Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 51: What is the normal value of arterial blood oxygen in mL of O2 per dL?

A. 12.1
B. 19.8 (Correct Answer)
C. 15.6
D. 27.8

Explanation: The oxygen content of arterial blood ($CaO_2$) is the sum of oxygen bound to hemoglobin and oxygen dissolved in plasma. This is a high-yield calculation for NEET-PG. ### **The Calculation** 1. **Bound to Hemoglobin:** 1 gram of pure $Hb$ can carry **1.34 mL** of $O_2$. Assuming a normal $Hb$ of **15 g/dL** and an arterial saturation ($SaO_2$) of **98%**: * $15 \times 1.34 \times 0.98 = \mathbf{19.7 \text{ mL/dL}}$ 2. **Dissolved in Plasma:** Oxygen is poorly soluble. At a normal $PaO_2$ of 100 mmHg, only **0.3 mL/dL** is dissolved ($0.003 \times 100$). 3. **Total Content:** $19.7 + 0.3 = \mathbf{20 \text{ mL/dL}}$ (approximately). Option B (**19.8**) is the most accurate value representing this physiological total. ### **Analysis of Incorrect Options** * **A (12.1):** This value is too low for arterial blood; it may be seen in severe anemia or significant hypoxia. * **C (15.6):** This represents the typical oxygen content of **mixed venous blood** ($CvO_2$), where saturation is approximately 75%. * **D (27.8):** This value is physiologically impossible under normal atmospheric conditions and standard hemoglobin levels. ### **NEET-PG High-Yield Pearls** * **Utilization Coefficient:** The fraction of oxygen given up to tissues (normally ~25%). * **Arterio-venous $O_2$ difference:** Normally **5 mL/dL** ($20 - 15$). This increases during exercise as tissues extract more oxygen. * **Hüfner's Constant:** While 1.34 is used clinically, the theoretical maximum is **1.39 mL/g**. * **Dissolved $O_2$:** Though small (0.3 mL), this is the only form of oxygen that exerts partial pressure ($PaO_2$) and determines hemoglobin saturation.

Question 52: A 49-year-old man has a pulmonary embolism that completely blocks blood flow to his left lung. As a result, which of the following will occur?

A. Ventilation/perfusion (V/Q) ratio in the left lung will be zero
B. Systemic arterial PO2 will be elevated
C. V/Q ratio in the left lung will be lower than in the right lung
D. Alveolar PO2 in the left lung will be approximately equal to the PO2 in inspired air (Correct Answer)

Explanation: ### Explanation This question tests the understanding of **Ventilation/Perfusion (V/Q) relationships** and their impact on alveolar gas composition. #### 1. Why Option D is Correct When blood flow to the left lung is completely blocked by a pulmonary embolism, perfusion ($Q$) becomes zero. This creates **Alveolar Dead Space**, where the V/Q ratio becomes **infinite** ($V/0 = \infty$). In this state, the alveoli are ventilated but not perfused. Since no gas exchange occurs between the blood and the alveoli, the composition of alveolar air does not change. Therefore, the **Alveolar $PO_2$ ($P_AO_2$)** will rise to equal the $PO_2$ of inspired air (~150 mmHg), and the **Alveolar $PCO_2$ ($P_ACO_2$)** will fall to zero. #### 2. Why Other Options are Incorrect * **Option A:** A V/Q ratio of **zero** occurs in a **Shunt** (perfusion without ventilation), such as an airway obstruction. In this case, perfusion is blocked, making the ratio infinite. * **Option B:** Systemic arterial $PO_2$ will **decrease**, not increase. This is due to the redistribution of blood flow to the right lung (over-perfusion relative to ventilation) and the potential for V/Q mismatch in the remaining functional lung tissue. * **Option C:** The V/Q ratio in the left lung ($\infty$) is significantly **higher** than in the right lung (which will have a normal or slightly decreased V/Q ratio due to increased compensatory blood flow). #### 3. NEET-PG High-Yield Pearls * **Dead Space ($V/Q = \infty$):** Ventilation without perfusion. $P_AO_2$ = 150 mmHg; $P_ACO_2$ = 0 mmHg. * **Shunt ($V/Q = 0$):** Perfusion without ventilation. Alveolar gas equilibrates with mixed venous blood ($P_AO_2$ = 40 mmHg; $P_ACO_2$ = 46 mmHg). * **West Zones:** In a normal upright lung, the V/Q ratio is highest at the **apex** (highest $PO_2$) and lowest at the **base**. * **Clinical Sign:** Pulmonary embolism increases the **physiological dead space**, leading to an increased **A-a gradient**.

Question 53: Maximum voluntary ventilation is measured for what duration?

A. 30 seconds
B. 60 seconds (Correct Answer)
C. 75 seconds
D. 120 seconds

Explanation: ### Explanation **Maximum Voluntary Ventilation (MVV)** is defined as the maximum volume of air that can be inhaled and exhaled from the lungs per minute by maximum conscious effort. **1. Why 60 seconds is the correct answer:** By definition, MVV is a measure of the **ventilatory capacity per minute**. Therefore, the standard unit of measurement is liters per minute (L/min). While the actual clinical test is performed for a shorter duration (usually 12–15 seconds) to prevent fatigue and hypocapnia, the value is mathematically extrapolated to **60 seconds** to represent the total breathing capacity over a full minute. **2. Why other options are incorrect:** * **30 seconds:** This duration is too short for a standard physiological "per minute" definition and is not used as the reference timeframe for MVV. * **75 and 120 seconds:** These durations are excessively long. Performing maximal respiratory effort for this long would lead to severe respiratory muscle fatigue and significant respiratory alkalosis due to the "washout" of CO₂, potentially causing syncope or tetany. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Normal Values:** In a healthy young adult male, the average MVV is approximately **140–180 L/min**. * **Clinical Significance:** MVV assesses the entire respiratory pump, including airway resistance, lung compliance, and the integrity of respiratory muscles. * **The 15-second Rule:** In clinical practice, the patient breathes as deeply and rapidly as possible for only **12 to 15 seconds**. This result is then multiplied by 4 (or 5) to calculate the 60-second MVV. * **Relationship with FEV1:** MVV can be indirectly estimated using the formula: **MVV = FEV1 × 35 (or 40)**. This is a common numerical MCQ in physiology.

Question 54: What is asthma?

A. Obstructive lung disease (Correct Answer)
B. Restrictive lung disease
C. Both
D. None

Explanation: **Explanation:** **1. Why Option A is Correct:** Asthma is a chronic inflammatory disorder of the airways characterized by **reversible airway obstruction**. The underlying pathophysiology involves bronchial hyperresponsiveness and inflammation, leading to bronchoconstriction, mucosal edema, and excessive mucus production. In obstructive lung diseases, the primary physiological defect is **increased resistance to airflow**, particularly during expiration. This is reflected in pulmonary function tests (PFTs) by a **decreased FEV1/FVC ratio (<0.7)**. **2. Why Other Options are Incorrect:** * **Option B (Restrictive Lung Disease):** These conditions (e.g., Pulmonary Fibrosis, Sarcoidosis) are characterized by reduced lung compliance and "stiff" lungs, leading to decreased lung volumes (TLC). In restriction, the FEV1/FVC ratio is typically normal or increased, unlike in asthma. * **Option C & D:** These are incorrect as asthma fits the specific physiological profile of obstruction. **3. NEET-PG High-Yield Clinical Pearls:** * **Hallmark PFT Finding:** Reversibility is key. An increase in FEV1 of **>12% and >200 ml** after inhaling a bronchodilator (e.g., Salbutamol) confirms the diagnosis. * **Airway Remodeling:** Chronic untreated inflammation can lead to permanent structural changes (subepithelial fibrosis), which may eventually cause irreversible obstruction. * **Pathology:** Look for **Curschmann spirals** (mucus plugs) and **Charcot-Leyden crystals** (eosinophil breakdown products) in sputum samples. * **Gold Standard for Diagnosis:** Spirometry is the preferred method to demonstrate variable expiratory airflow limitation.

Question 55: What is the approximate normal amount of pleural fluid?

A. 5 ml
B. 15 ml (Correct Answer)
C. 50 ml
D. 100 ml

Explanation: **Explanation:** The pleural space is a potential space between the visceral and parietal pleura. In a healthy adult, it contains a small amount of serous fluid that acts as a lubricant, reducing friction between the lungs and the chest wall during respiration. **Why Option B is Correct:** The normal volume of pleural fluid is approximately **0.1 to 0.2 ml/kg body weight**, which translates to roughly **7 to 15 ml** in a 70 kg adult. While some textbooks provide a range (e.g., 10–20 ml), **15 ml** is the most widely accepted "average" value in standard medical physiology (such as Guyton and Ganong). This fluid is constantly produced by the parietal pleura and drained by the lymphatic system. **Why Other Options are Incorrect:** * **Option A (5 ml):** This is slightly below the physiological average for a standard adult, though it may be seen in very small individuals. * **Option C (50 ml) & Option D (100 ml):** These volumes are pathologically high. Any accumulation of fluid in the pleural space exceeding 20–25 ml is generally considered a **pleural effusion**, which can be detected on imaging (e.g., blunting of the costophrenic angle on a chest X-ray usually requires at least 150–200 ml of fluid). **High-Yield Clinical Pearls for NEET-PG:** * **Protein Content:** Pleural fluid is a transudate with low protein content (< 1.5 g/dL). * **Drainage:** The primary route for pleural fluid clearance is through the **lymphatic stomata** in the parietal pleura. * **Pressure:** The intrapleural pressure is normally **negative** (approx. -5 cm H₂O) due to the opposing elastic recoil of the lungs and the chest wall. * **Light’s Criteria:** Used to differentiate between transudative and exudative pleural effusions (High yield for Medicine/Surgery).

Question 56: Normal pulmonary capillary wedge pressure (PCWP) with pulmonary edema is seen in which of the following conditions?

A. Left atrial myxoma
B. High altitude (Correct Answer)
C. Pulmonary vein obstruction
D. Pulmonary artery obstruction

Explanation: **Explanation:** The **Pulmonary Capillary Wedge Pressure (PCWP)** is a clinical surrogate for left atrial pressure and left ventricular end-diastolic pressure. Pulmonary edema occurs when the hydrostatic pressure in the pulmonary capillaries exceeds the oncotic pressure, causing fluid to leak into the interstitium and alveoli. **Why High Altitude is correct:** At high altitudes, the low partial pressure of oxygen ($FiO_2$) triggers **Hypoxic Pulmonary Vasoconstriction (HPV)**. This constriction occurs at the level of the **pulmonary arterioles** (pre-capillary). Because the obstruction/constriction is *proximal* to the capillaries, the pressure measured downstream (PCWP) remains **normal** (usually <12 mmHg). However, the uneven nature of this vasoconstriction leads to high pressure in non-constricted vessels, causing "stress failure" of the capillary membrane and resulting in **High-Altitude Pulmonary Edema (HAPE)**. **Analysis of Incorrect Options:** * **Left Atrial Myxoma:** This creates a mechanical obstruction at the mitral valve. This increases left atrial pressure, which is transmitted backwards, leading to an **elevated PCWP**. * **Pulmonary Vein Obstruction:** Since PCWP measures the pressure distal to the pulmonary capillaries (reflecting the veins and left atrium), any obstruction in the pulmonary veins will result in an **elevated PCWP**. * **Pulmonary Artery Obstruction (e.g., PE):** While this also features a normal PCWP, it typically causes decreased blood flow to the lungs and does **not** cause pulmonary edema. **Clinical Pearls for NEET-PG:** * **Normal PCWP:** 6–12 mmHg. * **Cardiogenic Edema:** PCWP >18 mmHg (e.g., CHF, Mitral Stenosis). * **Non-Cardiogenic Edema (Normal PCWP):** Includes HAPE, ARDS (due to increased permeability), and Neurogenic pulmonary edema. * **HAPE Treatment:** Rapid descent, supplemental oxygen, and **Nifedipine** (to reduce pulmonary artery pressure).

Question 57: In a healthy subject, under what condition is the partial pressure of oxygen (PO2) in the blood leaving a pulmonary capillary lower than the PO2 in the alveolus served by that capillary?

A. When breathing a low oxygen gas mixture
B. When performing an exercise that doubles cardiac output
C. At the base of the lungs, where perfusion exceeds ventilation
D. None of the above (Correct Answer)

Explanation: ### Explanation The core concept tested here is the **diffusion-limited vs. perfusion-limited** nature of gas exchange. In a healthy subject at rest or during moderate exercise, oxygen transfer is **perfusion-limited**. **1. Why "None of the above" is correct:** Under normal physiological conditions, the equilibration of oxygen between the alveolus and the pulmonary capillary is incredibly efficient. It takes approximately **0.25 seconds** for capillary $PO_2$ to match alveolar $PO_2$ ($P_AO_2$). Since the total transit time of a red blood cell in the capillary is about **0.75 seconds**, there is a large "safety factor." Even under stress, the blood leaving the individual capillary has fully equilibrated with the alveolar gas. Therefore, the $PO_2$ in the blood leaving a healthy capillary is **equal** to the $PO_2$ in the served alveolus. **2. Analysis of Incorrect Options:** * **Option A (Low $O_2$ mixture):** While the absolute $PO_2$ values will be lower, the *gradient* still favors equilibration. The blood will still reach the (lower) alveolar $PO_2$ before leaving the capillary. * **Option B (Exercise):** Doubling cardiac output reduces transit time (e.g., from 0.75s to 0.40s). However, since equilibration only requires 0.25s, the blood still reaches equilibrium with alveolar $PO_2$ before exiting. * **Option C (Lung Base):** While the V/Q ratio is lower at the base, the blood that *does* pass through a ventilated capillary still equilibrates with the gas present in that specific alveolus. (Note: The *mixed* venous blood in the pulmonary vein may have a lower $PO_2$ due to physiological shunting, but the question specifies blood leaving a capillary served by a specific alveolus). **High-Yield NEET-PG Pearls:** * **Perfusion-limited gases:** $O_2$ (normal), $CO_2$, and $N_2O$. They reach equilibrium early in the capillary. * **Diffusion-limited gases:** Carbon Monoxide ($CO$). It never reaches equilibrium; its uptake is limited only by the diffusion capacity of the membrane. * **Pathological shift:** $O_2$ can become diffusion-limited in cases of **severe pulmonary fibrosis** or **extreme exercise at high altitudes**, where the blood moves too fast and the membrane is too thick for equilibration to occur within the transit time.

Question 58: Given the Haldane effect, carbon dioxide uptake is 2 ml/100 ml of blood in arteries. What will be the carbon dioxide uptake in veins in the absence of the Haldane effect?

A. 2 ml/100 ml of blood
B. 4 ml/100 ml of blood (Correct Answer)
C. 6 ml/100 ml of blood
D. 8 ml/100 ml of blood

Explanation: ### Explanation The **Haldane Effect** describes how the oxygenation of hemoglobin in the lungs displaces carbon dioxide from the blood. Conversely, in systemic tissues, the deoxygenation of hemoglobin increases its ability to carry $CO_2$. **Why Option B is Correct:** Under normal physiological conditions, the total amount of $CO_2$ picked up by the blood as it passes from arteries to veins is approximately **4 ml/100 ml** (4 vol%). This uptake is the result of two factors: 1. **The change in $PCO_2$:** The rise in $PCO_2$ from 40 mmHg (arterial) to 46 mmHg (venous) accounts for about **2 ml** of $CO_2$ uptake. 2. **The Haldane Effect:** The deoxygenation of hemoglobin (as $O_2$ is released to tissues) accounts for the remaining **2 ml** of $CO_2$ uptake. The question states that in the presence of the Haldane effect, the uptake is 2 ml (referring to the portion contributed specifically by the effect). Therefore, the **total** $CO_2$ uptake in the veins is the sum of the $PCO_2$ gradient (2 ml) + the Haldane effect (2 ml) = **4 ml/100 ml**. **Analysis of Incorrect Options:** * **Option A (2 ml):** This represents only the $CO_2$ dissolved/carried due to the partial pressure gradient, ignoring the total physiological transport. * **Options C & D (6 ml & 8 ml):** These values exceed the normal physiological $CO_2$ dissociation curve shifts seen between arterial and venous blood. **High-Yield Clinical Pearls for NEET-PG:** * **Haldane Effect vs. Bohr Effect:** The Haldane effect describes $O_2$ affecting $CO_2$ transport (occurs in lungs/tissues), while the **Bohr Effect** describes $CO_2/H^+$ affecting $O_2$ affinity (occurs in tissues). * **Mechanism:** Deoxygenated hemoglobin is a **weaker acid** and a better proton acceptor than oxyhemoglobin, which facilitates the formation of carbamino compounds and bicarbonate. * **Significance:** The Haldane effect doubles the amount of $CO_2$ released from the blood in the lungs and doubles the $CO_2$ picked up in the tissues.

Question 59: All of the following affect resting ventilation except?

A. Stretch receptors
B. J receptors (Correct Answer)
C. Oxygen
D. PCO2

Explanation: **Explanation:** The regulation of resting ventilation is primarily governed by chemical control (PCO2, pH, and PO2) and neural feedback from the lungs. **1. Why J Receptors is the correct answer:** **Juxtacapillary (J) receptors** are sensory nerve endings located in the alveolar walls, adjacent to the pulmonary capillaries. They are **silent during normal, resting ventilation**. They are only activated during pathological states such as pulmonary edema, congestion, pneumonia, or microembolism. When stimulated, they trigger the "J-reflex," characterized by rapid shallow breathing (tachypnea), apnea, bradycardia, and hypotension. Since they do not fire under physiological conditions, they do not contribute to resting ventilation. **2. Why the other options are incorrect:** * **PCO2 (Option D):** This is the **most important** stimulus for resting ventilation. Central chemoreceptors are exquisitely sensitive to changes in arterial PCO2 (via H+ ions in the CSF), adjusting the rate and depth of breathing to maintain homeostasis. * **Oxygen (Option C):** While less dominant than PCO2 at rest, peripheral chemoreceptors (carotid and aortic bodies) constantly monitor PO2. They provide a "hypoxic drive" that contributes to the baseline respiratory rhythm. * **Stretch Receptors (Option A):** Located in the smooth muscle of the airways, these receptors mediate the **Hering-Breuer reflex**. They respond to lung inflation and help terminate inspiration, thereby influencing the timing and pattern of resting breaths. **High-Yield Clinical Pearls for NEET-PG:** * **Hering-Breuer Inflation Reflex:** In adults, it is typically active only when tidal volume exceeds **1.5 liters** (e.g., during exercise), but it plays a role in regulating the breathing frequency in neonates. * **Location of Chemoreceptors:** Central chemoreceptors are located on the **ventrolateral surface of the medulla**, while peripheral chemoreceptors are located in the **carotid bodies** (via CN IX) and **aortic bodies** (via CN X). * **J-Receptor Stimulus:** The most potent stimulus for J receptors is **pulmonary capillary engorgement** or interstitial edema.

Question 60: What is the cut-off limit for deoxygenated hemoglobin for the appearance of cyanosis?

A. >4 gm%
B. >5 gm% (Correct Answer)
C. < 2 gm%
D. <4 gm%

Explanation: **Explanation:** **Core Concept:** Cyanosis is defined as the bluish discoloration of the skin and mucous membranes. It is not determined by the total hemoglobin level or the oxygen saturation percentage alone, but specifically by the **absolute concentration of reduced (deoxygenated) hemoglobin** in the subpapillary venous plexus. The clinical threshold for the appearance of cyanosis is when the deoxygenated hemoglobin concentration exceeds **5 gm/dL (or 5 gm%)**. **Analysis of Options:** * **Option B (>5 gm%):** This is the correct physiological threshold. When more than 5 gm/dL of hemoglobin is in the deoxygenated state, the blood loses its bright red color and takes on a dark, bluish hue that becomes visible through the skin. * **Option A (>4 gm%):** While 4 gm% represents a significant amount of desaturation, it is generally insufficient to produce the classic clinical appearance of cyanosis in a patient with normal hemoglobin levels. * **Options C & D (<2 gm% and <4 gm%):** These values represent normal or near-normal levels of deoxygenated hemoglobin. In a healthy individual with a total Hb of 15 gm/dL and 95% saturation, the deoxygenated Hb is only 0.75 gm/dL, which is far below the cyanotic threshold. **High-Yield Clinical Pearls for NEET-PG:** 1. **Anemia Paradox:** A severely anemic patient (e.g., Hb = 6 gm/dL) may never manifest cyanosis even if dangerously hypoxic, because they cannot physically reach the 5 gm/dL threshold of deoxygenated Hb. 2. **Polycythemia:** Patients with high red cell counts can develop cyanosis more easily (at higher oxygen saturation levels) because they have a higher total pool of hemoglobin. 3. **Central vs. Peripheral:** Central cyanosis (tongue/lips) indicates systemic arterial desaturation, whereas peripheral cyanosis (fingertips) often indicates sluggish blood flow and increased oxygen extraction. 4. **Methemoglobinemia:** Cyanosis can appear at lower levels (**1.5 gm/dL**) if the abnormal pigment is methemoglobin rather than reduced hemoglobin.

Practice by Chapter

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Pulmonary Ventilation

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Pulmonary Circulation

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Gas Exchange in the Lungs

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Oxygen and Carbon Dioxide Transport

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Control of Breathing

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Respiratory Adjustments in Health and Disease

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High Altitude Physiology

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