Respiratory System — MCQs

A 54-year-old man presents with dyspnea and a cough. He is a non-smoker with no relevant occupational exposures. His pulmonary function test results are as follows: Pre-Bronchodilator (BD) Post-BD Test Actual Predicted % Predicted Actual % Change FVC (L) 3.19 4.22 76 4.00 25 FEV1 (L) 2.18 3.39 64 2.83 30 FEV1/FVC (%) 68 80 714. What is the most probable diagnosis based on these results?

Q206Easy

Which of the following is inactivated in the lung?

Q207Easy

The Hering Breuer reflex is mediated by which of the following receptors?

Q208Easy

What is the term for the increased oxygen delivery to tissues in response to increased CO2?

Q209Easy

The intrapleural pressure is negative both during inspiration and expiration because:

Q210Easy

Extrapulmonary restrictive defects lead to the development of which acid-base disturbance?

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 201: What is the normal alveolar pressure during inspiration?

A. -1 cm water (Correct Answer)
B. -1 cm mercury
C. +1 cm water
D. +1 cm mercury

Explanation: **Explanation:** The movement of air into and out of the lungs is governed by pressure gradients between the atmosphere and the alveoli. **1. Why Option A is Correct:** According to Boyle’s Law, as the volume of the thoracic cavity increases (due to the contraction of the diaphragm and external intercostal muscles), the pressure within the alveoli (**alveolar pressure**) decreases. To facilitate the inflow of air, alveolar pressure must become slightly lower than atmospheric pressure (which is 0 cm H₂O). During normal, quiet inspiration, alveolar pressure drops to approximately **-1 cm H₂O**. This small negative pressure gradient is sufficient to pull about 500 mL of air (Tidal Volume) into the lungs. **2. Why the Other Options are Incorrect:** * **Option B & D:** Mercury (Hg) is much denser than water. A pressure of -1 cm Hg would be roughly -13.6 cm H₂O, which is an excessively high gradient for quiet breathing. Respiratory pressures are typically measured in **cm H₂O**, while cardiovascular pressures use mm Hg. * **Option C:** A pressure of **+1 cm H₂O** occurs during **expiration**. As the thoracic cavity volume decreases, alveolar pressure becomes positive relative to the atmosphere, forcing air out of the lungs. **3. High-Yield Clinical Pearls for NEET-PG:** * **At the end of inspiration/expiration:** Alveolar pressure equals atmospheric pressure (**0 cm H₂O**), and airflow ceases. * **Intrapleural Pressure:** Always remains **negative** during quiet breathing (approx. -5 cm H₂O at start and -7.5 cm H₂O at the end of inspiration) to keep the lungs inflated. * **Transpulmonary Pressure:** The difference between alveolar and intrapleural pressure ($P_{alv} - P_{ip}$). It is a measure of the elastic forces of the lungs. * **Surfactant:** Prevents alveolar collapse at the end of expiration by reducing surface tension.

Question 202: What will be hemoglobin saturation if P02 is 60 mm Hg at pH 7.4 and temperature 37°C?

A. 50%
B. 60%
C. 75%
D. 90% (Correct Answer)

Explanation: The correct answer is **90%**. This question tests your understanding of the **Oxyhemoglobin Dissociation Curve (ODC)**, which describes the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin ($SaO_2$). ### 1. Why 90% is Correct The ODC is sigmoid-shaped due to the "cooperative binding" of hemoglobin. There are three high-yield "anchor points" on the curve that every NEET-PG aspirant must memorize: * **$P_{50}$:** At a $PO_2$ of **27 mmHg**, hemoglobin is **50%** saturated. * **Venous Point:** At a $PO_2$ of **40 mmHg**, hemoglobin is **75%** saturated. * **The "Shoulder" Point:** At a $PO_2$ of **60 mmHg**, hemoglobin is **90%** saturated. At $PO_2$ 60 mmHg, the curve begins to flatten. This is clinically significant because it means that even if $PO_2$ drops from 100 to 60 mmHg, oxygen saturation remains relatively high (90%), providing a safety buffer for oxygen delivery. ### 2. Why Other Options are Incorrect * **50%:** This occurs at a $PO_2$ of 27 mmHg (the $P_{50}$ value). * **60%:** This is a distractor; students often confuse $PO_2$ values with saturation percentages. * **75%:** This corresponds to a $PO_2$ of 40 mmHg, which is typical for mixed venous blood. ### 3. Clinical Pearls for NEET-PG * **Right Shift (Decreased Affinity):** Caused by "CADET, face Right!" (**C**O2 increase, **A**cidosis/Low pH, **D**PG (2,3-BPG) increase, **E**xercise, **T**emperature increase). * **Left Shift (Increased Affinity):** Caused by Alkalosis, Hypothermia, decreased 2,3-BPG, and Fetal Hemoglobin (HbF). * **Critical Threshold:** A $PO_2$ below 60 mmHg marks the beginning of the "steep" portion of the curve, where small further drops in $PO_2$ lead to drastic desaturation and potential tissue hypoxia.

Question 203: Which of the following statements regarding the pneumotaxic center is true?

A. It contains both inspiratory (I) and expiratory (E) neurons.
B. It may play a role in the fine-tuning of breathing.
C. Both of the above. (Correct Answer)
D. None of the above.

Explanation: The respiratory center in the brainstem is divided into the medullary and pontine centers. The **Pneumotaxic Center**, located in the upper pons (specifically the nucleus parabrachialis and Kolliker-Fuse nucleus), plays a critical role in regulating the respiratory pattern. ### **Explanation of the Correct Answer** **Option C (Both A and B) is correct because:** 1. **Neuronal Composition:** Unlike the Dorsal Respiratory Group (DRG), which is primarily inspiratory, the pneumotaxic center contains a heterogeneous population of **both inspiratory (I) and expiratory (E) neurons**. These neurons fire at different phases of the respiratory cycle to coordinate the transition between inspiration and expiration. 2. **Fine-tuning Function:** Its primary physiological role is to act as an **"off-switch" for inspiration**. By limiting the duration of inspiration, it indirectly controls the tidal volume and respiratory rate. This "fine-tuning" ensures a smooth, rhythmic transition, preventing gasping or irregular breathing patterns. ### **Analysis of Options** * **Option A:** True. Electrophysiological studies confirm the presence of both I and E neurons in the pontine nuclei. * **Option B:** True. It modulates the medullary rhythm generators to adapt to various physiological needs, effectively fine-tuning the breath. ### **High-Yield Facts for NEET-PG** * **Location:** Upper Pons. * **Effect of Stimulation:** Strong stimulation of the pneumotaxic center **increases respiratory rate** (by shortening inspiration) and **decreases tidal volume**. * **Lesion Effect:** A lesion in the pneumotaxic center (especially if the Vagus nerve is also cut) leads to **Apneusis**—prolonged inspiratory gasps with brief expiratory movements. * **Antagonist:** It functions antagonistically to the **Apneustic Center** (located in the lower pons), which promotes inspiration.

Question 204: In body plethysmography, a person is asked to expire against a closed glottis. What will be the change in the pressure in the lung and the box?

A. Increase in both
B. Decrease in both
C. Increase in lung, decrease in box (Correct Answer)
D. Decrease in lung, increase in box

Explanation: ### Explanation **1. Underlying Concept: Boyle’s Law** Body plethysmography is based on **Boyle’s Law**, which states that at a constant temperature, the pressure ($P$) and volume ($V$) of a gas are inversely proportional ($P \times V = K$). When a person attempts to **expire against a closed glottis** (a maneuver similar to the Valsalva maneuver), they are compressing the air within their lungs. * **In the Lungs:** The expiratory effort decreases the volume of the thoracic cavity. According to Boyle’s Law, as volume decreases, the **pressure in the lungs increases**. * **In the Box:** As the chest wall and lungs compress (occupying less space), the volume of the air *outside* the body but *inside* the airtight box increases. Consequently, the **pressure in the box decreases**. **2. Analysis of Incorrect Options** * **Option A & B:** These are incorrect because the system is closed. A change in the thoracic volume must result in an equal and opposite change in the box volume. Therefore, pressure changes cannot occur in the same direction for both. * **Option D:** This describes the opposite physiological event—**inspiration** against a closed glottis (Müller’s maneuver). During inspiration, lung volume increases (decreasing lung pressure) and the chest expands, compressing the air in the box (increasing box pressure). **3. Clinical Pearls & High-Yield Facts** * **Gold Standard:** Body plethysmography is the gold standard for measuring **Functional Residual Capacity (FRC)**, especially in patients with obstructive lung diseases (e.g., COPD) where helium dilution methods underestimate volume due to "trapped air." * **Total Lung Capacity (TLC):** Once FRC is determined via plethysmography, TLC and Residual Volume (RV) can be calculated. * **Maneuver:** The specific maneuver used during the test is often referred to as "panting" against a shutter.

Question 205: A 54-year-old man presents with dyspnea and a cough. He is a non-smoker with no relevant occupational exposures. His pulmonary function test results are as follows: Pre-Bronchodilator (BD) Post-BD Test Actual Predicted % Predicted Actual % Change FVC (L) 3.19 4.22 76 4.00 25 FEV1 (L) 2.18 3.39 64 2.83 30 FEV1/FVC (%) 68 80 714. What is the most probable diagnosis based on these results?

A. Normal pulmonary function tests
B. Moderate airflow obstruction (Correct Answer)
C. Severe airflow obstruction
D. Restrictive lung disease

Explanation: ### Explanation To interpret Pulmonary Function Tests (PFTs) for NEET-PG, follow a systematic stepwise approach: **1. Identify Obstruction (FEV1/FVC Ratio):** The primary indicator of obstructive lung disease is a decreased FEV1/FVC ratio (typically <70% or below the Lower Limit of Normal). In this patient, the pre-bronchodilator ratio is **68%**, confirming an **obstructive pattern**. **2. Determine Severity (FEV1 % Predicted):** Once obstruction is confirmed, severity is graded based on the **FEV1 % predicted**: * >80%: Mild * **50–80%: Moderate** * 30–50%: Severe * <30%: Very Severe This patient’s FEV1 is **64% of the predicted value**, placing him squarely in the **Moderate** category. **3. Assess Reversibility:** A significant bronchodilator response is defined as an increase in FEV1 of **>12% AND >200 mL**. This patient shows a **30% increase** in FEV1, suggesting a highly reversible airway disease like Asthma. --- ### Why the other options are incorrect: * **Option A (Normal):** The FEV1/FVC ratio is below 70%, and FEV1 is below 80% predicted, which is pathological. * **Option C (Severe):** Severe obstruction requires an FEV1 between 30% and 50%. At 64%, this patient is in the moderate range. * **Option D (Restrictive):** Restrictive lung disease is characterized by a **normal or increased** FEV1/FVC ratio and a decrease in Total Lung Capacity (TLC) or FVC. Here, the ratio is low, pointing to obstruction. --- ### High-Yield Clinical Pearls for NEET-PG: * **Gold Standard for Obstruction:** FEV1/FVC ratio < 0.7. * **Gold Standard for Restriction:** Total Lung Capacity (TLC) < 80% predicted. * **Reversibility:** Essential to differentiate Asthma (usually reversible) from COPD (largely irreversible). * **Flow-Volume Loops:** In obstruction, the loop shows a "scooped-out" appearance; in restriction, the loop is narrow and tall ("witch’s hat").

Question 206: Which of the following is inactivated in the lung?

A. Angiotensin I
B. Angiotensin II
C. Bradykinin (Correct Answer)
D. Serotonin

Explanation: **Explanation:** The lungs serve a vital non-respiratory metabolic function by acting as a selective filter for substances circulating in the blood. This is primarily mediated by enzymes located on the luminal surface of the pulmonary capillary vascular endothelium. **Why Bradykinin is Correct:** Approximately 80% of **Bradykinin** is inactivated during a single pass through the pulmonary circulation. This inactivation is catalyzed by **Angiotensin-Converting Enzyme (ACE)** (also known as kininase II). ACE breaks down bradykinin into inactive peptides, preventing this potent vasodilator from causing systemic hypotension. **Analysis of Incorrect Options:** * **Angiotensin I:** This is not inactivated; rather, it is **activated/converted** into Angiotensin II by ACE in the pulmonary capillaries. * **Angiotensin II:** This peptide passes through the lungs **unchanged**. It is not metabolized by the pulmonary endothelium, allowing it to exert its systemic vasoconstrictive effects. * **Serotonin (5-HT):** While serotonin is almost completely **removed** from the circulation by the lungs via high-affinity uptake and subsequent storage or degradation by MAO, the question specifically asks for "inactivation" in the context of enzymatic degradation of circulating vasoactive peptides. In standard physiological texts (like Ganong), Bradykinin is the classic example of a substance inactivated by ACE. **High-Yield Clinical Pearls for NEET-PG:** * **ACE Inhibitors (ACEIs):** Drugs like Enalapril inhibit ACE, leading to increased levels of Bradykinin. This accumulation in the lungs is responsible for the common side effect of a **dry cough**. * **Substances 100% removed/inactivated:** Bradykinin, Serotonin, and Prostaglandins (E1, E2, F2α). * **Substances unaffected by lungs:** Angiotensin II, Epinephrine, Oxytocin, and ADH. * **Substance produced/activated in lungs:** Angiotensin II and Surfactant.

Question 207: The Hering Breuer reflex is mediated by which of the following receptors?

A. Pulmonary stretch receptors (Correct Answer)
B. Bronchial stretch receptors
C. J receptors
D. Chest wall proprioceptors

Explanation: **Explanation:** The **Hering-Breuer Inflation Reflex** is a protective mechanism designed to prevent over-inflation of the lungs. It is mediated by **Pulmonary Stretch Receptors (PSRs)**, specifically the slowly adapting stretch receptors located in the smooth muscle of the large and small airways. When the lungs inflate to a high tidal volume (typically >1.5 liters in adults), these receptors are stimulated. They send inhibitory signals via the **Vagus nerve (Cranial Nerve X)** to the inspiratory center in the medulla (Dorsal Respiratory Group). This terminates inspiration and initiates expiration, effectively "switching off" the inspiratory ramp. **Analysis of Incorrect Options:** * **B. Bronchial stretch receptors:** While receptors exist in the bronchi, the term "Pulmonary stretch receptors" is the standard physiological nomenclature for the specific receptors mediating this reflex. * **C. J receptors (Juxtacapillary receptors):** Located in the alveolar walls near capillaries, these are stimulated by pulmonary congestion, edema, or irritants, leading to rapid shallow breathing (tachypnea), not the Hering-Breuer reflex. * **D. Chest wall proprioceptors:** These receptors (in muscles/joints) monitor the work of breathing and chest movement but do not trigger the Hering-Breuer inflation reflex. **High-Yield Clinical Pearls for NEET-PG:** * **Afferent Pathway:** Vagus Nerve. * **Effect:** Decreases respiratory rate by increasing expiratory time. * **Infants:** The reflex is much more active in neonates than in adults, playing a role in regulating normal tidal breathing. * **Hering-Breuer Deflation Reflex:** A separate reflex where lung atelectasis/deflation triggers an increase in respiratory rate to prevent lung collapse.

Question 208: What is the term for the increased oxygen delivery to tissues in response to increased CO2?

A. Bohr effect (Correct Answer)
B. Haldane Effect
C. Hamburger effect
D. Chloride shift

Explanation: **Explanation:** The correct answer is the **Bohr Effect**. This physiological phenomenon describes how an increase in blood CO2 concentration and a decrease in pH (increased H+ ions) lead to a **rightward shift** of the oxyhemoglobin dissociation curve. This shift decreases hemoglobin's affinity for oxygen, facilitating the unloading of oxygen into metabolically active tissues where CO2 levels are high. **Analysis of Options:** * **Bohr Effect (Correct):** Occurs at the **tissue level**. Increased CO2/H+ binds to hemoglobin, causing it to release O2. Think: "Bohr = Binding of H+ leads to Release of O2." * **Haldane Effect:** This is the opposite of the Bohr effect and occurs at the **lung level**. It describes how the binding of oxygen to hemoglobin promotes the release of CO2. Deoxygenated blood has an increased capacity to carry CO2. * **Hamburger Effect / Chloride Shift:** This refers to the exchange of bicarbonate (HCO3-) out of the RBC and chloride (Cl-) into the RBC to maintain electrical neutrality. This occurs primarily in systemic capillaries. **NEET-PG High-Yield Pearls:** * **Right Shift Factors (CADET, face Right!):** **C**O2 increase, **A**cidosis (H+), **D**PG (2,3-BPG) increase, **E**xercise, and **T**emperature increase. * **P50 Value:** The partial pressure of O2 at which hemoglobin is 50% saturated. A right shift (Bohr effect) **increases** the P50. * **Double Bohr Effect:** Occurs in the placenta, where maternal blood releases O2 (Bohr) and fetal blood picks it up, facilitating efficient fetal oxygenation.

Question 209: The intrapleural pressure is negative both during inspiration and expiration because:

A. Intrapulmonary pressure is always negative
B. The thoracic cage and lungs are elastic structures (Correct Answer)
C. Transpulmonary pressure determines the negativity
D. Surfactant prevents the lungs from collapsing

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The intrapleural pressure (IPP) is negative (sub-atmospheric) due to the **opposing elastic recoil forces** of the lungs and the thoracic cage. * The **lungs** have a natural tendency to recoil inward (collapse) due to elastic fibers and surface tension. * The **thoracic cage** has a natural tendency to recoil outward (expand). These two structures are held together by the thin film of pleural fluid. As they pull in opposite directions, they create a "vacuum" effect in the potential space between them, resulting in a negative IPP. This negativity persists even during expiration because the equilibrium point (Functional Residual Capacity) is only reached when these opposing forces are balanced, not eliminated. **2. Why Incorrect Options are Wrong:** * **Option A:** Intrapulmonary pressure is *not* always negative. It becomes negative during inspiration (to pull air in) and positive during expiration (to push air out). * **Option C:** Transpulmonary pressure is the *result* of the difference between alveolar and intrapleural pressure ($P_{tp} = P_{alv} - P_{ip}$); it is a measure of the distending force, not the primary cause of IPP negativity. * **Option D:** While surfactant reduces surface tension and prevents alveolar collapse, it does not generate the negative pressure in the pleural cavity; that is a function of the chest wall-lung interaction. **3. NEET-PG High-Yield Pearls:** * **Normal Values:** IPP is approximately **-5 cm $H_2O$** at the start of inspiration and drops to **-7.5 cm $H_2O$** at the end of inspiration. * **Pneumothorax:** If the pleural cavity is breached, air enters the space, IPP becomes equal to atmospheric pressure, and the lung collapses due to its unopposed inward recoil. * **Gravity Effect:** IPP is **most negative at the apex** of the lung and least negative at the base in an upright position.

Question 210: Extrapulmonary restrictive defects lead to the development of which acid-base disturbance?

A. Respiratory alkalosis
B. Respiratory acidosis (Correct Answer)
C. Increased DLco
D. Reduced DLco

Explanation: ### Explanation **Why Respiratory Acidosis is Correct:** Extrapulmonary restrictive defects (e.g., obesity, kyphoscoliosis, neuromuscular disorders like Myasthenia Gravis or Guillain-Barré Syndrome) impair the mechanical ability of the chest wall or respiratory muscles to expand. This leads to **alveolar hypoventilation**. When ventilation is inadequate, the lungs cannot effectively eliminate carbon dioxide ($CO_2$), leading to its retention in the blood (hypercapnia). According to the Henderson-Hasselbalch equation, an increase in $PaCO_2$ lowers the pH, resulting in **respiratory acidosis**. **Analysis of Incorrect Options:** * **A. Respiratory alkalosis:** This occurs due to hyperventilation (e.g., high altitude, anxiety, or early pulmonary embolism), which decreases $PaCO_2$. Restrictive defects typically cause hypoventilation, not hyperventilation. * **C. Increased DLco:** DLco (Diffusing Capacity of the Lungs for Carbon Monoxide) is never increased by restrictive defects. It may be increased in conditions like alveolar hemorrhage or polycythemia. * **D. Reduced DLco:** While DLco is reduced in **intrapulmonary** restrictive diseases (like Idiopathic Pulmonary Fibrosis) due to membrane thickening, it remains **normal** in **extrapulmonary** restrictive defects because the alveolar-capillary membrane itself is healthy. **High-Yield Clinical Pearls for NEET-PG:** * **The "Normal DLco" Rule:** In a patient with a restrictive pattern (Low FVC, Low TLC, Normal/High FEV1/FVC ratio), a **normal DLco** points toward an extrapulmonary cause (chest wall/neuromuscular), while a **low DLco** points toward an intrinsic parenchymal lung disease. * **PFT Pattern:** Both intra- and extrapulmonary restriction show a "Witch’s Hat" appearance on the flow-volume loop (narrow and tall). * **Chronic Compensation:** In chronic extrapulmonary restriction (e.g., Pickwickian Syndrome), the kidneys compensate for the respiratory acidosis by retaining bicarbonate ($HCO_3^-$).

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