Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 441: The following illustration of the Spirogram showing the arrow indicates:

A. Tidal volume
B. Expiratory reserve volume
C. Vital capacity (Correct Answer)
D. Total lung capacity

Explanation: ***Vital capacity*** - **Vital capacity** is the maximum volume of air that can be exhaled after maximum inspiration, representing the sum of **inspiratory reserve volume (IRV)**, **tidal volume (TV)**, and **expiratory reserve volume (ERV)**. - It typically measures around **4800 mL** in healthy adults and is a key indicator of **respiratory muscle strength** and **lung function**. *Tidal volume* - **Tidal volume** represents only the normal volume of air inhaled or exhaled during quiet breathing, typically around **500 mL**. - It is a much smaller component within the vital capacity and would not encompass the entire range shown by the arrow in a spirogram. *Expiratory reserve volume* - **Expiratory reserve volume** is the additional air that can be exhaled after normal expiration, typically around **1100 mL**. - It represents only one component of vital capacity and would be shown as a smaller segment below the tidal volume baseline. *Total lung capacity* - **Total lung capacity** includes the **residual volume** (air remaining in lungs after maximum expiration) which cannot be measured by spirometry. - It represents the sum of vital capacity plus **residual volume** (~1200 mL), making it larger than what can be demonstrated on a standard spirogram.

Question 442: Which is responsible for respiratory drive?

A. O2
B. CO
C. CO2 (Correct Answer)
D. Bicarbonate ions

Explanation: **Explanation:** The primary physiological stimulus for the respiratory drive in a healthy individual is the partial pressure of arterial carbon dioxide (**PaCO2**). **Why CO2 is the correct answer:** The respiratory center in the medulla is exquisitely sensitive to changes in PaCO2. This occurs via two mechanisms: 1. **Central Chemoreceptors:** Located on the ventral surface of the medulla, these are the most important. While H+ ions cannot cross the blood-brain barrier (BBB), CO2 diffuses readily. Once in the CSF, CO2 reacts with water to form H+ and HCO3-. The resulting drop in pH directly stimulates the chemoreceptors, increasing the rate and depth of respiration. 2. **Peripheral Chemoreceptors:** Located in the carotid and aortic bodies, these also respond to hypercapnia (high CO2) and acidosis, though they are primarily known for sensing hypoxia. **Analysis of Incorrect Options:** * **O2:** Under normal conditions, oxygen plays a secondary role. The "hypoxic drive" only becomes the primary stimulus when PaO2 drops significantly (below 60 mmHg), often seen in chronic lung diseases like COPD. * **CO (Carbon Monoxide):** CO has a high affinity for hemoglobin but does not affect the dissolved PaO2 or PaCO2 significantly. Therefore, it does not stimulate the respiratory centers, which is why CO poisoning is often "silent." * **Bicarbonate ions:** While part of the buffering system, HCO3- ions do not cross the BBB easily and do not act as the primary trigger for the respiratory drive. **High-Yield NEET-PG Pearls:** * **Most potent stimulus for Central Chemoreceptors:** H+ ions in the CSF (derived from CO2). * **Most potent stimulus for Peripheral Chemoreceptors:** Low PaO2 (< 60 mmHg). * **Breaking Point of Breath-holding:** This is reached when PaCO2 rises to about 50 mmHg. * **COPD Clinical Note:** In chronic hypercapnia, central receptors desensitize, and the drive shifts to O2 (Hypoxic Drive). Administering high-flow O2 to these patients can suppress their respiratory drive.

Question 443: Exocytosis requires which ion?

A. Na+
B. K+
C. Ca+ (Correct Answer)
D. Mg+2

Explanation: **Explanation:** **1. Why Calcium (Ca²⁺) is the Correct Answer:** Exocytosis is the process by which a cell transports secretory vesicles to the cell membrane to release their contents into the extracellular space. This process is fundamentally **Calcium-dependent**. When an action potential reaches a nerve terminal or a secretory cell, it triggers the opening of **voltage-gated calcium channels**. The resulting influx of Ca²⁺ ions acts as a second messenger. These ions bind to specific calcium-sensing proteins (such as **synaptotagmin**) located on the vesicle membrane. This binding triggers the **SNARE complex** to facilitate the fusion of the vesicle with the plasma membrane, leading to the release of neurotransmitters, hormones, or enzymes. **2. Why Other Options are Incorrect:** * **Na⁺ (Sodium):** Primarily involved in the depolarization phase of the action potential. While it initiates the electrical signal, it does not directly trigger the fusion of vesicles. * **K⁺ (Potassium):** Responsible for the repolarization phase and maintaining the resting membrane potential. High extracellular K⁺ can cause depolarization, but it is the subsequent Ca²⁺ entry that causes exocytosis. * **Mg²⁺ (Magnesium):** Magnesium actually acts as a **natural calcium channel blocker**. High levels of Mg²⁺ can inhibit exocytosis by competing with Ca²⁺ at the presynaptic terminals (e.g., inhibiting Acetylcholine release at the neuromuscular junction). **3. NEET-PG High-Yield Pearls:** * **SNARE Proteins:** Remember **V-SNARE** (Synaptobrevin) on the vesicle and **T-SNARE** (Syntaxin, SNAP-25) on the target membrane. * **Toxins:** *Clostridium botulinum* and *Clostridium tetani* toxins work by cleaving these SNARE proteins, thereby preventing exocytosis of neurotransmitters. * **Lambert-Eaton Syndrome:** Antibodies against voltage-gated Ca²⁺ channels lead to reduced exocytosis of Acetylcholine, causing muscle weakness.

Question 444: Tidal volume is calculated by?

A. Inspiratory capacity minus the inspiratory reserve volume (Correct Answer)
B. Total lung capacity minus the residual volume
C. Functional residual capacity minus residual volume
D. Vital capacity minus expiratory reserve volume

Explanation: **Explanation** To solve this question, one must understand the relationship between **Lung Volumes** (primary measurements) and **Lung Capacities** (sums of two or more volumes). **1. Why Option A is Correct:** Inspiratory Capacity (IC) is the maximum volume of air that can be inspired after a normal tidal expiration. It is the sum of the **Tidal Volume (TV)** and the **Inspiratory Reserve Volume (IRV)**. * **Formula:** $IC = TV + IRV$ * Therefore, **$TV = IC - IRV$**. This mathematical derivation makes Option A the correct calculation for Tidal Volume. **2. Analysis of Incorrect Options:** * **Option B:** Total Lung Capacity (TLC) minus Residual Volume (RV) equals **Vital Capacity (VC)**, not TV. ($VC = TLC - RV$) * **Option C:** Functional Residual Capacity (FRC) minus Residual Volume (RV) equals **Expiratory Reserve Volume (ERV)**. ($FRC = ERV + RV$) * **Option D:** Vital Capacity (VC) minus Expiratory Reserve Volume (ERV) equals **Inspiratory Capacity (IC)**. ($VC = IC + ERV$) **Clinical Pearls for NEET-PG:** * **Tidal Volume (TV):** Normal value is approximately **500 mL** in a healthy adult. * **Dead Space:** Out of 500 mL TV, only ~350 mL reaches the alveoli (Alveolar ventilation); ~150 mL remains in the conducting airways (Anatomical Dead Space). * **Spirometry:** Remember that **Residual Volume (RV)**, **Functional Residual Capacity (FRC)**, and **Total Lung Capacity (TLC)** cannot be measured by simple spirometry (they require helium dilution or body plethysmography). * **High-Yield Equation:** $Minute Ventilation = TV \times Respiratory Rate$.

Question 445: The curve indicated by the question mark is caused by which condition?

A. Bronchial asthma
B. Emphysema
C. Pulmonary fibrosis (Correct Answer)
D. Chronic bronchitis

Explanation: ***Pulmonary fibrosis*** - Shows a **restrictive pattern** with a small, proportionally reduced flow-volume loop where both **FVC** and **peak expiratory flow rate (PEFR)** are decreased, but the **PEFR/FVC ratio** remains normal or elevated. - The loop maintains its **normal shape** but is smaller overall due to **reduced lung compliance** and **decreased total lung capacity**. *Bronchial asthma* - Produces an **obstructive pattern** with a characteristic **scooped-out expiratory limb** due to **airway narrowing** and **bronchospasm**. - Shows **reduced peak expiratory flow** with **prolonged expiration** and potential **air trapping**, creating a concave expiratory curve. *Emphysema* - Demonstrates **severe obstructive pattern** with **markedly reduced peak flow** and a **severely scooped expiratory limb** due to **loss of elastic recoil**. - Features **significant air trapping** with **increased residual volume** and a **flattened, prolonged expiratory phase**. *Chronic bronchitis* - Shows **obstructive pattern** with **reduced expiratory flow rates** and a **scooped-out expiratory limb** due to **airway inflammation** and **mucus hypersecretion**. - Exhibits **impaired airway clearance** leading to **flow limitation** and **abnormal expiratory curve morphology**.

Question 446: What is the volume of air in the lungs after a normal expiration called?

A. Functional Residual Capacity (FRC) (Correct Answer)
B. Expiratory Reserve Volume (ERV)
C. Residual Volume (RV)
D. Tidal Capacity (TC)

Explanation: ### Explanation The correct answer is **Functional Residual Capacity (FRC)**. **1. Why FRC is Correct:** Functional Residual Capacity is defined as the volume of air remaining in the lungs at the end of a **normal (passive) expiration**. It represents the equilibrium point of the respiratory system where the inward elastic recoil of the lungs exactly balances the outward chest wall recoil. Mathematically, it is the sum of Expiratory Reserve Volume and Residual Volume (**FRC = ERV + RV**). **2. Why Other Options are Incorrect:** * **Expiratory Reserve Volume (ERV):** This is the maximum volume of air that can be exhaled *after* a normal tidal expiration. It is a component of FRC, not the total volume remaining. * **Residual Volume (RV):** This is the volume of air remaining in the lungs after a **maximal** forced expiration. It cannot be measured by simple spirometry. * **Tidal Capacity (TC):** This is a distractor term. The correct term is **Tidal Volume (TV)**, which is the volume of air inspired or expired during a single normal breath (approx. 500 mL). **3. NEET-PG High-Yield Pearls:** * **Measurement:** FRC cannot be measured by spirometry (because it contains RV). It is measured via **Helium Dilution**, **Nitrogen Washout**, or **Body Plethysmography**. * **Clinical Significance:** FRC acts as a buffer to prevent large fluctuations in alveolar gas tensions ($PaO_2$) during the respiratory cycle. * **Positioning:** FRC **decreases** in the supine position (due to abdominal contents pushing against the diaphragm) and in obesity, pregnancy, and restrictive lung diseases. * **Closing Capacity:** If FRC falls below the "Closing Capacity," small airways collapse during normal breathing, leading to V/Q mismatch.

Question 447: A man has an intrapleural pressure of -5 cm H2O before inspiration and -10 cm H2O at the end of inspiration. He inspires 1200 ml. What is the compliance of the lungs?

A. 50 ml/cm H2O
B. 120 ml/cm H2O
C. 240 ml/cm H2O (Correct Answer)
D. 250 ml/cm H2O

Explanation: ### Explanation **1. Understanding the Correct Answer (C: 240 ml/cm H₂O)** Lung compliance ($C_L$) is defined as the change in lung volume ($\Delta V$) per unit change in transpulmonary pressure ($\Delta P$). In a static state (at the beginning and end of inspiration), the change in intrapleural pressure reflects the change in pressure required to expand the lungs. The formula is: $$Compliance (C) = \frac{\Delta V}{\Delta P}$$ * **Change in Volume ($\Delta V$):** 1200 ml * **Change in Pressure ($\Delta P$):** $(-5 \text{ cm H}_2\text{O}) - (-10 \text{ cm H}_2\text{O}) = 5 \text{ cm H}_2\text{O}$ * **Calculation:** $1200 \text{ ml} / 5 \text{ cm H}_2\text{O} = \mathbf{240 \text{ ml/cm H}_2\text{O}}$ **2. Why Other Options are Incorrect** * **A (50 ml/cm H₂O):** This value is too low for normal human lungs and would represent a highly "stiff" lung (e.g., severe fibrosis). * **B (120 ml/cm H₂O):** This would result if the pressure change was 10 cm H₂O instead of 5. * **D (250 ml/cm H₂O):** While close to the average normal value (200–250 ml/cm H₂O), it does not match the specific mathematical calculation derived from the data provided in the question. **3. Clinical Pearls & High-Yield Facts** * **Normal Value:** The average compliance of the combined human lung-chest wall system is approximately **110 ml/cm H₂O**, while the lungs alone are roughly **200 ml/cm H₂O**. * **Increased Compliance:** Seen in **Emphysema** due to the loss of elastic fibers (the lung is "easy to blow up" but has poor elastic recoil). * **Decreased Compliance:** Seen in **Pulmonary Fibrosis**, Pulmonary Edema, and Lack of Surfactant (the lung is "stiff"). * **Surfactant:** Increases compliance by reducing surface tension, preventing alveolar collapse at low volumes.

Question 448: What substance in the human body performs the action of surfactant?

A. Sugar and salt
B. Soap and water
C. Lipid and protein (Correct Answer)
D. Base and lipid

Explanation: **Explanation:** Pulmonary surfactant is a surface-active lipoprotein complex secreted by **Type II alveolar epithelial cells** (pneumocytes). Its primary function is to reduce surface tension at the air-liquid interface of the alveoli, preventing them from collapsing during expiration (atelectasis) and increasing lung compliance. **Why Lipid and Protein is correct:** The composition of surfactant is approximately **90% lipids and 10% proteins**. * **Lipids:** The most abundant and functional component is **Dipalmitoylphosphatidylcholine (DPPC)**, also known as lecithin. It is responsible for the reduction of surface tension. * **Proteins:** It contains surfactant-specific proteins (SP-A, SP-B, SP-C, and SP-D). SP-B and SP-C are essential for the spreading of the surfactant film. **Why other options are incorrect:** * **A & B:** Sugar, salt, soap, and water are not physiological components of the alveolar lining. While soap acts as a surfactant in a laboratory setting by breaking surface tension, it is not the biological substance found in the human body. * **D:** While lipids are a major component, "base" is a generic chemical term and does not represent the specific protein-lipid complex required for respiratory function. **High-Yield Clinical Pearls for NEET-PG:** * **L/S Ratio:** A Lecithin-to-Sphingomyelin ratio of **>2:1** in amniotic fluid indicates fetal lung maturity. * **NRDS:** Deficiency of surfactant in premature infants leads to **Neonatal Respiratory Distress Syndrome** (Hyaline Membrane Disease). * **Storage:** Surfactant is stored in intracellular organelles of Type II pneumocytes called **Lamellar bodies**. * **Law of Laplace:** Surfactant counteracts the Law of Laplace ($P = 2T/r$), ensuring that smaller alveoli do not collapse into larger ones.

Question 449: Kussmaul respiration occurs in response to what condition?

A. Decrease in blood pH (Correct Answer)
B. Increase in blood pH
C. Obstructive pulmonary disease
D. Carbon monoxide poisoning

Explanation: **Explanation:** **Kussmaul respiration** is a deep, rapid, and labored breathing pattern. It is a physiological compensatory mechanism for **metabolic acidosis**, characterized by a **decrease in blood pH**. 1. **Why Option A is Correct:** When blood pH drops (acidemia), the increased concentration of hydrogen ions ($H^+$) stimulates **peripheral chemoreceptors** (carotid and aortic bodies) and **central chemoreceptors**. This triggers the respiratory center in the medulla to increase the rate and depth of ventilation. The goal is to "blow off" excess Carbon Dioxide ($CO_2$), thereby reducing volatile acid levels and attempting to return the blood pH toward its normal range (7.35–7.45). This is classically seen in **Diabetic Ketoacidosis (DKA)**. 2. **Why Other Options are Incorrect:** * **Option B:** An increase in blood pH (alkalosis) leads to hypoventilation (slow, shallow breathing) to retain $CO_2$ and lower the pH. * **Option C:** Obstructive diseases (like Asthma or COPD) typically present with wheezing, prolonged expiration, or pursed-lip breathing, rather than the rhythmic deep gasping of Kussmaul’s. * **Option D:** Carbon monoxide poisoning causes tissue hypoxia but often does not stimulate chemoreceptors initially because $PaO_2$ remains normal; it does not typically present with Kussmaul breathing unless severe lactic acidosis develops. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Kussmaul Causes (KUSSMAUL):** **K**etones (DKA), **U**remia (Renal failure), **S**epsis, **S**alicylates, **M**ethanol, **A**ldehydes, **U**ndetermined (Lactic) **L**actic acidosis. * **Distinction:** Do not confuse Kussmaul **respiration** (breathing) with Kussmaul **sign** (paradoxical rise in JVP on inspiration, seen in constrictive pericarditis). * **Key Feature:** Unlike Cheyne-Stokes breathing, Kussmaul respiration is **constant and rhythmic** without periods of apnea.

Question 450: What is an important non-respiratory function of the lungs?

A. Anion balance
B. Defense against inhaled air (Correct Answer)
C. Potassium balance
D. Calcium balance

Explanation: **Explanation:** The lungs are not only responsible for gas exchange but also serve as a vital immunological barrier. **Defense against inhaled air** is a primary non-respiratory function because the respiratory tract is constantly exposed to pathogens, dust, and pollutants. This defense is mediated by: 1. **Mucociliary Escalator:** Ciliated epithelium moves mucus-trapped particles upward toward the pharynx. 2. **Alveolar Macrophages (Dust Cells):** These phagocytose small particles that reach the alveoli. 3. **Secretory IgA and Lysozymes:** Present in the airway surface liquid to neutralize pathogens. **Analysis of Incorrect Options:** * **A. Anion balance:** While the lungs regulate acid-base balance by exhaling $CO_2$ (volatile acid), they do not directly regulate specific anion concentrations (like chloride or phosphate); this is primarily a renal function. * **C & D. Potassium and Calcium balance:** These are strictly regulated by the kidneys and endocrine system (e.g., Aldosterone for $K^+$, Parathyroid hormone for $Ca^{2+}$). The lungs have no physiological role in the homeostasis of these electrolytes. **High-Yield NEET-PG Pearls:** * **Metabolic Functions:** The lungs are the primary site for the conversion of **Angiotensin I to Angiotensin II** via Angiotensin-Converting Enzyme (ACE) located in the pulmonary capillary endothelium. * **Inactivation:** The lungs inactivate Bradykinin, Serotonin, and Prostaglandins ($E_1, E_2, F_{2\alpha}$), but notably **do not** inactivate Epinephrine or Histamine. * **Surfactant:** Produced by Type II Pneumocytes, it prevents alveolar collapse and also plays a role in innate immunity (SP-A and SP-D).

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