Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 221: The approximate amount of air left in the lungs after maximal forced expiration in a normal woman is:

A. 0.5 L
B. 2.0 L
C. 1.1 L (Correct Answer)
D. 1.8 L

Explanation: ### Explanation The volume of air remaining in the lungs after a maximal forced expiration is defined as the **Residual Volume (RV)**. This volume cannot be exhaled because the small airways collapse at low lung volumes, trapping air, and the chest wall reaches its inward limit of compression. **Why 1.1 L is correct:** In a healthy adult, the average Residual Volume is approximately **1.2 L in males** and **1.1 L in females**. Since the question specifically asks for the value in a **normal woman**, 1.1 L is the most accurate physiological estimate. **Analysis of Incorrect Options:** * **0.5 L (Option A):** This value corresponds to the **Tidal Volume (TV)**, which is the volume of air inspired or expired during a single normal breath. * **2.0 L (Option B) & 1.8 L (Option D):** These values are too high for RV. However, a value around 2.2–2.4 L would represent the **Functional Residual Capacity (FRC)**—the air remaining after a *normal* (passive) expiration (FRC = RV + ERV). **High-Yield NEET-PG Pearls:** 1. **Measurement:** Residual Volume **cannot** be measured by simple spirometry because the air never leaves the lungs. It must be measured using indirect methods: **Helium Dilution**, **Nitrogen Washout**, or **Body Plethysmography**. 2. **Clinical Correlation:** RV is significantly **increased** in obstructive lung diseases (e.g., Emphysema, Asthma) due to air trapping, leading to hyperinflation. 3. **Formula:** $FRC - ERV = RV$ (where ERV is Expiratory Reserve Volume). 4. **Aging:** RV typically increases with age as the elastic recoil of the lungs decreases.

Question 222: What is the normal vital capacity of the lungs?

A. 500 ml
B. 1200 ml
C. 3000 ml
D. 4700 ml (Correct Answer)

Explanation: **Explanation:** **Vital Capacity (VC)** is the maximum volume of air a person can exhale from the lungs after a maximum inhalation. It is a key indicator of pulmonary function and reflects the total "usable" gas exchange surface. **Why Option D is Correct:** The normal Vital Capacity in a healthy adult male is approximately **4600–4700 ml**. It is calculated by the sum of three primary lung volumes: * **Tidal Volume (TV):** 500 ml * **Inspiratory Reserve Volume (IRV):** 3000 ml * **Expiratory Reserve Volume (ERV):** 1100–1200 ml * **Calculation:** $500 + 3000 + 1200 = 4700 \text{ ml}$. **Analysis of Incorrect Options:** * **Option A (500 ml):** This represents the **Tidal Volume (TV)**, which is the volume of air inspired or expired during a normal, quiet breath. * **Option B (1200 ml):** This represents the **Residual Volume (RV)**—the air remaining in the lungs after forceful expiration—or the **Expiratory Reserve Volume (ERV)**. * **Option C (3000 ml):** This represents the **Inspiratory Reserve Volume (IRV)**, the additional volume that can be inspired above the normal tidal volume. **High-Yield NEET-PG Pearls:** 1. **VC vs. TLC:** Vital Capacity does *not* include Residual Volume. Total Lung Capacity (TLC) = VC + RV (approx. 5800–6000 ml). 2. **Clinical Significance:** VC is decreased in **Restrictive Lung Diseases** (e.g., Pulmonary Fibrosis, Kyphoscoliosis) but remains relatively normal or is only slightly reduced in obstructive diseases. 3. **Timed VC:** The most clinically significant measurement is **FEV1** (Forced Expiratory Volume in 1 second), which is normally 80% of the FVC (Forced Vital Capacity). 4. **Factors:** VC is higher in males, taller individuals, and athletes; it decreases with age and in the supine position.

Question 223: Respiratory distress syndrome is due to a defect in the biosynthesis of which substance?

A. Dipalmitoyl lecithin (Correct Answer)
B. Dipalmitoyl cephalin
C. Dipalmitoyl serine
D. Dipalmitoyl inositol

Explanation: ### Explanation **Correct Option: A. Dipalmitoyl lecithin** **Mechanism and Concept:** Respiratory Distress Syndrome (RDS), also known as Hyaline Membrane Disease, is primarily caused by a deficiency of **pulmonary surfactant**. Surfactant is a surface-active lipoprotein complex secreted by **Type II pneumocytes**. Its primary function is to reduce surface tension at the air-liquid interface of the alveoli, preventing alveolar collapse (atelectasis) during expiration. The most critical functional component of surfactant is **Dipalmitoylphosphatidylcholine (DPPC)**, commonly known as **Dipalmitoyl lecithin**. It accounts for approximately 50–60% of the surfactant composition. Chemically, it is a phospholipid with two palmitic acid chains. A defect in its biosynthesis or premature birth (before 34 weeks) leads to high surface tension, decreased lung compliance, and subsequent respiratory failure. **Why Other Options are Incorrect:** * **B, C, and D:** While Cephalin (Phosphatidylethanolamine), Serine (Phosphatidylserine), and Inositol (Phosphatidylinositol) are all phospholipids found in cell membranes and in minor quantities within surfactant, they do not possess the unique surface-tension-reducing properties of lecithin. They are not the primary functional molecules whose deficiency leads to RDS. **High-Yield Clinical Pearls for NEET-PG:** * **L/S Ratio:** Fetal lung maturity is assessed by the **Lecithin-Sphingomyelin ratio** in amniotic fluid. A ratio **> 2.0** indicates mature lungs. * **Glucocorticoids:** Antenatal administration of steroids (e.g., Betamethasone or Dexamethasone) accelerates surfactant synthesis by inducing enzymes in Type II pneumocytes. * **Surfactant Proteins:** SP-A and SP-D are involved in innate immunity, while **SP-B and SP-C** are crucial for the physical properties of surfactant. * **Radiology:** RDS typically presents with a "ground-glass appearance" and air bronchograms on a chest X-ray.

Question 224: Respiration stops in the last stage of expiration, during forced expiration, because of:

A. Respiratory muscle fatigue
B. Collapse of alveoli
C. Dynamic compression of airways (Correct Answer)
D. Breaking effect of inspiratory muscles

Explanation: ### Explanation **Correct Answer: C. Dynamic compression of airways** **Mechanism:** During forced expiration, the intrapleural pressure becomes highly positive (exceeding atmospheric pressure) to drive air out. As air moves from the alveoli toward the mouth, pressure within the airways drops due to resistance and frictional loss. At a certain point, known as the **Equal Pressure Point (EPP)**, the pressure inside the airway equals the positive intrapleural pressure outside it. Beyond this point (closer to the mouth), the external pleural pressure exceeds the internal airway pressure, causing the non-cartilaginous bronchioles to collapse. This phenomenon is called **Dynamic Compression**. Once this occurs, any further increase in expiratory effort only increases the compression, limiting the flow rate. This is why forced expiration is "effort-independent" at low lung volumes. **Why other options are incorrect:** * **A. Respiratory muscle fatigue:** While muscles can tire, the immediate cessation of flow during a single forced breath is a mechanical limitation of the airways, not a failure of muscle contractility. * **B. Collapse of alveoli:** Alveoli are kept patent by **surfactant** and the radial traction of surrounding lung tissue. They do not collapse during forced expiration; rather, it is the small conducting airways that compress. * **D. Braking effect of inspiratory muscles:** This refers to the gradual release of inspiratory muscle tone at the *beginning* of passive expiration to ensure smooth airflow; it does not stop flow at the end of forced expiration. **High-Yield Facts for NEET-PG:** * **Equal Pressure Point (EPP):** In healthy lungs, the EPP occurs in airways supported by cartilage. In **Emphysema** (loss of elastic recoil), the EPP moves deeper into smaller, collapsible airways, leading to early airway closure and air trapping. * **Starling Resistor Effect:** This is the physiological model used to describe dynamic airway compression. * **Flow-Volume Loop:** The "effort-independent" portion of the expiratory curve is a direct clinical representation of dynamic compression.

Question 225: Carbon dioxide retention is seen in all, except?

A. Carbon monoxide poisoning (Correct Answer)
B. Respiratory failure
C. Ventilator failure
D. Pulmonary edema

Explanation: **Explanation:** The core concept here is the difference between **Type I** and **Type II respiratory failure**. Carbon dioxide retention (Hypercapnia) occurs when there is an issue with **ventilation** (the movement of air in and out of the lungs), whereas oxygenation issues can occur even when ventilation is preserved. **Why Carbon Monoxide (CO) Poisoning is the correct answer:** In CO poisoning, the primary pathology is **impaired oxygen transport**, not a failure of ventilation. CO binds to hemoglobin with 210 times the affinity of oxygen, causing a leftward shift of the oxygen-dissociation curve. However, the lungs' ability to exhale $CO_2$ remains intact. In fact, patients often hyperventilate due to tissue hypoxia, which leads to **decreased** $PaCO_2$ (hypocapnia) rather than retention. **Analysis of Incorrect Options:** * **Respiratory Failure:** Specifically Type II respiratory failure (Ventilatory failure) is defined by the inability to eliminate $CO_2$, leading to hypercapnia. * **Ventilator Failure:** Any mechanical failure or setting mismatch that reduces alveolar ventilation will directly result in $CO_2$ accumulation. * **Pulmonary Edema:** While initially causing Type I failure (hypoxia with low $CO_2$), severe or end-stage pulmonary edema leads to respiratory muscle fatigue and increased dead space, eventually resulting in $CO_2$ retention. **High-Yield Clinical Pearls for NEET-PG:** * **CO Poisoning Triad:** Cherry-red skin (rarely seen clinically), headache, and normal $SpO_2$ (pulse oximeters cannot distinguish between carboxyhemoglobin and oxyhemoglobin). * **Type I Respiratory Failure:** Hypoxia with normal/low $PaCO_2$ (e.g., Pneumonia, early Pulmonary Edema). * **Type II Respiratory Failure:** Hypoxia with high $PaCO_2$ (e.g., COPD, Neuromuscular weakness, Opioid overdose).

Question 226: Carbon monoxide poisoning causes which of the following types of hypoxia?

A. Anemic hypoxia (Correct Answer)
B. Histotoxic hypoxia
C. Anoxic hypoxia
D. Stagnant hypoxia

Explanation: **Explanation:** **Why Anemic Hypoxia is Correct:** Anemic hypoxia occurs when the oxygen-carrying capacity of the blood is reduced, even though the partial pressure of arterial oxygen ($PaO_2$) remains normal. In Carbon Monoxide (CO) poisoning, CO binds to hemoglobin with an affinity **200–250 times greater** than oxygen, forming **carboxyhemoglobin**. This effectively reduces the amount of hemoglobin available to transport oxygen. Furthermore, CO causes a **leftward shift** of the oxygen-hemoglobin dissociation curve, meaning the remaining oxygen binds more tightly to hemoglobin and is not easily released to the tissues. **Why Other Options are Incorrect:** * **Histotoxic Hypoxia:** Occurs when tissues cannot utilize oxygen despite adequate delivery (e.g., **Cyanide poisoning** inhibiting cytochrome oxidase). * **Anoxic (Hypoxic) Hypoxia:** Characterized by low arterial $PaO_2$ due to external factors like high altitude, airway obstruction, or alveolar hypoventilation. * **Stagnant (Ischemic) Hypoxia:** Occurs when blood flow to tissues is reduced despite normal oxygen content (e.g., heart failure, shock, or local embolism). **High-Yield Clinical Pearls for NEET-PG:** * **Pulse Oximetry:** Standard pulse oximeters cannot distinguish between oxyhemoglobin and carboxyhemoglobin, often giving **falsely normal** $SpO_2$ readings. * **Clinical Sign:** Patients may present with "cherry-red" skin discoloration (a classic but late sign). * **Treatment:** 100% Hyperbaric oxygen is the treatment of choice to displace CO from hemoglobin. * **Curve Shift:** CO poisoning is a classic cause of a **Left Shift** (along with Alkalosis, decreased 2,3-DPG, and Hypothermia).

Question 227: In hyperventilation, what happens to the P50 and Hb affinity for O2?

A. P50 and Hb affinity for O2 increases
B. P50 and Hb affinity for O2 decreases
C. P50 increases and affinity O2 decreases (Correct Answer)
D. P50 decreases and affinity O2 increases

Explanation: ### Explanation **Underlying Concept: The Bohr Effect and Oxygen-Hemoglobin Dissociation Curve** The correct answer is based on the relationship between blood gases, pH, and hemoglobin’s affinity for oxygen. 1. **Hyperventilation:** This process involves excessive breathing, which leads to the "washout" of Carbon Dioxide ($CO_2$). 2. **Respiratory Alkalosis:** A decrease in $PaCO_2$ (hypocapnia) leads to an increase in blood pH (alkalosis). 3. **The Bohr Effect:** According to the Bohr effect, a decrease in $H^+$ ions (alkalosis) and a decrease in $PCO_2$ causes the Oxygen-Hemoglobin dissociation curve to **shift to the LEFT**. 4. **Affinity and P50:** A leftward shift signifies an **increased affinity** of hemoglobin for oxygen (it holds onto $O_2$ more tightly). Consequently, the **P50** (the partial pressure of oxygen at which 50% of hemoglobin is saturated) **decreases**. **Note on the Provided Key:** There appears to be a discrepancy in the provided option marking. In standard physiology, hyperventilation causes a **Left Shift**, which **decreases P50** and **increases affinity**. If the question intended to ask about *exercise* or *hypoventilation*, the answer would differ. However, based on the physiological definition of hyperventilation: * **Correct Physiological Fact:** P50 decreases; Affinity increases (Option D). * **Explanation for Option C (if marked correct):** This would only occur in conditions of **Right Shift** (e.g., high $CO_2$, high temperature, or high 2,3-DPG), which is the opposite of hyperventilation. --- ### Why Other Options are Incorrect: * **Option A:** P50 and affinity have an inverse relationship; they cannot both increase simultaneously. * **Option B:** While P50 decreases in hyperventilation, affinity must increase. * **Option C:** Describes a **Right Shift** (seen in fever, acidosis, or high altitude), not hyperventilation. --- ### High-Yield Clinical Pearls for NEET-PG: * **Left Shift (Increased Affinity/Decreased P50):** "HALT" — **H**ypocapnia, **A**lkalosis, **L**ow 2,3-DPG, **T**emperature (low). Also, Carboxyhemoglobin and Fetal Hb (HbF). * **Right Shift (Decreased Affinity/Increased P50):** "CADET, face Right!" — **C**O2 (high), **A**cidosis, **D**PG (2,3-DPG high), **E**xercise, **T**emperature (high). * **P50 Value:** The normal P50 for an adult is approximately **26.6 mmHg**.

Question 228: Which of the following has the maximum smooth muscle relative to its wall thickness?

A. Respiratory bronchiole
B. Alveoli
C. Terminal bronchiole (Correct Answer)
D. Alveolar ducts

Explanation: **Explanation:** The correct answer is **Terminal bronchiole**. **1. Why Terminal Bronchiole is Correct:** In the respiratory tree, as we move from the trachea toward the alveoli, the amount of cartilage decreases while the relative proportion of smooth muscle increases. The **terminal bronchiole** marks the end of the conducting zone. It contains a complete layer of smooth muscle but lacks cartilage entirely. Because its wall is thin compared to larger bronchi, the smooth muscle constitutes the **maximum percentage of its total wall thickness**. This high muscle-to-wall ratio allows terminal bronchioles to significantly alter airway resistance through bronchoconstriction and bronchodilation. **2. Why Other Options are Incorrect:** * **Respiratory bronchiole:** These mark the beginning of the respiratory zone. Their walls are interrupted by occasional alveoli, leading to a fragmented and reduced smooth muscle layer compared to terminal bronchioles. * **Alveoli:** These are the primary sites of gas exchange. Their walls consist almost entirely of Type I and Type II pneumocytes and a basement membrane to facilitate diffusion; they **lack smooth muscle** entirely. * **Alveolar ducts:** These are passages lined with alveoli. While they contain small "knobs" of smooth muscle at the alveolar openings, the overall muscle content relative to the wall is significantly less than that of the terminal bronchiole. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Maximum Airway Resistance:** While terminal bronchioles have the most muscle, the **medium-sized bronchi** (generations 2-5) are the site of maximum airway resistance. * **Asthma Pathophysiology:** The abundance of smooth muscle in the bronchioles is the anatomical basis for bronchospasm in asthma. * **Anatomical Landmark:** The terminal bronchiole is the last structure supplied by the **bronchial circulation**; structures distal to it are supplied by the pulmonary circulation.

Question 229: Which of the following statements regarding the vascularity of the lungs is TRUE?

A. Hypoxia causes pulmonary vasodilation
B. The ratio of pulmonary to systemic vascular resistance is usually greater than 0.3
C. Pulmonary perfusion is greater in the apical lobe than in the base
D. Pulmonary blood flow is less in the upper parts of the lungs compared to the lower parts (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Pulmonary blood flow is less in the upper parts of the lungs compared to the lower parts.** **Why Option D is Correct:** In an upright individual, pulmonary blood flow is significantly influenced by **gravity**. The lungs are a low-pressure system; therefore, hydrostatic pressure increases as we move from the apex (top) to the base (bottom). This results in greater recruitment and distension of pulmonary capillaries at the base. According to **West’s Zones of the Lung**, Zone 3 (the base) receives the highest blood flow, while Zone 1 (the apex) receives the least. **Analysis of Incorrect Options:** * **A. Hypoxia causes pulmonary vasodilation:** This is incorrect. Unlike systemic vessels (which dilate in response to hypoxia), pulmonary vessels undergo **Hypoxic Pulmonary Vasoconstriction (HPV)**. This mechanism shunts blood away from poorly ventilated alveoli to well-ventilated ones to optimize gas exchange (V/Q matching). * **B. The ratio of pulmonary to systemic vascular resistance is > 0.3:** This is incorrect. The pulmonary circulation is a **low-resistance** system. Normal Pulmonary Vascular Resistance (PVR) is about 1/10th of Systemic Vascular Resistance (SVR). The ratio is typically around **0.1 to 0.15**. * **C. Pulmonary perfusion is greater in the apical lobe than in the base:** This is incorrect. As explained above, gravity ensures that perfusion ($Q$) is significantly higher at the base than at the apex. **High-Yield Clinical Pearls for NEET-PG:** * **V/Q Ratio:** While both ventilation ($V$) and perfusion ($Q$) increase from apex to base, perfusion increases more steeply. Thus, the **V/Q ratio is highest at the apex** (~3.3) and lowest at the base (~0.6). * **West Zones:** In a healthy person, Zone 1 (where Alveolar pressure > Arterial pressure) usually does not exist but can occur during hemorrhage (low BP) or positive pressure ventilation. * **Apex Predilection:** Secondary Tuberculosis thrives in the lung apices because the **high V/Q ratio** results in higher local oxygen concentration ($PAO_2$), favoring the growth of aerobic *M. tuberculosis*.

Question 230: An increase in which of the following parameters will shift the oxygen dissociation curve to the left?

A. Temperature
B. Partial pressure of CO2
C. 2,3 DPG concentration
D. Oxygen affinity of hemoglobin (Correct Answer)

Explanation: **Explanation:** The Oxygen Dissociation Curve (ODC) represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin ($SaO_2$). **Why the Correct Answer is Right:** A **Left Shift** indicates that hemoglobin has an **increased affinity for oxygen**. This means hemoglobin binds oxygen more tightly and is less willing to release it to the tissues. By definition, any factor that increases the oxygen affinity of hemoglobin will shift the curve to the left. At any given $PO_2$, the hemoglobin saturation will be higher than normal. **Why the Other Options are Wrong:** Options A, B, and C all cause a **Right Shift** (decreased affinity), which facilitates oxygen unloading to tissues (the Bohr Effect). * **A. Temperature:** Increased temperature (e.g., during fever or exercise) decreases affinity, shifting the curve to the right. * **B. Partial pressure of $CO_2$ ($PCO_2$):** Increased $CO_2$ leads to increased $H^+$ concentration (decreased pH). This stabilizes the "Tense" (T) state of hemoglobin, shifting the curve to the right. * **C. 2,3 DPG concentration:** This byproduct of glycolysis binds to the beta chains of hemoglobin, decreasing its affinity for $O_2$ and shifting the curve to the right. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Right Shift:** "**CADET**, face Right!" (**C**-$CO_2$, **A**-Acid/H+, **D**-2,3 DPG, **E**-Exercise, **T**-Temperature). * **Left Shift Causes:** Fetal Hemoglobin (HbF), Carbon Monoxide poisoning (COHb), Methemoglobinemia, and Alkalosis. * **$P_{50}$ Value:** The $PO_2$ at which hemoglobin is 50% saturated. A **Left Shift decreases $P_{50}$**, while a **Right Shift increases $P_{50}$**. Normal $P_{50}$ is approximately 26-27 mmHg.

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