**, not a chemical one. While significant changes in blood pressure can influence respiration via the **baroreceptor reflex** (e.g., severe hypotension can stimulate hyperpnea), it is categorized under **neural/reflex regulation**, not chemical regulation. Chemical regulation specifically refers to the feedback loop involving pH, $pCO_2$, and $pO_2$. **Analysis of Incorrect Options:** * **$pCO_2$ (Option B):** This is the **most potent** chemical stimulus for respiration. Carbon dioxide diffuses across the blood-brain barrier, decreasing the pH of the CSF, which stimulates **central chemoreceptors** in the medulla. * **pH (Option A):** A decrease in arterial pH (acidosis) stimulates **peripheral chemoreceptors** (carotid and aortic bodies). Central chemoreceptors also respond to pH changes in the brain interstitial fluid. * **$pO_2$ (Option C):** Peripheral chemoreceptors are sensitive to a fall in arterial $pO_2$ (hypoxia). However, this only becomes a significant respiratory drive when $pO_2$ drops below **60 mmHg** (the "hypoxic drive"). **High-Yield Clinical Pearls for NEET-PG:** * **Central Chemoreceptors:** Located in the ventral medulla; sensitive to **$H^+$ ions** (derived from $CO_2$), but **not** sensitive to arterial $pO_2$. * **Peripheral Chemoreceptors:** Located in carotid and aortic bodies; sensitive to **low $pO_2$**, **high $pCO_2$**, and **low pH**. * **Breaking Point:** During breath-holding, the urge to breathe is triggered primarily by rising $pCO_2$ (hypercapnia), not lack of oxygen. * **Hering-Breuer Reflex:** A neural (not chemical) reflex that prevents over-inflation of the lungs via stretch receptors.

Question 196: Which of the following best characterizes alveolar ventilation and blood flow at the base, compared with the apex, of the lungs of a healthy standing person?

A. Ventilation - Higher, Blood flow - Higher, V/Q ratio - Lower (Correct Answer)
B. Ventilation - Lower, Blood flow - Higher, V/Q ratio - Higher
C. Ventilation - Lower, Blood flow - Lower, V/Q ratio - Lower
D. Ventilation - Higher, Blood flow - Lower, V/Q ratio - Higher

Explanation: ### Explanation In a healthy standing person, gravity exerts a significant influence on both air and blood distribution within the lungs. **1. Why the Correct Answer (A) is Right:** * **Blood Flow (Q):** Due to gravity, hydrostatic pressure is much higher at the base than the apex. This keeps capillaries open and recruited, leading to significantly higher perfusion at the base. * **Ventilation (V):** At the base, the lung tissue is compressed by its own weight, making the alveoli smaller and more compliant (easier to expand). At the apex, alveoli are already stretched open by high negative intrapleural pressure, making them less compliant. Therefore, during inspiration, more air enters the basal alveoli. * **V/Q Ratio:** While both V and Q increase from apex to base, **blood flow increases much more steeply than ventilation**. Consequently, the ratio (V divided by Q) is lowest at the base (~0.6) and highest at the apex (~3.0). **2. Why Other Options are Wrong:** * **Option B & C:** These are incorrect because ventilation is actually **higher** at the base due to increased compliance of the basal alveoli compared to the over-distended apical alveoli. * **Option D:** This is incorrect because blood flow is **highest** at the base due to gravity. A "Higher V/Q ratio" is a characteristic of the apex, where blood flow is disproportionately low compared to ventilation. **3. NEET-PG High-Yield Pearls:** * **West Zones:** The lung is divided into Zone 1 (Apex: $P_A > P_a > P_v$), Zone 2 (Middle), and Zone 3 (Base: $P_a > P_v > P_A$). * **Gas Exchange:** Because the V/Q ratio is lower at the base, the $P_{O2}$ is lower and $P_{CO2}$ is higher at the base compared to the apex. * **Clinical Correlation:** *Mycobacterium tuberculosis* prefers the **apex** because the high V/Q ratio there results in a higher local $P_{O2}$, favoring the growth of this aerobe.

Question 197: Peripheral and central chemoreceptors respond to changes in which of the following?

A. Increased arterial pH
B. Increased arterial Oxygen
C. Increased arterial CO2 (Correct Answer)
D. Decreased arterial CO2

Explanation: **Explanation:** The primary stimulus for both central and peripheral chemoreceptors is an increase in arterial $PCO_2$ (Hypercapnia). 1. **Central Chemoreceptors:** Located on the ventral surface of the medulla, these are the most sensitive regulators of ventilation. While they do not respond to $H^+$ ions in the blood (as $H^+$ cannot cross the blood-brain barrier), $CO_2$ diffuses readily into the cerebrospinal fluid (CSF). There, it reacts with water to form carbonic acid, which dissociates into $H^+$ and $HCO_3^-$. The resulting **increase in $H^+$ concentration in the CSF** directly stimulates the central chemoreceptors. 2. **Peripheral Chemoreceptors:** Located in the carotid and aortic bodies, these respond primarily to **decreased $PO_2$ (Hypoxia)**, but they are also stimulated by **increased $PCO_2$** and **decreased pH**. **Analysis of Incorrect Options:** * **A & D:** Increased pH (alkalosis) and decreased $CO_2$ (hypocapnia) act as inhibitors of respiration, reducing the firing rate of chemoreceptors to prevent over-ventilation. * **B:** Increased arterial oxygen (hyperoxia) actually suppresses peripheral chemoreceptor activity. **NEET-PG High-Yield Pearls:** * **Main Drive:** Under normal physiological conditions, the **central chemoreceptors** provide the main drive for ventilation (approx. 70-80%) via $CO_2$ changes. * **Hypoxic Drive:** Peripheral chemoreceptors only become the dominant driver when $PO_2$ falls below **60 mmHg**. * **COPD Clinical Note:** In chronic hypercapnia (e.g., COPD), central receptors become desensitized, and the body relies on the "hypoxic drive" from peripheral receptors. Administering high-flow oxygen can suppress this drive, leading to respiratory arrest.

Question 198: What is surfactant?

A. Protein
B. Glycoprotein
C. Sphingolipid
D. Alpha palmitoyl lecithin (Correct Answer)

Explanation: **Explanation:** **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 the correct answer is right:** The major constituent of surfactant (comprising about 90% lipids) is phospholipids. The most abundant and physiologically active component is **Dipalmitoylphosphatidylcholine (DPPC)**, also known as **Dipalmitoyl lecithin**. Chemically, this is an **alpha-palmitoyl lecithin**. It is this specific phospholipid that is responsible for the significant reduction in surface tension. **Why the incorrect options are wrong:** * **Protein:** While surfactant contains specific proteins (SP-A, B, C, and D), they only make up about 10% of its composition. They are crucial for immunity and spreading but are not the primary chemical definition of surfactant. * **Glycoprotein:** Surfactant is a lipoprotein, not a glycoprotein. * **Sphingolipid:** Sphingomyelin is found in the amniotic fluid, but it is not the active component of surfactant. The **Lecithin/Sphingomyelin (L/S) ratio** is used to assess fetal lung maturity; a ratio >2 indicates mature lungs. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** Secreted by Type II alveolar epithelial cells (contain **Lamellar bodies**). * **Key Stimulus:** Hyperinflation of lungs (deep breathing/sighing) triggers surfactant release. * **Clinical Correlation:** Deficiency leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease, common in premature infants. * **Glucocorticoids:** These are administered to mothers in preterm labor to accelerate surfactant synthesis in the fetus.

Question 199: What is the maximum air volume in the lungs?

A. 1200 ml
B. 2400 ml
C. 3000 ml
D. 5900 ml (Correct Answer)

Explanation: **Explanation:** The question asks for the maximum air volume the lungs can hold, which is physiologically defined as the **Total Lung Capacity (TLC)**. TLC is the volume of air in the lungs after a maximal inspiratory effort. It is the sum of all lung volumes: **TLC = VC + RV** (Vital Capacity + Residual Volume) or **TLC = IRV + TV + ERV + RV**. In a healthy adult male, this value is approximately **5800 ml to 6000 ml** (average 5900 ml). **Analysis of Options:** * **Option A (1200 ml):** This represents the **Residual Volume (RV)**—the volume of air remaining in the lungs after a forceful expiration. It cannot be measured by simple spirometry. * **Option B (2400 ml):** This corresponds to the **Functional Residual Capacity (FRC)**—the volume of air remaining in the lungs after a normal tidal expiration (ERV + RV). * **Option C (3000 ml):** This is approximately the **Inspiratory Reserve Volume (IRV)**, which is the extra volume of air that can be inspired over and above the normal tidal volume. * **Option D (5900 ml):** This is the correct value for **Total Lung Capacity (TLC)**, representing the maximum capacity of the respiratory system. **High-Yield NEET-PG Pearls:** 1. **Spirometry Limitations:** RV, FRC, and TLC **cannot** be measured by simple spirometry because they include the Residual Volume. They are measured using Helium Dilution, Nitrogen Washout, or Body Plethysmography. 2. **Clinical Correlation:** TLC is **decreased in Restrictive Lung Diseases** (e.g., Pulmonary Fibrosis) and can be **increased (hyperinflation)** in Obstructive Lung Diseases like Emphysema. 3. **Vital Capacity (VC):** Usually around 4600 ml; it is the maximum amount of air a person can expel from the lungs after first filling the lungs to their maximum.

Question 200: Which of the following is true about Head's paradoxical reflex?

A. This reflex causes inspiration to be inhibited when lung inflation exceeds a certain threshold.
B. This reflex causes inspiration to be inhibited when lung inflation exceeds a certain threshold. (Correct Answer)
C. This reflex causes inspiration to be inhibited when lung inflation exceeds a certain threshold.
D. This reflex causes inspiration to be inhibited when lung inflation exceeds a certain threshold.

Explanation: ### Explanation: Head’s Paradoxical Reflex **Understanding the Concept:** Head’s Paradoxical Reflex is a physiological response where **lung inflation triggers further inspiratory effort** rather than inhibiting it. This is "paradoxical" because it opposes the classic Hering-Breuer Inflation Reflex, which terminates inspiration to prevent over-distension. The reflex is mediated by **rapidly adapting receptors (RARs)** in the lungs, with impulses traveling via the **vagus nerve**. When the lungs are suddenly inflated, these receptors stimulate the inspiratory center to produce a deeper breath (a gasp). **Analysis of Options:** * **Correct Answer:** While the provided options in the prompt appear identical, the true definition of Head’s Paradoxical Reflex is that **inflation leads to further inspiration**. If the correct answer choice in a standard exam states "inspiration is inhibited," it is actually describing the **Hering-Breuer Reflex**. * *Note:* In a standard NEET-PG format, the correct description for Head's reflex would be: **"Inflation of the lungs produces a further increase in inspiratory effort."** **Why it is "Paradoxical":** 1. **Hering-Breuer Reflex:** Inflation → Inhibition of inspiration (Protective). 2. **Head’s Reflex:** Inflation → Stimulation of inspiration (Augmenting). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **First Breath of Newborn:** Head’s reflex is believed to play a crucial role in the **initial expansion of the lungs at birth**, helping the neonate take deep gasps to overcome high surface tension. * **Sighing/Yawning:** In adults, this reflex is thought to be responsible for periodic "sighs," which help prevent atelectasis (alveolar collapse) by intermittently over-inflating the lungs. * **Receptors:** It is mediated by **Rapidly Adapting Receptors (Irritant receptors)**, whereas the Hering-Breuer reflex is mediated by **Slowly Adapting Receptors (Stretch receptors)** in the smooth muscles of the airways. * **Vagus Nerve:** Both reflexes are abolished if the vagus nerves are cooled or transected.

Practice by Chapter

Mechanics of Breathing

Practice Questions

Pulmonary Ventilation

Practice Questions

Pulmonary Circulation

Practice Questions

Gas Exchange in the Lungs

Practice Questions

Oxygen and Carbon Dioxide Transport

Practice Questions

Control of Breathing

Practice Questions

Respiratory Adjustments in Health and Disease

Practice Questions

High Altitude Physiology

Practice Questions

Diving Physiology

Practice Questions

Respiratory Function Tests

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free