Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 691: A pulmonary disorder causes the alveoli to break down and coalesce into large air spaces. The lungs also lose elasticity and compliance is increased. A person who suffers from this disease will have what characteristic?

A. Increased dead air space (Correct Answer)
B. Increased vital capacity
C. Decreased PCO2 in the blood
D. Decreased anteroposterior diameter

Explanation: The clinical scenario describes **Emphysema**, a type of Chronic Obstructive Pulmonary Disease (COPD). In emphysema, the destruction of alveolar septa leads to the formation of large, permanent air spaces (bullae), reducing the surface area for gas exchange and causing a loss of elastic recoil. ### Why Option A is Correct **Increased Dead Air Space:** The breakdown and coalescence of alveoli create large air sacs with a significantly reduced capillary interface. While these areas are still ventilated, they are poorly perfused (or the surface area is insufficient for exchange). This increases **physiological dead space** (wasted ventilation). Additionally, the loss of elastic recoil leads to air trapping and hyperinflation, further increasing the volume of air that does not participate in gas exchange. ### Why Other Options are Incorrect * **B. Increased Vital Capacity:** In emphysema, the Vital Capacity (VC) actually **decreases**. This is because air trapping increases the Residual Volume (RV), which "encroaches" upon the VC. * **C. Decreased PCO2:** Due to the loss of exchange surface and airway obstruction, patients typically experience hypoventilation and ventilation-perfusion (V/Q) mismatch, leading to **increased PCO2** (hypercapnia), not a decrease. * **D. Decreased Anteroposterior (AP) Diameter:** Loss of elasticity leads to hyperinflation of the lungs. To accommodate this, the chest wall expands, leading to an **increased AP diameter**, classically known as a **"Barrel Chest."** ### NEET-PG High-Yield Pearls * **Compliance:** Emphysema is the classic example of **increased lung compliance** due to the loss of elastic fibers (elastin). * **Flow-Volume Loop:** Shows a characteristic **"scooped-out"** appearance during expiration. * **Diffusion Capacity:** Emphysema is one of the few obstructive diseases where the **DLCO (Diffusion Capacity of Carbon Monoxide) is decreased** due to alveolar wall destruction.

Question 692: What is the formula for normal respiratory minute volume?

A. Tidal volume × Respiratory Rate (Correct Answer)
B. Tidal volume / Respiratory Rate
C. Total Lung Capacity / Respiratory Rate
D. Functional Residual Capacity / Respiratory Rate

Explanation: ### Explanation **Respiratory Minute Volume (RMV)**, also known as Minute Ventilation ($\dot{V}_E$), is the total volume of gas entering (or leaving) the lungs per minute. It is a primary indicator of pulmonary ventilation efficiency. **1. Why Option A is Correct:** The formula for RMV is: $$\text{Minute Volume} = \text{Tidal Volume (TV)} \times \text{Respiratory Rate (RR)}$$ * **Tidal Volume:** The amount of air inspired or expired during a single normal breath (approx. 500 mL in a healthy adult). * **Respiratory Rate:** The number of breaths per minute (approx. 12–16 breaths/min). * **Calculation:** $500 \text{ mL} \times 12 \text{ breaths/min} = 6,000 \text{ mL/min}$ or **6 L/min**. **2. Why Other Options are Incorrect:** * **Option B:** Dividing TV by RR has no physiological significance. * **Option C:** Total Lung Capacity (TLC) represents the maximum air the lungs can hold (approx. 6L). Dividing this by RR does not measure dynamic ventilation. * **Option D:** Functional Residual Capacity (FRC) is the air remaining after a normal expiration. It acts as a buffer for gas exchange but is not used to calculate minute ventilation. **3. NEET-PG High-Yield Clinical Pearls:** * **Alveolar Ventilation ($\dot{V}_A$):** This is more clinically significant than RMV as it accounts for **Anatomic Dead Space ($V_D$)**. * Formula: $\dot{V}_A = (TV - V_D) \times RR$. * **Dead Space:** In a healthy individual, anatomic dead space is roughly **2 mL/kg** of body weight (approx. 150 mL). * **Rapid Shallow Breathing:** If a patient has a low TV and high RR, the RMV might remain normal, but Alveolar Ventilation will drop significantly, leading to hypoxia and hypercapnia.

Question 693: A 43-year-old, 190 cm tall man presents with left-sided chest discomfort and dyspnea following a flight. Chest X-ray reveals a small area devoid of lung markings in the apex of the left lung. What is the most likely diagnosis?

A. Spontaneous pneumothorax (Correct Answer)
B. Myocardial infarction
C. Acute cor pulmonale
D. Aortic dissection

Explanation: ### Explanation **Correct Option: A. Spontaneous Pneumothorax** The clinical presentation is classic for **Primary Spontaneous Pneumothorax (PSP)**. The patient is a tall, thin male (190 cm), which is a significant risk factor due to higher pleural pressure gradients at the lung apex, leading to the formation of subpleural blebs. Rupture of these blebs (often triggered by pressure changes, such as during a flight) causes air to enter the pleural space. The pathognomonic radiological finding is a **visible visceral pleural line** with an **absence of distal lung markings** (peripheral lucency), typically seen at the apex in upright films. **Why Incorrect Options are Wrong:** * **B. Myocardial Infarction:** While it causes chest pain and dyspnea, it would not show "absence of lung markings" on a chest X-ray. ECG and cardiac enzymes are the diagnostic tools here. * **C. Acute Cor Pulmonale:** Usually secondary to massive pulmonary embolism. While it causes sudden dyspnea, the X-ray typically shows enlarged pulmonary arteries or Westermark sign, not a peripheral void of lung markings. * **D. Aortic Dissection:** Presents with "tearing" chest pain radiating to the back. The classic X-ray finding is a **widened mediastinum**, not a pneumothorax pattern. **NEET-PG High-Yield Pearls:** * **Risk Factors:** Tall, thin young males ("Asthenic build"), smoking, and Marfan syndrome. * **Diagnosis:** Chest X-ray in **upright expiration** is the most sensitive routine film for small pneumothoraces. * **Deep Sulcus Sign:** A high-yield radiological sign of pneumothorax seen on a **supine** chest X-ray (common in ICU/trauma patients). * **Management:** Small, asymptomatic PSP (<2 cm) can be managed conservatively with observation and oxygen; large or symptomatic cases require needle aspiration or chest tube insertion (intercostal drain).

Question 694: What diagnosis is suggested by these spirography findings?

A. Intrathoracic localized obstruction (Correct Answer)
B. Fixed inspiratory obstruction
C. Pneumothorax
D. Restrictive lung disease

Explanation: ### Explanation The diagnosis of **Intrathoracic localized (variable) obstruction** is based on the characteristic morphology of the Flow-Volume loop. **1. Why the Correct Answer is Right:** In **variable intrathoracic obstructions** (e.g., tracheomalacia or a localized tumor in the lower trachea), the obstruction is influenced by changes in pleural pressure. During **expiration**, the positive intrapleural pressure compresses the airway, worsening the obstruction and resulting in a **flattening or "plateau" of the expiratory limb**. During inspiration, the negative pleural pressure helps pull the airway open, leaving the inspiratory limb relatively normal. **2. Why the Incorrect Options are Wrong:** * **Fixed inspiratory/expiratory obstruction:** This occurs in conditions like tracheal stenosis or goiter. It results in **flattening of both** the inspiratory and expiratory limbs (the loop looks like a "box"). * **Pneumothorax:** This is an acute clinical emergency. While it causes a restrictive pattern, it is not typically diagnosed via routine spirography. * **Restrictive lung disease:** Characterized by a "miniature" version of a normal loop. Both volumes (FVC) and flow rates are reduced, but the **shape remains preserved** (tall and narrow) without flattening of the limbs. **3. High-Yield Clinical Pearls for NEET-PG:** * **Extrathoracic Variable Obstruction:** (e.g., Vocal cord palsy) Flattening occurs during **Inspiration** (atmospheric pressure collapses the airway). * **Intrathoracic Variable Obstruction:** Flattening occurs during **Expiration**. * **Fixed Obstruction:** Flattening occurs in **Both** phases. * **Scooped-out appearance:** Classic for **Obstructive** diseases like Asthma and COPD (due to reduced effort-independent flow).

Question 695: Which of the following is true about ventilation and perfusion in alveoli in an erect posture?

A. Ventilation/perfusion ratio is maximum at the apex (Correct Answer)
B. Ventilation/perfusion ratio is maximum at the base
C. Ventilation is maximum at the apex
D. Perfusion is maximum at the apex

Explanation: In the erect posture, gravity significantly influences the distribution of air and blood in the lungs. Understanding the regional differences in ventilation (V) and perfusion (Q) is a high-yield concept for NEET-PG. ### **Explanation of the Correct Answer** **Option A is correct.** While both ventilation and perfusion increase as we move from the apex to the base of the lung, they do not increase at the same rate. Perfusion (Q) increases much more steeply than ventilation (V) towards the base. * At the **apex**, both V and Q are low, but Q is disproportionately lower. This results in a **high V/Q ratio (~3.3)**. * At the **base**, both V and Q are high, but Q is disproportionately higher. This results in a **low V/Q ratio (~0.6)**. ### **Analysis of Incorrect Options** * **Option B:** Incorrect. The V/Q ratio is lowest at the base because the denominator (perfusion) increases significantly more than the numerator (ventilation). * **Option C:** Incorrect. Ventilation is **maximum at the base**. Due to gravity, the basal alveoli are more compressed (less expanded) at the end of expiration, making them more compliant and able to receive more air during inspiration compared to the already stretched apical alveoli. * **Option D:** Incorrect. Perfusion is **maximum at the base** due to the effects of gravity and hydrostatic pressure, which pull blood toward the lower parts of the lung. ### **NEET-PG High-Yield Pearls** * **West Zones:** The lung is divided into Zone 1 (Apex: $P_A > P_a > P_v$), Zone 2 (Mid: $P_a > P_A > P_v$), and Zone 3 (Base: $P_a > P_v > P_A$). * **Gas Exchange:** Because the V/Q ratio is highest at the apex, $P_{O2}$ is highest and $P_{CO2}$ is lowest at the apex. * **Clinical Correlation:** *Mycobacterium tuberculosis* prefers the apex of the lung because the high V/Q ratio provides a high-oxygen environment favorable for its growth.

Question 696: All of the following factors increase the level of respiratory neuron activity in the medulla, EXCEPT:

A. Rise in the PCO2
B. Rise in H+ concentration of arterial blood
C. Rise in PO2 (Correct Answer)
D. Drop in PO2

Explanation: The respiratory center in the medulla is responsible for the rhythmic generation of breathing. Its activity is primarily regulated by chemical stimuli sensed by central and peripheral chemoreceptors. **Why "Rise in PO2" is the correct answer:** A **Rise in PO2 (Hyperoxia)** actually **decreases** respiratory drive. When arterial oxygen levels are high, the peripheral chemoreceptors (carotid and aortic bodies) decrease their firing rate to the medulla. This leads to a reduction in the activity of respiratory neurons. Therefore, it is the only factor among the options that does not increase medullary activity. **Explanation of incorrect options:** * **Rise in PCO2 (Hypercapnia):** This is the most potent stimulus for respiration. CO2 diffuses across the blood-brain barrier, forming H+ ions in the CSF that directly stimulate **central chemoreceptors**, significantly increasing medullary activity. * **Rise in H+ concentration (Acidosis):** Increased arterial H+ stimulates **peripheral chemoreceptors**. Although H+ does not cross the blood-brain barrier easily, the peripheral signal strongly increases medullary respiratory output to blow off CO2 (Kussmaul breathing). * **Drop in PO2 (Hypoxia):** A decrease in arterial PO2 (specifically below 60 mmHg) is sensed by peripheral chemoreceptors, which send excitatory signals via the glossopharyngeal and vagus nerves to the medulla to increase ventilation. **High-Yield NEET-PG Pearls:** * **Central Chemoreceptors:** Located in the ventral medulla; sensitive to **H+ changes in CSF** (driven by arterial CO2). They do NOT respond to hypoxia. * **Peripheral Chemoreceptors:** Located in carotid and aortic bodies; primarily sensitive to **low PO2**, high PCO2, and low pH. * **Breaking Point:** The urge to breathe during breath-holding is primarily due to the **rise in PCO2**, not the fall in PO2.

Question 697: At an altitude of 6500 meters, where the atmospheric pressure is 347 mmHg, what is the inspired partial pressure of oxygen (PO2)?

A. 73 mm Hg
B. 63 mm Hg (Correct Answer)
C. 53 mm Hg
D. 83 mm Hg

Explanation: ### Explanation To calculate the inspired partial pressure of oxygen ($PiO_2$), we must account for the fact that as air enters the respiratory tract, it is warmed and fully saturated with water vapor before reaching the alveoli. **The Formula:** $PiO_2 = (P_{atm} - PH_2O) \times FiO_2$ * **$P_{atm}$ (Atmospheric Pressure):** 347 mmHg (given) * **$PH_2O$ (Water Vapor Pressure):** At normal body temperature (37°C), this is a constant **47 mmHg**. * **$FiO_2$ (Fraction of Inspired Oxygen):** Oxygen makes up approximately **21%** (0.21) of the atmosphere, a percentage that remains constant regardless of altitude. **Calculation:** 1. Subtract water vapor pressure from total pressure: $347 - 47 = 300 \text{ mmHg}$ 2. Multiply by the fraction of oxygen: $300 \times 0.21 = \mathbf{63 \text{ mmHg}}$ --- ### Analysis of Options * **B (63 mmHg) is Correct:** This correctly accounts for the humidification of air in the conducting zone. * **A (73 mmHg) is Incorrect:** This is the result if you forget to subtract the water vapor pressure ($347 \times 0.21 \approx 73$). * **C & D (53 & 83 mmHg) are Incorrect:** These values do not correlate with standard physiological calculations at this specific atmospheric pressure. --- ### High-Yield Clinical Pearls for NEET-PG * **The Constant $FiO_2$:** A common misconception is that the *percentage* of oxygen decreases at altitude. It does not; it remains 21%. Only the *total barometric pressure* (and thus the partial pressure) decreases. * **Water Vapor Pressure:** Always remember to subtract **47 mmHg** when calculating *inspired* or *alveolar* gas pressures. * **Alveolar Gas Equation:** To find Alveolar $PO_2$ ($PAO_2$), the formula extends further: $PAO_2 = PiO_2 - (PaCO_2 / R)$. * **Critical Altitude:** At the summit of Mt. Everest (~8848m), $P_{atm}$ is ~253 mmHg, making $PiO_2$ roughly 43 mmHg, which is near the limit of human tolerance without supplemental oxygen.

Question 698: What is the normal expiratory reserve volume of an adult?

A. 500 ml
B. 3000 ml
C. 1200 ml (Correct Answer)
D. 4500 ml

Explanation: **Explanation:** **Expiratory Reserve Volume (ERV)** is defined as the maximum volume of air that can be exhaled from the lungs by forceful expiration after the end of a normal tidal expiration. In a healthy adult male, the average value of ERV is approximately **1100 ml to 1200 ml**. **Analysis of Options:** * **Option C (1200 ml):** This is the correct physiological average for ERV. It represents the additional air available for expiration beyond the resting expiratory level. * **Option A (500 ml):** This represents the **Tidal Volume (TV)**, which is the volume of air inspired or expired during a single normal, quiet breath. * **Option B (3000 ml):** This corresponds to the **Inspiratory Reserve Volume (IRV)**, the maximum volume of air that can be inspired over and above the normal tidal volume. * **Option D (4500 ml):** This value represents the average **Vital Capacity (VC)**, which is the sum of IRV + TV + ERV. It is the maximum amount of air a person can expel from the lungs after a maximum inspiration. **High-Yield NEET-PG Clinical Pearls:** 1. **Functional Residual Capacity (FRC):** This is the sum of **ERV + Residual Volume (RV)**. It is the volume of air remaining in the lungs at the end of a normal expiration. 2. **Clinical Significance:** ERV is significantly reduced in **obstructive lung diseases** (due to air trapping) and **obesity**, where the chest wall compliance is decreased. 3. **Measurement:** Lung volumes like ERV and TV can be measured via simple **Spirometry**, whereas Residual Volume (RV), FRC, and Total Lung Capacity (TLC) require helium dilution or body plethysmography.

Question 699: The spirometer can estimate all of the following except?

A. Total lung capacity (Correct Answer)
B. Forced expiratory volume in 1 second
C. Peak expiratory flow
D. Vital capacity

Explanation: ### Explanation The correct answer is **Total Lung Capacity (TLC)**. **Why is TLC the correct answer?** A standard spirometer measures the volume of air moving into and out of the lungs. However, it cannot measure air that remains in the lungs after a maximal expiration. This "trapped" air is known as the **Residual Volume (RV)**. Since Total Lung Capacity is the sum of Vital Capacity and Residual Volume ($TLC = VC + RV$), and Functional Residual Capacity includes Residual Volume ($FRC = ERV + RV$), any lung volume or capacity containing RV **cannot** be measured by simple spirometry. To measure these, specialized techniques like Helium Dilution, Nitrogen Washout, or Body Plethysmography are required. **Analysis of Incorrect Options:** * **Forced Expiratory Volume in 1 second (FEV1):** This is a dynamic volume measured during a forced expiratory maneuver into a spirometer. It is the gold standard for diagnosing obstructive lung diseases. * **Peak Expiratory Flow (PEF):** This represents the maximum speed of expiration. While often measured by a Peak Flow Meter, it can be accurately derived from a spirometric flow-volume loop. * **Vital Capacity (VC):** This is the maximum volume of air a person can exhale after a maximum inhalation. Since it involves only "movable" air, it is easily measured by spirometry. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of RV":** Spirometry cannot measure **RV, FRC, or TLC**. * **Obstructive vs. Restrictive:** In obstructive diseases (e.g., Asthma, COPD), the FEV1/FVC ratio is **decreased** (<0.7). In restrictive diseases (e.g., Fibrosis), the ratio is **normal or increased**, but the TLC is decreased. * **Gold Standard:** Body Plethysmography is the most accurate method for measuring FRC and TLC as it accounts for air trapped behind closed airways.

Question 700: Which of the following relaxes bronchial smooth muscles?

A. Cold air
B. Leukotriene
C. Acetylcholine
D. Vasoactive intestinal peptide (Correct Answer)

Explanation: **Explanation:** The tone of bronchial smooth muscle is regulated by the autonomic nervous system and various local mediators. Bronchodilation is primarily mediated by the sympathetic nervous system (via $\beta_2$ receptors) and the **Non-Adrenergic Non-Cholinergic (NANC)** inhibitory system. **Why Vasoactive Intestinal Peptide (VIP) is Correct:** VIP is the primary neurotransmitter of the **inhibitory NANC system** in the airways. It is co-released with Nitric Oxide (NO) from parasympathetic postganglionic neurons. VIP acts on specific receptors to increase intracellular cAMP and decrease calcium levels, leading to the **relaxation of bronchial smooth muscle**. It is considered one of the most potent endogenous bronchodilators. **Analysis of Incorrect Options:** * **Cold Air:** Acts as a physical trigger that induces bronchoconstriction, often via a reflex arc or by causing mast cell degranulation. This is a classic trigger for Exercise-Induced Bronchospasm (EIB). * **Leukotrienes (LTC4, LTD4, LTE4):** These are potent bronchoconstrictors produced via the lipoxygenase pathway of arachidonic acid metabolism. They are significantly more potent than histamine in inducing airway narrowing in asthma. * **Acetylcholine (ACh):** This is the primary neurotransmitter of the parasympathetic nervous system. It acts on **M3 muscarinic receptors** in the airways to cause bronchoconstriction and increased mucus secretion. **NEET-PG High-Yield Pearls:** * **Mnemonic for Bronchoconstrictors:** "HALT" (Histamine, Acetylcholine, Leukotrienes, Thromboxane A2). * **NANC System:** The excitatory NANC system uses Substance P and Neurokinin A to cause constriction; the inhibitory NANC system uses **VIP and NO** to cause relaxation. * **Clinical Correlation:** Anticholinergics (e.g., Ipratropium bromide) work by blocking the bronchoconstrictor effect of Acetylcholine.

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Pulmonary Ventilation

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Pulmonary Circulation

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Control of Breathing

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