Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 261: In the presence of vasopressin, what is the greatest fraction of filtered water absorbed?

A. Proximal tubule (Correct Answer)
B. Loop of Henle
C. Distal tubule
D. Collecting duct

Explanation: **Explanation:** The correct answer is **Proximal Tubule (A)**. This question tests the understanding of obligatory versus facultative water reabsorption in the nephron. 1. **Why Proximal Tubule is Correct:** Regardless of the presence or absence of Vasopressin (ADH), the **Proximal Convoluted Tubule (PCT)** is the site of the greatest water reabsorption. Approximately **65-70%** of the filtered water is reabsorbed here via osmosis, following the active transport of sodium (iso-osmotic reabsorption). This is known as **obligatory water reabsorption** and occurs independently of ADH levels. 2. **Why Incorrect Options are Wrong:** * **Loop of Henle (B):** About 15% of filtered water is reabsorbed in the descending limb. The ascending limb is impermeable to water. * **Distal Tubule (C):** Only a small fraction (approx. 5%) of water is reabsorbed here. * **Collecting Duct (D):** This is the site of **facultative water reabsorption** mediated by Vasopressin (via V2 receptors and Aquaporin-2 channels). While Vasopressin significantly increases the permeability of this segment, the total volume reabsorbed here is only about **10-15%** of the filtered load. Even under maximal ADH stimulation, it never exceeds the volume reabsorbed by the PCT. **High-Yield Clinical Pearls for NEET-PG:** * **Iso-osmotic Reabsorption:** The fluid leaving the PCT is always iso-osmotic to plasma (290-300 mOsm/L). * **V2 Receptors:** Vasopressin acts on V2 receptors in the late distal tubule and collecting ducts to insert **Aquaporin-2** channels. * **Free Water Clearance:** In the absence of ADH (e.g., Diabetes Insipidus), the collecting duct remains impermeable to water, leading to the excretion of large volumes of dilute urine.

Question 262: Which parameter best denotes airway resistance?

A. Vital Capacity
B. Mid respiratory flow rates (Correct Answer)
C. FEV1
D. Total volume

Explanation: **Explanation:** **Airway resistance (Raw)** is the resistance to airflow in the respiratory tract. While several parameters provide information about lung function, **Mid-respiratory flow rates (specifically FEF 25-75%)** are considered the most sensitive indicators of airway resistance, particularly in the **small airways** (bronchioles < 2mm in diameter). 1. **Why Mid-respiratory flow rates (FEF 25-75%) is correct:** This parameter measures the average flow rate during the middle half of a forced expiration. Unlike the initial part of expiration, which is effort-dependent, the middle phase is **effort-independent** and reflects the status of the peripheral, small airways. Since the small airways are the earliest sites of resistance changes in obstructive diseases (like early COPD or asthma), this is the most accurate clinical marker for resistance. 2. **Why other options are incorrect:** * **Vital Capacity (VC):** This is a **volume** measurement (static lung volume), not a flow measurement. It indicates lung size and expansion but does not measure the resistance encountered during airflow. * **FEV1:** While FEV1 (Forced Expiratory Volume in 1 second) is used to diagnose obstructive disease, it is highly **effort-dependent** and primarily reflects resistance in the **large, central airways**. It is less sensitive than FEF 25-75% for early small-airway disease. * **Tidal Volume (TV):** This is simply the volume of air moved in or out during a normal breath. It does not provide information regarding the resistance or flow dynamics of the airways. **High-Yield Clinical Pearls for NEET-PG:** * The **major site of airway resistance** in the normal lung is the **medium-sized bronchi** (not the smallest bronchioles, due to their massive total cross-sectional area). * **FEF 25-75%** is often the first parameter to decline in smokers, making it the "gold standard" for detecting **early small airway obstruction**. * Airway resistance is **inversely proportional** to lung volume (as lung volume increases, Raw decreases due to radial traction).

Question 263: What change occurs in blood as it passes through systemic capillaries, EXCEPT for one of the following?

A. Increase in hematocrit
B. pH decreases
C. Shift of oxygen dissociation curve to the left (Correct Answer)
D. Increase in protein content

Explanation: ### Explanation In systemic capillaries, blood undergoes specific physiological changes as it exchanges gases and solutes with tissues. **Why Option C is the Correct Answer (The Exception):** As blood passes through systemic capillaries, it picks up **CO₂** and **H⁺ ions** (metabolic byproducts). This increase in $PCO_2$ and decrease in pH causes a **Rightward Shift** of the Oxygen Dissociation Curve (ODC), known as the **Bohr Effect**. A right shift decreases hemoglobin's affinity for oxygen, facilitating oxygen unloading to the tissues. Therefore, a "shift to the left" is incorrect in this context. **Analysis of Incorrect Options:** * **A. Increase in hematocrit:** As $CO_2$ enters RBCs, it is converted to $HCO_3^-$ and $H^+$. The $HCO_3^-$ exits the cell in exchange for $Cl^-$ (**Chloride Shift/Hamburger Phenomenon**). This increase in intracellular osmotically active particles causes water to enter the RBCs, making them swell and slightly increasing the hematocrit in venous blood. * **B. pH decreases:** Tissues produce $CO_2$, which reacts with water to form carbonic acid ($H_2CO_3$), which dissociates into $H^+$ and $HCO_3^-$. This increase in $H^+$ concentration lowers the blood pH. * **D. Increase in protein content:** Due to hydrostatic pressure in the capillaries, some fluid (plasma) filters into the interstitial space, while large proteins remain in the vessel. This slight loss of fluid leads to a relative increase in the concentration of plasma proteins in the venous end. **High-Yield NEET-PG Pearls:** * **Right Shift (CADET, face Right!):** **C**O₂, **A**cidosis, **D**PG (2,3-BPG), **E**xercise, and **T**emperature all increase in systemic capillaries and shift the ODC to the right. * **Chloride Shift:** Occurs in systemic capillaries ($Cl^-$ moves into RBCs). * **Reverse Chloride Shift:** Occurs in pulmonary capillaries ($Cl^-$ moves out of RBCs).

Question 264: If pulmonary arterial pressure > alveolar pressure > venous pressure (Pa > Palv > Pv), under normal circumstances, which zone of the lung is being described?

A. Zone I
B. Zone II (Correct Answer)
C. Zone III
D. Zone IV

Explanation: **Explanation:** The distribution of pulmonary blood flow is determined by the relationship between three pressures: **Pulmonary Arterial Pressure (Pa)**, **Alveolar Pressure (Palv)**, and **Pulmonary Venous Pressure (Pv)**. This concept is known as West’s Zones of the lung. **Why Zone II is correct:** In **Zone II (the middle zone)**, the relationship is **Pa > Palv > Pv**. Here, arterial pressure is high enough to overcome alveolar pressure, but alveolar pressure is higher than venous pressure. This creates a "Starling Resistor" or "Waterfall effect," where blood flow is determined by the difference between arterial and alveolar pressure, rather than the usual arterial-venous gradient. Flow occurs intermittently, primarily during systole. **Analysis of Incorrect Options:** * **Zone I (Palv > Pa > Pv):** Alveolar pressure exceeds arterial pressure, compressing the capillaries and resulting in no blood flow (Physiological Dead Space). This is not found under normal conditions but occurs in hemorrhage or positive pressure ventilation. * **Zone III (Pa > Pv > Palv):** Both arterial and venous pressures exceed alveolar pressure. The capillaries remain permanently open, and blood flow is continuous and highest here due to gravity. * **Zone IV:** This is a pathological zone (often seen in pulmonary edema) where interstitial pressure exceeds other pressures, reducing flow at the extreme lung bases. **High-Yield Pearls for NEET-PG:** * **Gravity Effect:** Blood flow and ventilation both increase as you move from the apex to the base, but blood flow increases more steeply. * **V/Q Ratio:** Highest at the **Apex** (~3.3) and lowest at the **Base** (~0.6). * **Postural Change:** In a supine position, the lung behaves entirely as Zone III. Zone I is typically only seen in upright individuals with low pulmonary pressures.

Question 265: What reflex is responsible for tachycardia during right ventricle distention?

A. Bezold-Jaisch reflex
B. Bainbridge reflex (Correct Answer)
C. Cushing reflex
D. J-reflex

Explanation: ### Explanation **Correct Answer: B. Bainbridge Reflex** The **Bainbridge reflex** (also known as the atrial distension reflex) is a compensatory mechanism where an increase in venous return leads to an increase in heart rate (tachycardia). * **Mechanism:** When the right atrium or right ventricle is distended due to increased blood volume, stretch receptors (low-pressure baroreceptors) located in the veno-atrial junctions are stimulated. * **Pathway:** Afferent signals travel via the **vagus nerve** to the medulla (nucleus tractus solitarius). This triggers an increase in sympathetic activity and a decrease in parasympathetic tone, resulting in tachycardia to pump the excess volume forward and prevent pulmonary congestion. --- ### Analysis of Incorrect Options: * **A. Bezold-Jarisch Reflex:** This is the "cardio-inhibitory" reflex. It involves a triad of **bradycardia, hypotension, and apnea** in response to noxious stimuli (like chemical triggers or myocardial infarction) sensed by ventricular receptors. It is essentially the opposite of the Bainbridge reflex. * **C. Cushing Reflex:** This is a nervous system response to **increased intracranial pressure (ICP)**. It presents as a triad of hypertension, bradycardia, and irregular respiration. * **D. J-reflex (Juxtacapillary reflex):** Triggered by J-receptors located in the alveolar walls near pulmonary capillaries. Stimulation (due to pulmonary edema or congestion) leads to **rapid shallow breathing (tachypnea)**, bradycardia, and hypotension. --- ### High-Yield Clinical Pearls for NEET-PG: * **Bainbridge vs. Baroreceptor Reflex:** These two often work in opposition. If blood volume increases, Bainbridge increases HR; however, the resulting increase in BP triggers the Baroreceptor reflex to decrease HR. The final heart rate depends on the net effect. * **Respiratory Sinus Arrhythmia:** The Bainbridge reflex is partially responsible for the increase in heart rate during inspiration (as venous return increases). * **Afferent/Efferent:** For the Bainbridge reflex, both the afferent and efferent limbs involve the **Vagus nerve** (though the efferent effect is sympathetic dominance).

Question 266: In an obese patient, what is the most likely respiratory change?

A. Functional residual capacity (FRC) is unchanged
B. Alveolar hypoventilation (Correct Answer)
C. Residual volume is decreased
D. Normal work of breathing

Explanation: ### Explanation Obesity significantly impacts respiratory mechanics due to the accumulation of adipose tissue around the thorax and abdomen, leading to a restrictive pattern of lung disease. **Why "Alveolar Hypoventilation" is correct:** In morbid obesity, the excess weight on the chest wall and diaphragm reduces **chest wall compliance**. This increases the elastic work required to breathe, leading to a breathing pattern characterized by low tidal volumes and high respiratory rates. This shallow breathing, combined with ventilation-perfusion (V/Q) mismatching in the lower lung zones (due to airway closure), leads to **alveolar hypoventilation**. In severe cases, this progresses to Obesity Hypoventilation Syndrome (Pickwickian Syndrome), characterized by hypercapnia ($PaCO_2 > 45$ mmHg) and chronic hypoxia. **Analysis of Incorrect Options:** * **A. Functional Residual Capacity (FRC) is unchanged:** This is incorrect. FRC is the most significantly affected lung volume in obesity. It **decreases** because the outward elastic recoil of the chest wall is compromised by excess fat, shifting the equilibrium point of the respiratory system downward. * **C. Residual Volume (RV) is decreased:** This is generally incorrect. While FRC and Expiratory Reserve Volume (ERV) decrease significantly, the **RV usually remains relatively preserved** or unchanged in simple obesity. * **D. Normal work of breathing:** This is incorrect. The work of breathing is **markedly increased** (often 3–4 times higher) due to decreased compliance and increased airway resistance. **High-Yield NEET-PG Pearls:** * **Most sensitive indicator** of obesity-related lung change: **Decreased ERV** (Expiratory Reserve Volume). * **FRC < Closing Capacity:** In obesity, FRC often falls below the closing capacity, leading to small airway collapse and shunting during normal tidal breathing. * **Lung Compliance:** Usually normal (unless there is pulmonary congestion), but **Chest Wall Compliance** is significantly decreased.

Question 267: What is true for restrictive lung disease?

A. FEV1/FVC decreased, compliance normal
B. FEV1/FVC increased, compliance increased
C. FEV1/FVC decreased, compliance increased
D. FEV1/FVC increased, compliance decreased (Correct Answer)

Explanation: **Explanation** In **Restrictive Lung Disease (RLD)**, such as interstitial lung disease or pulmonary fibrosis, the primary pathology is a reduction in lung expansion. 1. **Compliance:** The lungs become "stiff" due to fibrosis or chest wall restrictions. This leads to a **decrease in lung compliance** (the ability of the lungs to stretch), making it harder to inhale and reducing the Total Lung Capacity (TLC). 2. **FEV1/FVC Ratio:** While both the Forced Expiratory Volume in 1 second (FEV1) and the Forced Vital Capacity (FVC) decrease, the **FVC decreases more significantly** than the FEV1. Additionally, increased radial traction from fibrotic tissue keeps the airways open during expiration. Consequently, the **FEV1/FVC ratio is either normal or increased** (typically >0.7 or 70%). **Analysis of Incorrect Options:** * **Option A & C:** A **decreased FEV1/FVC ratio** is the hallmark of **Obstructive Lung Diseases** (e.g., Asthma, COPD), where airway resistance is increased. * **Option B & C:** **Increased compliance** is characteristic of **Emphysema**, where the destruction of alveolar elastic fibers makes the lungs overly distensible but prone to collapse during expiration. **High-Yield Clinical Pearls for NEET-PG:** * **Flow-Volume Loop:** In RLD, the loop shifts to the **right** and appears "tall and narrow" (Witch’s Hat appearance). * **Total Lung Capacity (TLC):** A reduction in TLC is the gold standard for diagnosing restriction. * **Radial Traction:** In fibrosis, increased elastic recoil (radial traction) increases expiratory flow rates relative to lung volume, explaining the supranormal FEV1/FVC ratio.

Question 268: Which of the following statements best describes the factors affecting the diffusion of a gas across the respiratory membrane?

A. Diffusion is directly proportional to the thickness of the respiratory membrane.
B. Diffusion is inversely proportional to the molecular weight of the gas. (Correct Answer)
C. Diffusion is independent of the lipid-water solubility of the gas.
D. Diffusion is directly proportional to the pressure gradient across the membrane.

Explanation: The diffusion of gases across the respiratory membrane is governed by **Fick’s Law of Diffusion**. Understanding this law is crucial for mastering pulmonary physiology and clinical gas exchange. ### Why Option B is Correct According to **Graham’s Law**, the rate of diffusion of a gas is **inversely proportional to the square root of its molecular weight** ($D \propto 1/\sqrt{MW}$). While the option simplifies this to "inversely proportional," it correctly identifies that heavier molecules diffuse more slowly than lighter ones. ### Analysis of Incorrect Options * **Option A:** Diffusion is **inversely proportional** to the thickness of the membrane. Conditions like pulmonary fibrosis increase membrane thickness, thereby reducing gas exchange. * **Option C:** Diffusion is **directly proportional** to the solubility of the gas. Carbon dioxide ($CO_2$) is approximately 20–24 times more soluble in water/lipids than Oxygen ($O_2$), allowing it to diffuse much faster despite having a higher molecular weight. * **Option D:** While this statement is technically true (Diffusion $\propto \Delta P$), the question asks for the "best" description among the choices provided. In many standardized formats, Option B is the classic physiological principle tested under Graham's Law. *Note: In some interpretations, D is also a primary factor; however, B specifically tests the physical property of the gas itself.* ### NEET-PG High-Yield Pearls * **Diffusion Capacity ($D_L$):** Carbon monoxide ($CO$) is used to measure the diffusing capacity of the lung ($DL_{CO}$) because it is **diffusion-limited**, not perfusion-limited. * **CO2 vs. O2:** Even though $O_2$ is lighter, $CO_2$ diffuses faster because its **solubility coefficient** is much higher. * **Surface Area:** Diffusion is directly proportional to surface area. Emphysema reduces the surface area, leading to impaired gas exchange. * **Formula to Remember:** $Rate \propto \frac{Area \times \Delta P \times Solubility}{Thickness \times \sqrt{MW}}$

Question 269: What is the effect of prolonged oxygen therapy?

A. Endothelial damage
B. Increased pulmonary compliance
C. Atelectasis (Correct Answer)
D. Decreased vital capacity

Explanation: **Explanation:** The correct answer is **Atelectasis**, specifically **Absorption Atelectasis**. 1. **Why it is correct:** In normal room air, nitrogen (which is poorly soluble) stays in the alveoli and acts as a "stent" to keep them open. When a patient receives 100% oxygen for a prolonged period, the nitrogen is "washed out" and replaced by oxygen. Since oxygen is rapidly absorbed into the pulmonary capillaries, the volume of the alveoli decreases rapidly. If the rate of absorption exceeds the rate of ventilation, the alveoli collapse, leading to absorption atelectasis. 2. **Why the other options are incorrect:** * **Endothelial damage:** While prolonged high-dose oxygen can cause oxygen toxicity (via free radicals), the primary damage occurs at the **alveolar epithelium** and capillary endothelium leading to pulmonary edema/fibrosis, but "Absorption Atelectasis" is the more immediate and classic physiological consequence described in respiratory mechanics. * **Increased pulmonary compliance:** Prolonged oxygen therapy actually **decreases** compliance. Oxygen toxicity damages Type II pneumocytes, leading to decreased surfactant production, making the lungs stiffer. * **Decreased vital capacity:** While vital capacity may eventually decrease due to fibrosis or collapse, it is a *result* of the underlying pathology (like atelectasis), not the primary physiological effect usually tested in this context. **High-Yield Clinical Pearls for NEET-PG:** * **Nitrogen Washout:** The process of replacing alveolar nitrogen with oxygen is the physiological basis for pre-oxygenation before intubation. * **Oxygen Toxicity (Lorrain Smith Effect):** High partial pressures of $O_2$ lead to the formation of Reactive Oxygen Species (ROS), causing lung parenchymal damage similar to ARDS. * **Retinopathy of Prematurity (ROP):** In neonates, prolonged $O_2$ therapy causes vasoconstriction followed by abnormal vascular proliferation in the retina.

Question 270: Which nucleus is primarily responsible for the generation of the respiratory rhythm?

A. Pneumotaxic centre
B. Dorsal group of nucleus
C. Apneustic centre
D. Pre-Botzinger Complex (Correct Answer)

Explanation: **Explanation:** The generation of the respiratory rhythm is a complex process controlled by the medullary respiratory centers. **1. Why Pre-Bötzinger Complex is correct:** The **Pre-Bötzinger Complex (pre-BötC)**, located in the upper part of the Ventral Respiratory Group (VRG) in the medulla, is considered the **pacemaker of respiration**. It contains a cluster of interneurons that display spontaneous, rhythmic discharges. These neurons initiate the basic respiratory rhythm, which is then transmitted to the motor neurons controlling the diaphragm and other inspiratory muscles. **2. Why other options are incorrect:** * **Pneumotaxic Centre:** Located in the upper pons (Nucleus Parabrachialis), its primary role is to act as an "off-switch" for inspiration. It limits the duration of inspiration, thereby increasing the respiratory rate. * **Dorsal Respiratory Group (DRG):** Located in the Nucleus Tractus Solitarius (NTS), the DRG is primarily responsible for **inspiration**. While it sends the basic motor drive to the diaphragm, it does not generate the rhythm itself; it receives input from the pre-BötC. * **Apneustic Centre:** Located in the lower pons, it promotes long, deep inspirations (apneustic breathing) by delaying the "off-switch" signal. It is normally inhibited by the pneumotaxic center. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Pre-BötC is situated between the nucleus ambiguus and the lateral reticular nucleus. * **Opioid Sensitivity:** This complex is highly sensitive to **opioids and barbiturates**, which is why respiratory depression is a hallmark of overdose. * **Hering-Breuer Reflex:** This is a protective reflex that prevents over-inflation of the lungs, mediated by stretch receptors via the Vagus nerve (CN X) to the DRG.

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