Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 371: Hyperinflation of lungs is prevented by which reflex?

A. Hering-Breuer reflex (Correct Answer)
B. Irritation reflex
C. Cushing reflex
D. Bainbridge reflex

Explanation: **Explanation:** The **Hering-Breuer Inflation Reflex** is a protective mechanism designed to prevent the over-distension (hyperinflation) of the lungs. 1. **Mechanism:** When the lungs are inflated to a high tidal volume (typically >1.5 liters in adults), **stretch receptors** located in the muscular portions of the walls of the bronchi and bronchioles are activated. These receptors send inhibitory signals via the **Vagus nerve (CN X)** to the Dorsal Respiratory Group (DRG) in the medulla. This "switches off" the inspiratory ramp, stopping further inspiration and initiating expiration. **Analysis of Incorrect Options:** * **B. Irritation Reflex:** Triggered by receptors in the epithelium of the trachea and bronchi in response to noxious gases, dust, or smoke. It results in coughing, sneezing, or bronchoconstriction rather than regulating lung volume. * **C. Cushing Reflex:** A physiological nervous system response to increased intracranial pressure (ICP), characterized by the triad of hypertension, bradycardia, and irregular respiration. * **D. Bainbridge Reflex:** An atrial reflex where an increase in venous return (stretching the right atrium) leads to an increase in heart rate to prevent blood pooling in the venous system. **High-Yield Clinical Pearls for NEET-PG:** * **Receptors:** Slow-adapting stretch receptors. * **Afferent Pathway:** Vagus Nerve. * **Physiological Role:** In humans, this reflex is largely inactive during normal quiet breathing; it acts as a **protective mechanism** during heavy exercise or in neonates. * **Hering-Breuer Deflation Reflex:** A separate reflex where a sudden decrease in lung volume (atelectasis) triggers an increase in respiratory rate to prevent lung collapse.

Question 372: What is the total lung capacity?

A. 2.4 L
B. 3.6 L
C. 6 L (Correct Answer)
D. 10 L

Explanation: **Total Lung Capacity (TLC)** is the maximum volume of air that the lungs can hold after a maximal inspiratory effort. It represents the sum of all four primary lung volumes: **Tidal Volume (TV) + Inspiratory Reserve Volume (IRV) + Expiratory Reserve Volume (ERV) + Residual Volume (RV).** ### Why Option C is Correct: In a healthy adult male of average height and weight, the TLC is approximately **6,000 mL (6 Liters)**. It can also be calculated as the sum of **Vital Capacity (VC ~4.8 L)** and **Residual Volume (RV ~1.2 L)**. This value serves as a physiological baseline for assessing restrictive lung diseases, where TLC is characteristically decreased. ### Why Other Options are Incorrect: * **Option A (2.4 L):** This value roughly corresponds to the **Functional Residual Capacity (FRC)**, which is the volume of air remaining in the lungs after a normal tidal expiration (ERV + RV). * **Option B (3.6 L):** This is the approximate value for **Inspiratory Capacity (IC)**, which is the total amount of air one can breathe in starting from the resting expiratory level (TV + IRV). * **Option D (10 L):** This is physiologically impossible for a human; such high volumes are not seen even in extreme cases of hyperinflation (like severe emphysema). ### NEET-PG High-Yield Pearls: * **Measurement:** TLC cannot be measured by simple spirometry because it includes **Residual Volume**, which cannot be exhaled. It is measured using **Helium Dilution, Nitrogen Washout, or Body Plethysmography**. * **Gender Difference:** TLC is typically 20–25% lower in females (~4.2–4.7 L) due to smaller thoracic dimensions. * **Clinical Correlation:** TLC is **increased** in obstructive diseases (e.g., Emphysema due to hyperinflation) and **decreased** in restrictive diseases (e.g., Pulmonary Fibrosis, Kyphoscoliosis).

Question 373: Which of the following is NOT a respiratory cause of hypoxemia?

A. Hypoventilation
B. Physiological Shunt
C. Ventilation-Perfusion (V/Q) mismatch
D. Dead space ventilation (Correct Answer)

Explanation: **Explanation:** To understand this question, one must distinguish between **Hypoxemia** (low partial pressure of oxygen in arterial blood, $PaO_2$) and **Hypoxia** (low oxygen delivery to tissues). **Why Dead Space Ventilation is the correct answer:** Dead space refers to the volume of inspired air that does not participate in gas exchange (e.g., air in the trachea or non-perfused alveoli). While an increase in dead space decreases the **efficiency** of ventilation (wasting energy), it does not inherently cause hypoxemia as long as the remaining functional alveoli can maintain adequate gas exchange. In clinical practice, increased dead space primarily leads to **hypercapnia** (CO₂ retention) rather than isolated hypoxemia. **Analysis of Incorrect Options:** * **Hypoventilation:** Reduced alveolar ventilation leads to an increase in $PACO_2$, which displaces oxygen in the alveoli (Alveolar Gas Equation), directly lowering $PaO_2$. * **Physiological Shunt:** This occurs when blood bypasses ventilated alveoli (e.g., collapse or consolidation). Deoxygenated blood mixes with oxygenated blood, significantly lowering arterial $PaO_2$. * **V/Q Mismatch:** This is the **most common cause** of hypoxemia. When the ratio of ventilation to perfusion is imbalanced, the blood leaving the lungs is not fully oxygenated. **High-Yield NEET-PG Pearls:** 1. **A-a Gradient:** Hypoventilation and High Altitude are the only causes of hypoxemia with a **normal A-a gradient**. Shunt, V/Q mismatch, and Diffusion defects have an **increased A-a gradient**. 2. **Oxygen Response:** Hypoxemia caused by a **Shunt** is the only type that **does not correct** with 100% supplemental oxygen. 3. **V/Q Ratio:** In a standing position, both V and Q are highest at the **base** of the lung, but the V/Q ratio is highest at the **apex**.

Question 374: All of the following statements are true about the pneumotaxic center except?

A. The pneumotaxic center is located in the upper pons.
B. It decreases expiration and increases inspiration. (Correct Answer)
C. It controls the rate of respiration.
D. It helps in the smooth switch between inspiration and expiration.

Explanation: ### Explanation The **Pneumotaxic Center** is a key neural regulator located in the **nucleus parabrachialis** of the **upper pons**. Its primary function is to act as a "switch-off" mechanism for the inspiratory ramp. **1. Why Option B is the Correct Answer (The False Statement):** The pneumotaxic center actually **limits inspiration** and **shortens the inspiratory phase**. By switching off inspiration prematurely, it indirectly leads to a shorter respiratory cycle, which **increases the respiratory rate**. It does not increase inspiration; rather, it inhibits the apneustic center (which promotes inspiration). Therefore, the statement that it "increases inspiration" is physiologically incorrect. **2. Analysis of Other Options:** * **Option A:** Correct. It is anatomically situated in the dorsolateral nucleus of the upper (superior) pons. * **Option C:** Correct. By controlling the duration of the inspiratory ramp, it effectively determines the duration of each breath, thereby controlling the **respiratory rate**. A strong signal can increase the rate to 30–40 breaths/min, while a weak signal can reduce it to 3–5 breaths/min. * **Option D:** Correct. Its primary role is to fine-tune the transition between inspiration and expiration, ensuring a smooth, rhythmic breathing pattern. **Clinical Pearls for NEET-PG:** * **Apneustic Center:** Located in the **lower pons**. It stimulates the inspiratory neurons in the medulla. If the pneumotaxic center or vagus nerve is damaged, it leads to **Apneusis** (prolonged inspiratory gasps). * **Medullary Centers:** The **Dorsal Respiratory Group (DRG)** is primarily responsible for inspiration (rhythm generator), while the **Ventral Respiratory Group (VRG)** is active during forced expiration. * **Hering-Breuer Reflex:** This is a protective lung inflation reflex that also helps "switch off" inspiration, similar to the pneumotaxic center, but is triggered by stretch receptors in the bronchioles.

Question 375: All of the following statements concerning CO2 transport are TRUE, EXCEPT:

A. CO2 is rapidly converted to bicarbonate anions in plasma. (Correct Answer)
B. Chloride ions enter red blood cells in exchange for bicarbonate anions.
C. Compared to O2, dissolved CO2 plays a significant role in its transport.
D. The bulk of CO2 transport involves reversible combination of CO2 with water in red blood cells.

Explanation: **Explanation** The transport of carbon dioxide (CO₂) is a high-yield topic in respiratory physiology. To identify the incorrect statement, we must understand the kinetics of the **Carbonic Anhydrase (CA)** enzyme. **Why Option A is the Correct Answer (The False Statement):** While CO₂ does react with water to form bicarbonate ($HCO_3^-$), this reaction is **extremely slow in the plasma** because plasma lacks the enzyme Carbonic Anhydrase. The rapid conversion of CO₂ to bicarbonate occurs almost exclusively **inside the Red Blood Cells (RBCs)**, where CA is present in high concentrations. Therefore, saying it is "rapidly converted in plasma" is physiologically incorrect. **Analysis of Other Options:** * **Option B:** This describes the **Chloride Shift (Hamburger Phenomenon)**. As $HCO_3^-$ builds up inside the RBC, it diffuses out into the plasma. To maintain electrical neutrality, Chloride ($Cl^-$) ions move from the plasma into the RBC via the Anion Exchanger 1 (Band 3 protein). * **Option C:** CO₂ is approximately **20-24 times more soluble** than O₂. Consequently, dissolved CO₂ accounts for about 7% of total transport, whereas dissolved O₂ accounts for only 1.5-3%. * **Option D:** Approximately **70% of CO₂** is transported as bicarbonate formed within the RBCs, making it the primary (bulk) method of transport. **High-Yield Clinical Pearls for NEET-PG:** * **Haldane Effect:** Deoxygenation of blood increases its ability to carry CO₂ (occurs in tissues). * **Bohr Effect:** Increased CO₂/Acidity decreases hemoglobin's affinity for O₂ (shifts curve to the right). * **Carbaminohemoglobin:** About 23% of CO₂ binds to the *globin* chain (not heme) of hemoglobin. * **Enzyme Fact:** Carbonic Anhydrase contains **Zinc** as a cofactor.

Question 376: Cyanosis does not occur in severe anemia because why?

A. Hypoxia stimulates erythropoietin production
B. Oxygen carrying capacity of available Hemoglobin is increased
C. Critical concentration of Hemoglobin required to produce cyanosis is reduced (Correct Answer)
D. Oxygen Hemoglobin curve shifts to the right

Explanation: ### Explanation **1. Why the Correct Answer is Right:** Cyanosis is a clinical sign characterized by the bluish discoloration of the skin and mucous membranes. It is not determined by the total amount of hemoglobin (Hb), but by the **absolute concentration of deoxygenated (reduced) hemoglobin** in the capillaries. For cyanosis to become clinically visible, there must be at least **5 g/dL of reduced hemoglobin** in the capillary blood. In cases of severe anemia (e.g., Hb < 5 g/dL), even if all the hemoglobin is deoxygenated, the total amount cannot reach the critical threshold of 5 g/dL required to manifest the blue color. Therefore, anemic patients may be severely hypoxic but will appear pale rather than cyanotic. **2. Why the Other Options are Incorrect:** * **Option A:** While hypoxia does stimulate erythropoietin, this is a compensatory mechanism to increase red cell mass over time; it does not explain the immediate absence of cyanosis in anemic states. * **Option B:** The oxygen-carrying capacity per gram of hemoglobin remains constant (1.34 ml O₂/g). Anemia reduces the *total* oxygen content of the blood, not the capacity of the individual hemoglobin molecules. * **Option D:** A right shift in the Oxy-Hb curve (due to increased 2,3-BPG in anemia) facilitates oxygen unloading to tissues. While this helps combat hypoxia, it actually increases the amount of reduced hemoglobin, which would theoretically *promote* cyanosis if enough Hb were present. **3. High-Yield Clinical Pearls for NEET-PG:** * **Polycythemia:** Patients with polycythemia can show cyanosis even with mild hypoxia because they easily reach the 5 g/dL threshold of reduced Hb. * **Methemoglobinemia:** Cyanosis appears at much lower levels (**1.5 g/dL**) of methemoglobin. * **Central vs. Peripheral:** Central cyanosis (tongue/lips) indicates systemic arterial desaturation, while peripheral cyanosis (fingertips) often indicates reduced local blood flow (vasoconstriction).

Question 377: Total lung capacity is dependent upon which of the following factors?

A. Size of the airway
B. Closing volume
C. Lung compliance (Correct Answer)
D. Residual volume

Explanation: **Explanation:** **Total Lung Capacity (TLC)** is the maximum volume of air the lungs can hold after a maximal inspiratory effort. It is primarily determined by the balance between the outward pull of the chest wall and the inward elastic recoil of the lungs. **Why Lung Compliance is correct:** Compliance refers to the "distensibility" or the ease with which the lungs expand. TLC is directly dependent on the **compliance of the lungs and the chest wall**, as well as the strength of the inspiratory muscles. In restrictive lung diseases (e.g., pulmonary fibrosis), lung compliance decreases, making the lungs "stiff" and significantly reducing TLC. Conversely, in emphysema, compliance increases due to loss of elastic recoil, leading to an increased TLC (hyperinflation). **Why other options are incorrect:** * **Size of the airway:** This primarily affects airway resistance and flow rates (e.g., FEV1), not the total volume capacity of the lungs. * **Closing volume:** This is the volume at which small airways in the dependent parts of the lung begin to close during expiration. It relates to small airway patency, not the total expansion limit. * **Residual volume (RV):** While RV is a *component* of TLC (TLC = VC + RV), the total capacity itself is governed by the physical limits of expansion (compliance and muscle strength), rather than being "dependent" on the air left after maximal expiration. **High-Yield Clinical Pearls for NEET-PG:** * **TLC Formula:** TLC = Inspiratory Reserve Volume (IRV) + Tidal Volume (TV) + Expiratory Reserve Volume (ERV) + Residual Volume (RV). * **Restrictive Pattern:** Characterized by a **decrease in all lung volumes**, especially TLC. * **Obstructive Pattern:** TLC is often **normal or increased** (due to air trapping), but the FEV1/FVC ratio is decreased. * **Helium Dilution & Body Plethysmography:** These are the gold standard methods to measure TLC, as spirometry cannot measure RV.

Question 378: Depth of inspiration is increased by which of the following?

A. Pneumotaxic centre
B. Ventral group of neurons in medulla
C. Dorsal group of respiratory neurons
D. Apneustic centre (Correct Answer)

Explanation: The regulation of respiration is controlled by specific neural clusters in the brainstem. Understanding the interplay between these centers is crucial for NEET-PG. **Why the Apneustic Centre is correct:** Located in the lower pons, the **Apneustic centre** functions as the "gas pedal" for inspiration. It sends stimulatory signals to the Dorsal Respiratory Group (DRG) in the medulla, delaying the "off-switch" signal of the inspiratory ramp. This results in prolonged, deep inspiratory gasps, thereby **increasing the depth of inspiration**. Under normal physiological conditions, this center is inhibited by the pneumotaxic center and the vagus nerve. **Why the other options are incorrect:** * **Pneumotaxic Centre:** Located in the upper pons (nucleus parabrachialis), it acts as the "limit setter." It inhibits inspiration by switching off the inspiratory ramp, thereby **decreasing the depth** of inspiration and increasing the respiratory rate. * **Dorsal Respiratory Group (DRG):** Located in the medulla, these neurons are primarily responsible for the **basic rhythm** of respiration (normal quiet breathing). While they initiate inspiration, they do not independently increase its depth without apneustic influence. * **Ventral Respiratory Group (VRG):** These neurons remain inactive during quiet breathing. They are primarily involved in **forced expiration** and increased pulmonary ventilation during exercise. **High-Yield Clinical Pearls for NEET-PG:** * **Apneustic Breathing:** Characterized by prolonged inspiratory gasps followed by brief expiration; it occurs clinically due to lesions in the upper pons (removing pneumotaxic inhibition). * **Hering-Breuer Reflex:** Inflation of the lungs triggers pulmonary stretch receptors which inhibit the apneustic center via the Vagus nerve, preventing over-inflation. * **Location Summary:** Pons = Pneumotaxic (Upper) & Apneustic (Lower); Medulla = DRG (Dorsal) & VRG (Ventral).

Question 379: What is the typical total alveolar ventilation volume in L/min?

A. 1.5
B. 3.5
C. 4.2 (Correct Answer)
D. 5

Explanation: **Explanation:** **Alveolar Ventilation ($V_A$)** is the volume of fresh air that reaches the gas-exchange areas of the lungs (alveoli, alveolar sacs, and respiratory bronchioles) per minute. It is a more accurate measure of gas exchange than Minute Ventilation because it accounts for **Anatomic Dead Space**. The formula for Alveolar Ventilation is: $$V_A = (\text{Tidal Volume} - \text{Dead Space}) \times \text{Respiratory Rate}$$ Using standard physiological values for a healthy adult: * **Tidal Volume ($V_T$):** 500 mL * **Anatomic Dead Space ($V_D$):** 150 mL (approx. 2 mL/kg) * **Respiratory Rate (RR):** 12 breaths/min * **Calculation:** $(500 - 150) \times 12 = 350 \times 12 = \mathbf{4,200\text{ mL/min or } 4.2\text{ L/min}}.$ **Analysis of Options:** * **Option A (1.5):** This is too low and would represent severe hypoventilation or respiratory failure. * **Option B (3.5):** This value might be seen in individuals with a smaller body habitus or lower respiratory rate, but it is not the "typical" standard. * **Option D (5.0):** This value (approx. 6 L/min) represents the **Total Minute Ventilation** ($V_T \times RR = 500 \times 12$), which fails to subtract the dead space. **High-Yield Clinical Pearls for NEET-PG:** * **Dead Space:** The volume of the conducting airways where no gas exchange occurs. * **Alveolar vs. Minute Ventilation:** If a patient breathes rapidly and shallowly (e.g., $V_T$ 200 mL, RR 30), the Minute Ventilation remains 6 L/min, but Alveolar Ventilation drops significantly $[(200-150) \times 30 = 1.5\text{ L/min}]$, leading to hypercapnia. * **V/Q Ratio:** In a healthy lung, the ideal Ventilation-Perfusion ratio is approximately **0.8** (4.2 L/min ventilation / 5.0 L/min cardiac output).

Question 380: Which law is applicable for surfactant action?

A. Laplace law (Correct Answer)
B. Charlie's law
C. Thames law
D. Charles law

Explanation: ### Explanation **1. Why Laplace Law is Correct:** The **Law of Laplace** describes the relationship between pressure, surface tension, and the radius of a sphere (like an alveolus). The formula is **$P = 2T / r$** (where $P$ = distending pressure, $T$ = surface tension, and $r$ = radius). According to this law, smaller alveoli have a smaller radius, which would normally result in a higher collapsing pressure, causing them to empty into larger alveoli (atelectasis). **Surfactant** (secreted by Type II pneumocytes) reduces surface tension ($T$) more effectively in smaller alveoli than in larger ones. By decreasing $T$ as $r$ decreases, surfactant equalizes pressure across different-sized alveoli, preventing alveolar collapse and increasing lung compliance. **2. Why Other Options are Incorrect:** * **Charles’s Law:** A gas law stating that the volume of a gas is directly proportional to its absolute temperature ($V \propto T$) at constant pressure. It is not related to surface tension. * **Charlie’s Law / Thames Law:** These are not recognized physical or physiological laws relevant to the respiratory system; they are likely "distractor" options. **3. Clinical Pearls for NEET-PG:** * **Composition:** Surfactant is 90% lipids and 10% proteins. The most important component is **Dipalmitoylphosphatidylcholine (DPPC)** or Lecithin. * **Clinical Correlation:** Deficiency of surfactant in premature infants leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease. * **L/S Ratio:** A Lecithin/Sphingomyelin ratio of **>2:1** in amniotic fluid indicates fetal lung maturity. * **Glucocorticoids:** These are administered to mothers in preterm labor to accelerate surfactant production in the fetus.

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