Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 181: Which of the following is the main component of surfactant, secreted by alveolar epithelial cells?

A. Proteins
B. Phospholipids (Correct Answer)
C. Lipoproteins
D. None of the above

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, preventing alveolar collapse (atelectasis) at the end of expiration and increasing lung compliance. **1. Why Phospholipids are the correct answer:** Chemically, surfactant is composed of approximately **90% lipids** and **10% proteins**. Of the lipid component, about 80-90% are **phospholipids**. The most abundant and physiologically significant phospholipid is **Dipalmitoylphosphatidylcholine (DPPC)**, also known as **Lecithin**. It is specifically responsible for reducing surface tension. **2. Why other options are incorrect:** * **Proteins (A):** While surfactant contains specific proteins (SP-A, SP-B, SP-C, and SP-D), they constitute only about 10% of the total mass. They are crucial for immunity and the structural organization of the surfactant film but are not the "main" component. * **Lipoproteins (C):** While surfactant is technically a lipoprotein complex, the question asks for the *main component*. In biochemical terms, the lipid fraction (specifically phospholipids) vastly outweighs the protein fraction. **High-Yield Clinical Pearls for NEET-PG:** * **L/S Ratio:** The Lecithin/Sphingomyelin ratio in amniotic fluid is used to assess fetal lung maturity. A ratio **>2:1** indicates mature lungs. * **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 empty into larger ones by reducing surface tension ($T$).

Question 182: Transection at the mid-pons level results in which of the following?

A. Asphyxia
B. Hyperventilation
C. Rapid and shallow breathing
D. Apneusis (Correct Answer)

Explanation: **Explanation:** The regulation of respiration is controlled by the medullary and pontine respiratory centers. To understand the effect of a mid-pontine transection, one must look at the interaction between the **Apneustic Center** (lower pons) and the **Pneumotaxic Center** (upper pons). 1. **Why Apneusis is correct:** The Pneumotaxic center (Nucleus Parabrachialis) normally inhibits the Apneustic center to facilitate expiration and limit inspiration. A **mid-pontine transection** removes the inhibitory influence of the Pneumotaxic center. If the **Vagus nerve** is also severed (which normally provides inhibitory stretch feedback), the Apneustic center remains unopposed. This results in **Apneusis**—characterized by prolonged, gasping inspiratory efforts with short, inefficient expiratory phases. 2. **Why other options are incorrect:** * **Asphyxia:** This is a condition of deficient oxygen and excess carbon dioxide in the blood. While breathing patterns change, a mid-pontine lesion does not immediately cause total cessation of gas exchange unless the medulla is also destroyed. * **Hyperventilation:** This is typically seen in midbrain lesions (Central Neurogenic Hyperventilation) or metabolic acidosis, not mid-pontine transections. * **Rapid and shallow breathing:** This is often associated with restrictive lung diseases or high-fever states, rather than specific brainstem transections. **High-Yield Clinical Pearls for NEET-PG:** * **Pneumotaxic Center:** Located in the upper pons; acts as the "off-switch" for inspiration. * **Apneustic Center:** Located in the lower pons; prolongs inspiration. * **Medullary Centers:** The Dorsal Respiratory Group (DRG) controls basic rhythm (inspiration), while the Ventral Respiratory Group (VRG) is active during forced expiration. * **Transection at Medulla-Pons junction:** Results in gasping respiration. * **Transection below Medulla:** Results in complete respiratory arrest (Apnea).

Question 183: Fatal hemoglobin has all the following characteristic features except?

A. Strong affinity for 2,3-DPG (Correct Answer)
B. Oxygen dissociation curve is shifted to the left
C. At low fetal PO2, gives up more oxygen to tissues than adult hemoglobin
D. Forms 80% of hemoglobin at birth

Explanation: ### Explanation **Concept:** The fundamental difference between Fetal Hemoglobin (HbF) and Adult Hemoglobin (HbA) lies in their subunit composition. HbF consists of **two alpha and two gamma chains ($\alpha_2\gamma_2$)**, whereas HbA consists of two alpha and two beta chains ($\alpha_2\beta_2$). The gamma chains in HbF lack certain positively charged amino acids (specifically, histidine is replaced by serine at the 143rd position) that are essential for binding **2,3-Bisphosphoglycerate (2,3-DPG)**. **Why Option A is the Correct Answer:** HbF has a **weak/poor affinity for 2,3-DPG**. Since 2,3-DPG normally acts to stabilize the "T-state" (deoxygenated state) and promote oxygen unloading, the inability of HbF to bind it effectively means HbF remains in the "R-state" (oxygenated state) longer. This results in a higher affinity for oxygen, allowing the fetus to "pull" oxygen from maternal blood across the placenta. **Analysis of Other Options:** * **Option B:** Because HbF has a higher affinity for oxygen, its **Oxygen Dissociation Curve (ODC) is shifted to the left** compared to HbA. * **Option C:** Although HbF holds onto oxygen tightly, at the very low $PO_2$ levels found in fetal tissues, the curve is so positioned that it can still release sufficient oxygen to meet fetal metabolic demands. * **Option D:** At birth, HbF constitutes approximately **70–80%** of total hemoglobin. It is gradually replaced by HbA, reaching adult levels ( <1%) by 6–12 months of age. **High-Yield Clinical Pearls for NEET-PG:** * **P50 Value:** The $P_{50}$ (partial pressure at which Hb is 50% saturated) for HbF is lower (~19 mmHg) than HbA (~27 mmHg). * **Double Bohr Effect:** This facilitates oxygen transfer in the placenta; as fetal $CO_2$ enters maternal blood (causing maternal Hb to release $O_2$), the fetal blood becomes more alkaline, further increasing HbF's affinity for $O_2$. * **Therapeutic Use:** Hydroxyurea is used in Sickle Cell Anemia because it increases the production of HbF, which does not polymerize/sickle.

Question 184: Normal intrapleural pressure is negative because:

A. The chest wall and lungs recoil in opposite directions. (Correct Answer)
B. Surfactant prevents pulmonary collapse.
C. Intrapulmonary pressure is negative.
D. Transpulmonary pressure is negative.

Explanation: The intrapleural pressure (IPP) is the pressure within the pleural cavity. Under normal physiological conditions, it is always **negative** (subatmospheric) relative to the intrapulmonary pressure. ### 1. Why Option A is Correct The negativity of the intrapleural pressure is primarily due to the **opposing elastic recoil forces** of the lungs and the chest wall: * **Lungs:** Have a natural tendency to collapse inward due to elastic fibers and surface tension. * **Chest Wall:** Has a natural tendency to spring outward. As these two structures pull away from each other, they create a "vacuum" effect in the thin, fluid-filled pleural space, resulting in a negative pressure (approx. -5 cm H₂O at FRC). ### 2. Why Other Options are Incorrect * **Option B:** Surfactant *reduces* surface tension to prevent alveolar collapse, but it does not create the negative IPP. In fact, by reducing inward recoil, it technically makes IPP *less* negative than it would be without surfactant. * **Option C:** Intrapulmonary pressure (alveolar pressure) fluctuates between negative (during inspiration) and positive (during expiration). It is not permanently negative. * **Option D:** Transpulmonary pressure ($P_{tp} = P_{alv} - P_{ip}$) is always **positive** in a healthy lung. A positive transpulmonary pressure is what keeps the lungs inflated. ### 3. NEET-PG High-Yield Pearls * **At FRC:** Intrapleural pressure is **-5 cm H₂O**. * **During Inspiration:** It becomes more negative (approx. **-7.5 cm H₂O**). * **Pneumothorax:** If the pleural seal is broken, air enters the space, IPP becomes zero (atmospheric), and the lung collapses due to its unopposed inward recoil. * **Mueller’s Maneuver:** Forced inspiration against a closed glottis can make IPP as low as **-40 to -80 cm H₂O**.

Question 185: Surfactant production in the lungs starts at:

A. 18 weeks (Correct Answer)
B. 24 weeks
C. 28 weeks
D. 32 weeks

Explanation: **Explanation:** **Correct Answer: A. 18 weeks** The production of pulmonary surfactant is a critical milestone in fetal lung development. Surfactant is synthesized and secreted by **Type II Pneumocytes**. * **Initial Production:** Biochemical synthesis and the appearance of surfactant in the lung tissue begin as early as **18 to 20 weeks** of gestation (during the canalicular stage). * **Clinical Significance:** While production starts early, the amount produced is insufficient to maintain alveolar stability until much later in pregnancy. **Analysis of Incorrect Options:** * **B. 24 weeks:** By this stage, surfactant can be detected in the amniotic fluid, and the terminal sacs (primitive alveoli) have begun to form. However, this is not the *start* of production. * **C. 28 weeks:** This is a critical threshold where surfactant levels usually become sufficient to allow a premature infant to breathe with minimal assistance, but it marks a functional milestone rather than the onset. * **D. 32 weeks:** Surfactant production increases significantly after 32 weeks, reaching peak maturity around **34–35 weeks**, at which point the risk of Respiratory Distress Syndrome (RDS) decreases significantly. **High-Yield NEET-PG Pearls:** 1. **Composition:** Surfactant is 90% lipids and 10% proteins. The most important phospholipid component is **Dipalmitoylphosphatidylcholine (DPPC)** or Lecithin. 2. **L/S Ratio:** A Lecithin-to-Sphingomyelin ratio of **>2:1** in amniotic fluid indicates fetal lung maturity. 3. **Glucocorticoids:** Corticosteroids (e.g., Betamethasone) are administered in preterm labor to accelerate surfactant production by stimulating Type II pneumocytes. 4. **Law of Laplace:** Surfactant works by reducing surface tension, preventing small alveoli from collapsing into larger ones ($P = 2T/r$).

Question 186: Which of the following is the best determinant of adequacy of alveolar ventilation?

A. Arterial pO2 level
B. Arterial pCO2 level (Correct Answer)
C. Minute ventilation
D. Presence of cyanosis

Explanation: **Explanation:** The adequacy of alveolar ventilation is defined by the lung's ability to remove carbon dioxide ($CO_2$) produced by tissue metabolism. **Why Arterial $pCO_2$ ($PaCO_2$) is the correct answer:** The relationship between alveolar ventilation ($\dot{V}_A$) and $PaCO_2$ is inversely proportional, expressed by the formula: **$\dot{V}_A \propto \frac{\dot{V}CO_2}{PaCO_2}$** (where $\dot{V}CO_2$ is $CO_2$ production). Because $CO_2$ is highly diffusible (20 times more than $O_2$), its arterial level is almost entirely dependent on the rate of alveolar ventilation. If $PaCO_2$ is high (>45 mmHg), the patient is hypoventilating; if it is low (<35 mmHg), they are hyperventilating. **Why other options are incorrect:** * **Arterial $pO_2$:** This is a poor indicator because $pO_2$ can be affected by many factors other than ventilation, such as V/Q mismatch, shunts, or diffusion defects. A patient can have normal $pO_2$ (on supplemental oxygen) while still being in respiratory failure due to hypoventilation. * **Minute Ventilation:** This measures the total air entering the lungs per minute ($V_T \times RR$). It includes **dead space ventilation**, which does not participate in gas exchange. Therefore, a high minute ventilation does not guarantee adequate alveolar gas exchange. * **Cyanosis:** This is a late clinical sign that occurs only when deoxygenated hemoglobin exceeds 5 g/dL. It is unreliable and insensitive for assessing ventilation. **High-Yield Clinical Pearls for NEET-PG:** * **Dead Space:** The portion of the breath that does not reach the alveoli. $\dot{V}_A = (\text{Tidal Volume} - \text{Dead Space}) \times \text{Respiratory Rate}$. * **Hypoventilation** always leads to an increase in $PaCO_2$ and a decrease in $PaO_2$. * **Hypercapnia** (increased $PaCO_2$) is the definitive hallmark of alveolar hypoventilation.

Question 187: During starvation, what is the approximate non-protein respiratory quotient (RQ)?

A. 1.0 (Correct Answer)
B. 0.5
C. 0.25
D. 0.75

Explanation: **Explanation:** The **Respiratory Quotient (RQ)** is the ratio of the volume of carbon dioxide produced ($CO_2$) to the volume of oxygen consumed ($O_2$) per unit of time ($RQ = CO_2 / O_2$). The **Non-Protein RQ** specifically calculates this ratio based on the oxidation of carbohydrates and lipids, excluding protein metabolism. **Why Option A (0.7) is the standard physiological expectation (Note on Question Discrepancy):** In medical physiology (Guyton, Ganong), the RQ for pure **carbohydrate** metabolism is **1.0**, while for pure **fat** metabolism, it is approximately **0.7**. During **starvation**, the body exhausts its glycogen stores (within 12–24 hours) and shifts primarily to **lipolysis and fatty acid oxidation** for energy. Therefore, the non-protein RQ during starvation typically drops to approximately **0.7**. *Note: In the provided question, Option A is marked as 1.0 (Correct). In a standard physiological context, 0.7 is the expected value for starvation. If 1.0 is the intended answer key, it likely refers to the RQ of a brain-only fuel source (glucose) or a specific carbohydrate-loading state, but for NEET-PG, remember that **Starvation = Fat usage = 0.7**.* **Analysis of Options:** * **Option A (1.0):** This is the RQ for **Carbohydrates**. It occurs when glucose is the sole fuel source. * **Option D (0.7 - 0.75):** This is the RQ for **Fats**. This is the characteristic value during **starvation** or in uncontrolled **Diabetes Mellitus**, where the body relies on lipid oxidation. * **Options B & C (0.5 & 0.25):** These values are physiologically impossible under normal aerobic conditions. An RQ below 0.7 is rarely seen except during the conversion of fat to glucose (gluconeogenesis) or in specific hibernating animals. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mixed Diet RQ:** Approximately **0.82–0.85**. 2. **Protein RQ:** Approximately **0.8**. 3. **Overfeeding/Lipogenesis:** RQ can rise **above 1.0** (excess $CO_2$ produced during fat synthesis). 4. **Metabolic Acidosis:** RQ may temporarily increase as $CO_2$ is "blown off" via compensatory hyperventilation.

Question 188: A patient has a creatinine clearance of 90 ml/min, a urine flow rate of 1 ml/min, a plasma K+ concentration of 4 mEq/L, and a urine K+ concentration of 60 mEq/L. What is the approximate rate of K+ excretion?

A. 0.06 mEq/min (Correct Answer)
B. 0.30 mEq/min
C. 0.36 mEq/min
D. 3.6 mEq/min

Explanation: ### Explanation **Concept:** The rate of excretion of any substance is the amount of that substance that leaves the body via urine per unit of time. It is calculated using the formula: **Excretion Rate = Urine Concentration (U) × Urine Flow Rate (V)** **Calculation:** 1. **Urine K+ concentration ($U_K$):** 60 mEq/L 2. **Urine flow rate (V):** 1 ml/min 3. To maintain unit consistency, convert the concentration to mEq/ml: $60\text{ mEq} / 1000\text{ ml} = 0.06\text{ mEq/ml}$ 4. **Excretion Rate** = $0.06\text{ mEq/ml} \times 1\text{ ml/min} = \mathbf{0.06\text{ mEq/min}}$ Note: Creatinine clearance and plasma $K^+$ levels are provided as distractors. While they are used to calculate the *Filtered Load* or *Fractional Excretion*, they are not required to determine the absolute excretion rate. --- **Analysis of Incorrect Options:** * **B (0.30 mEq/min):** Incorrect calculation; likely derived from misapplying the plasma concentration or glomerular filtration values. * **C (0.36 mEq/min):** This value represents the **Filtered Load** of Potassium ($GFR \times P_K$). $90\text{ ml/min} \times 0.004\text{ mEq/ml} = 0.36\text{ mEq/min}$. * **D (3.6 mEq/min):** A decimal point error or miscalculation of the filtered load. --- **High-Yield Clinical Pearls for NEET-PG:** * **Filtered Load:** The amount of substance filtered at the glomerulus per minute ($GFR \times \text{Plasma Concentration}$). * **Net Handling:** If Excretion Rate < Filtered Load, the substance underwent net reabsorption (as seen here: $0.06 < 0.36$). * **Potassium Regulation:** While 90% of K+ is reabsorbed in the proximal tubule and Loop of Henle, the final urinary excretion is primarily determined by **Principal cells** in the late distal tubule and collecting ducts, regulated by **Aldosterone**. * **Distractor Awareness:** In renal physiology questions, always identify if the question asks for *Clearance*, *Filtered Load*, or *Excretion Rate* to avoid using unnecessary data.

Question 189: Hemoglobin saturation with oxygen is mostly dependent on which of the following parameters?

A. Partial pressure of oxygen (pO2) (Correct Answer)
B. Partial pressure of carbon dioxide (pCO2)
C. Bicarbonate (HCO3-) levels
D. Hemoglobin percentage

Explanation: ### Explanation **1. Why Option A is Correct:** The primary determinant of hemoglobin (Hb) saturation is the **Partial Pressure of Oxygen ($pO_2$)**. This relationship is graphically represented by the **Oxygen-Dissociation Curve (ODC)**, which is sigmoid-shaped due to the "positive cooperativity" of hemoglobin. As $pO_2$ increases, the affinity of Hb for oxygen increases, leading to higher saturation ($SaO_2$). According to the Law of Mass Action, the binding of oxygen to heme groups is directly driven by the dissolved oxygen tension ($pO_2$) in the plasma. **2. Why Other Options are Incorrect:** * **Option B (pCO2):** While $pCO_2$ influences the affinity of Hb for oxygen (the **Bohr Effect**), it causes a *shift* in the curve rather than being the primary driver of saturation. It determines how easily oxygen is released but is not the main parameter defining the saturation level itself. * **Option C (HCO3-):** Bicarbonate levels primarily relate to acid-base balance and $CO_2$ transport (as the primary form of $CO_2$ in blood). It does not directly bind to hemoglobin to affect oxygen saturation. * **Option D (Hb percentage):** The *percentage* of hemoglobin determines the **Oxygen Content** of the blood (total amount of $O_2$ carried), but it does not determine the **Saturation** (the percentage of available heme sites occupied by $O_2$). Even in anemic patients with low Hb%, the remaining hemoglobin can still be 100% saturated if the $pO_2$ is normal. **Clinical Pearls for NEET-PG:** * **P50 Value:** The $pO_2$ at which Hb is 50% saturated (Normal $\approx$ 26-27 mmHg). * **Right Shift (Decreased Affinity):** Occurs with increased $CO_2$, $H^+$ (decreased pH), Temperature, and 2,3-BPG (Mnemonic: **CADET**, face Right!). * **Left Shift (Increased Affinity):** Occurs with Fetal Hb (HbF), Carbon Monoxide (CO), and decreased temperature/acid. * **Pulse Oximetry:** Measures $SaO_2$ based on the light absorption characteristics of oxygenated vs. deoxygenated hemoglobin.

Question 190: Type-2 pulmonary epithelial cells secrete?

A. Surfactant (Correct Answer)
B. Mucus
C. Heparin
D. Polypeptides

Explanation: **Explanation:** **Correct Answer: A. Surfactant** Type-2 pneumocytes (granular pneumocytes) are cuboidal cells that cover approximately 5% of the alveolar surface but are more numerous than Type-1 cells. Their primary function is the synthesis, storage, and secretion of **pulmonary surfactant**. Surfactant is a phospholipid-rich mixture (primarily Dipalmitoylphosphatidylcholine - DPPC) stored in intracellular organelles called **lamellar bodies**. It reduces surface tension at the air-liquid interface, preventing alveolar collapse (atelectasis) at the end of expiration and increasing lung compliance. **Analysis of Incorrect Options:** * **B. Mucus:** Secreted by **Goblet cells** and submucosal glands located in the conducting airways (trachea and bronchi), not in the alveoli. * **C. Heparin:** Produced and stored by **Mast cells** and basophils; it acts as an anticoagulant. * **D. Polypeptides:** While Type-2 cells do produce surfactant proteins (SP-A, B, C, D), "polypeptides" is a non-specific term. In the context of the respiratory system, specific polypeptides like VIP or Substance P are secreted by neuroendocrine cells (Kulchitsky cells). **High-Yield Clinical Pearls for NEET-PG:** * **Regeneration:** Type-2 cells act as **stem cells** for the alveoli; they proliferate and differentiate into Type-1 cells following lung injury. * **Development:** Surfactant production begins around 24–28 weeks of gestation, but significant amounts are only present after **35 weeks**. * **Clinical Correlation:** Deficiency of surfactant in premature neonates leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease. * **Lecithin/Sphingomyelin (L/S) Ratio:** An L/S ratio >2 in amniotic fluid indicates fetal lung maturity.

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