Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 631: Which of the following statements are true about interstitial fibrosis?

A. FVC decreased
B. FEV1/FVC ratio is normal or increased
C. FRC is normal
D. FEV1/FVC ratio is decreased (Correct Answer)

Explanation: **Explanation:** Interstitial fibrosis is a classic example of a **Restrictive Lung Disease (RLD)**. In these conditions, the lung parenchyma becomes stiff and non-compliant, making it difficult for the lungs to expand during inspiration. **Why the Correct Answer (D) is actually a point of debate:** *Note: In standard medical physiology (Guyton/Ganong), Restrictive Lung Diseases typically present with a **normal or increased FEV1/FVC ratio**. This is because while both FEV1 and FVC decrease, the FVC decreases more significantly due to low lung volumes, and the increased radial traction of fibrotic tissue keeps airways open, allowing for rapid initial expiration.* However, if Option D is marked as correct in your specific question bank, it likely refers to a "mixed" pattern or a specific stage of disease. **Classically, for NEET-PG, remember: RLD = FEV1/FVC Ratio Normal or Increased.** **Analysis of Options:** * **A. FVC decreased:** This is **TRUE** and a hallmark of fibrosis. Total lung capacity and vital capacity are reduced because the lungs cannot expand fully. * **B. FEV1/FVC ratio is normal or increased:** This is the **physiologically classic finding** for interstitial fibrosis. The ratio stays high because the "stiffness" of the lung helps squeeze air out quickly (increased elastic recoil). * **C. FRC is normal:** This is **FALSE**. In restrictive diseases, the Functional Residual Capacity (FRC) is characteristically **decreased** due to the inward pulling of fibrotic tissue. **High-Yield Clinical Pearls for NEET-PG:** 1. **Restrictive Pattern:** ↓ TLC, ↓ FVC, ↓ FRC, and **↑ or Normal FEV1/FVC ratio**. 2. **Obstructive Pattern (Asthma/COPD):** ↑ TLC (hyperinflation), ↓ FEV1, and **↓ FEV1/FVC ratio (<0.7)**. 3. **Diffusion Capacity (DLCO):** In interstitial fibrosis, DLCO is characteristically **decreased** due to the thickened alveolar-capillary membrane. 4. **Compliance:** Lung compliance is significantly **decreased** in fibrosis ("stiff lungs").

Question 632: Surfactant acts on which part of the respiratory system?

A. Bronchi
B. Alveoli (Correct Answer)
C. Bronchioles
D. Trachea

Explanation: **Explanation:** **1. Why Alveoli is Correct:** Pulmonary surfactant is a surface-active lipoprotein complex (primarily Dipalmitoylphosphatidylcholine - DPPC) secreted by **Type II Pneumocytes** in the alveolar epithelium. Its primary function is to **reduce surface tension** at the air-liquid interface within the **alveoli**. According to the **Law of Laplace ($P = 2T/r$)**, smaller spheres have a higher collapsing pressure. By reducing surface tension ($T$), surfactant prevents the collapse of smaller alveoli during expiration (atelectasis), increases lung compliance, and reduces the work of breathing. **2. Why Other Options are Incorrect:** * **Bronchi and Trachea (Options A & D):** These are part of the conducting zone and are structurally supported by **cartilage**. They do not rely on surface tension management to remain patent; their rigid walls prevent collapse. * **Bronchioles (Option C):** While bronchioles lack cartilage, they are kept open by smooth muscle tone and the radial traction of surrounding lung parenchyma. Surfactant is specifically functional at the terminal gas-exchange units (alveoli) where the air-fluid interface is most critical. **3. High-Yield Clinical Pearls for NEET-PG:** * **Composition:** 90% lipids (mainly **DPPC/Lecithin**) and 10% proteins (SP-A, B, C, D). * **Synthesis:** Starts around 24–28 weeks of gestation; reaches maturity by **35 weeks**. * **L/S Ratio:** A Lecithin/Sphingomyelin ratio **> 2** in amniotic fluid indicates fetal lung maturity. * **Clinical Correlation:** Deficiency of surfactant leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease, characterized by widespread alveolar collapse. * **Glucocorticoids:** These are administered to mothers in preterm labor to accelerate surfactant production by stimulating Type II pneumocytes.

Question 633: What are the non-respiratory functions of the lungs?

A. Sodium balance.
B. Fibrinolytic function.
C. Secretion of heparin.
D. All of the above. (Correct Answer)

Explanation: The lungs are not merely organs for gas exchange; they serve as a vital metabolic and filtration hub. The correct answer is **D (All of the above)** because the pulmonary circulation is involved in several homeostatic processes: 1. **Sodium Balance:** The lungs are the primary site for the conversion of Angiotensin I to Angiotensin II via the **Angiotensin-Converting Enzyme (ACE)** located on the luminal surface of pulmonary capillary endothelial cells. Angiotensin II stimulates aldosterone secretion, which regulates sodium reabsorption in the kidneys. 2. **Fibrinolytic Function:** The pulmonary endothelium produces substances like **plasminogen activator**, which helps dissolve small clots (thrombi) before they can enter the systemic circulation and cause arterial emboli. 3. **Secretion of Heparin:** The lungs contain a high concentration of **mast cells** in the connective tissue. these cells secrete heparin, a potent anticoagulant that helps maintain blood fluidity and prevents micro-clot formation within the pulmonary vasculature. **Why other options are not "wrong":** In this "All of the above" format, each individual option represents a documented non-respiratory function. Therefore, selecting only one would be incomplete. **High-Yield Clinical Pearls for NEET-PG:** * **Reservoir Function:** The lungs act as a blood reservoir, holding ~500ml of blood (9% of total volume). * **Inactivation Site:** The lungs inactivate **Bradykinin, Serotonin, and Prostaglandins (E & F)**, but they do **not** inactivate Epinephrine or Angiotensin II. * **Filtration:** They act as a "sieve" for air bubbles, fat globules, and small emboli.

Question 634: The Bohr effect is associated with the effect of which of the following on oxygen affinity?

A. Temperature
B. pH (Correct Answer)
C. 2,3 BPG
D. None of the above

Explanation: **Explanation:** The **Bohr effect** describes the physiological phenomenon where an increase in blood CO₂ concentration or a decrease in **pH** (increased acidity) leads to a decrease in hemoglobin’s affinity for oxygen. This causes the oxygen-hemoglobin dissociation curve to shift to the **right**, facilitating the unloading of oxygen to metabolically active tissues. **Why pH is the correct answer:** In tissues with high metabolic activity, CO₂ is produced, which reacts with water to form carbonic acid, subsequently dissociating into H⁺ ions and bicarbonate. These H⁺ ions bind to specific amino acid residues on the hemoglobin molecule (primarily histidine), stabilizing the **T-state (Tense state)** or deoxygenated form. This reduces its affinity for O₂, allowing oxygen to be released where it is needed most. **Analysis of Incorrect Options:** * **A. Temperature:** While an increase in temperature does shift the curve to the right (decreasing affinity), this is a direct kinetic effect on the bond between hemoglobin and oxygen, not the Bohr effect. * **C. 2,3 BPG:** 2,3-Bisphosphoglycerate is a byproduct of glycolysis that stabilizes the T-state. While it decreases O₂ affinity, this is a distinct regulatory mechanism separate from the pH-driven Bohr effect. **High-Yield Clinical Pearls for NEET-PG:** 1. **Bohr vs. Haldane Effect:** Remember, **B**ohr = **B**lood/Tissues (CO₂/H⁺ affecting O₂ affinity). **H**aldane = **H**ematosis/Lungs (O₂ affecting CO₂ affinity). 2. **Right Shift Factors:** "CADET, face Right!" (**C**O₂, **A**cid/H⁺, **D**PG/2,3-BPG, **E**xercise, **T**emperature). 3. **P50:** The Bohr effect increases the P50 value (the partial pressure of O₂ at which hemoglobin is 50% saturated).

Question 635: CO2 is mainly transported in the blood in which form?

A. Dissolved form
B. Bicarbonate (HCO3-) (Correct Answer)
C. Carbamino compound
D. Gas form

Explanation: **Explanation:** Carbon dioxide ($CO_2$) is a metabolic waste product transported from the tissues to the lungs via three primary mechanisms. Understanding the distribution of these forms is a high-yield topic for NEET-PG. **1. Why Bicarbonate ($HCO_3^-$) is Correct:** The majority of $CO_2$ (**approximately 70%**) is transported as bicarbonate ions. When $CO_2$ enters red blood cells (RBCs), it reacts with water to form carbonic acid ($H_2CO_3$), a reaction catalyzed by the enzyme **Carbonic Anhydrase**. This acid then dissociates into $H^+$ and $HCO_3^-$. The bicarbonate then exits the RBC into the plasma in exchange for Chloride ions (known as the **Chloride Shift or Hamburger Phenomenon**). **2. Analysis of Incorrect Options:** * **A. Dissolved form:** Only about **7%** of $CO_2$ is transported dissolved in physical solution in the plasma. While $CO_2$ is 20 times more soluble than Oxygen, this remains a minor transport pathway. * **C. Carbamino compound:** About **23%** of $CO_2$ binds directly to the amino groups of hemoglobin to form **carbaminohemoglobin**. Note: $CO_2$ does *not* bind to the iron (heme) site, unlike Oxygen or Carbon Monoxide. * **D. Gas form:** $CO_2$ does not travel through the blood as free gas bubbles; it must be dissolved or chemically modified to maintain blood stability. **High-Yield Clinical Pearls for NEET-PG:** * **Haldane Effect:** Deoxygenation of blood increases its ability to carry $CO_2$. This occurs in the tissues. * **Chloride Shift:** To maintain electrical neutrality, as $HCO_3^-$ leaves the RBC, $Cl^-$ enters. This causes RBCs to swell slightly in venous blood. * **Enzyme:** Carbonic Anhydrase is one of the fastest enzymes in the body and is absent in plasma (it is localized within RBCs).

Question 636: What is the TRUE statement regarding the measurement of anatomical dead space?

A. 1 mL of dead space per pound of ideal body weight (Correct Answer)
B. 5 mL of dead space per pound of ideal body weight
C. 10 mL of dead space per pound of ideal body weight
D. 15 mL of dead space per pound of ideal body weight

Explanation: ### Explanation **Conceptual Basis** Anatomical dead space refers to the volume of the conducting airways (from the nose/mouth down to the terminal bronchioles) where no gas exchange occurs because there are no alveoli. In a healthy adult, this volume is remarkably consistent relative to body size. The standard physiological rule of thumb is that anatomical dead space is approximately **1 mL per pound (lb)** of ideal body weight (or roughly 2.2 mL/kg). For a typical 150 lb (70 kg) adult, the anatomical dead space is approximately 150 mL. **Analysis of Options** * **Option A (Correct):** This aligns with the established physiological constant. It is the most accurate estimation used in clinical practice and respiratory physiology equations (e.g., calculating alveolar ventilation: $\dot{V}_A = (V_T - V_D) \times f$). * **Options B, C, and D (Incorrect):** These values (5, 10, and 15 mL/lb) significantly overestimate the dead space. If dead space were this high, a person would need an impossibly large tidal volume just to ensure any fresh air reached the alveoli. **High-Yield NEET-PG Pearls** * **Fowler’s Method:** Anatomical dead space is measured using the **Single Breath Nitrogen Washout** technique. * **Bohr’s Equation:** This is used to measure **Physiological Dead Space** (which includes anatomical dead space plus alveolar dead space). It uses arterial $CO_2$ ($PaCO_2$) and expired $CO_2$ ($PeCO_2$). * **Anatomical vs. Physiological:** In healthy individuals, anatomical and physiological dead space are nearly equal. However, in diseases like COPD or Pulmonary Embolism, physiological dead space increases significantly due to ventilation-perfusion (V/Q) mismatch. * **Positioning:** Anatomical dead space increases in the upright position and during deep inspiration.

Question 637: Oxygen carrying capacity of blood is largely determined by what?

A. Hemoglobin level (Correct Answer)
B. Amount of CO2 in blood
C. Acidosis
D. Plasma concentration

Explanation: **Explanation:** The oxygen-carrying capacity of blood refers to the maximum amount of oxygen that can be transported by a specific volume of blood. In the human body, oxygen is transported in two forms: dissolved in plasma (approx. 1.5%) and bound to hemoglobin (approx. 98.5%). **Why Hemoglobin level is correct:** Each gram of pure hemoglobin (Hb) can bind approximately **1.34 mL of oxygen** (Hüfner's constant). The formula for oxygen content is: *Oxygen Content = (1.34 × [Hb] × SaO₂) + (0.003 × PaO₂)* Since the vast majority of oxygen is chemically bound to hemoglobin, the total concentration of hemoglobin is the primary limiting factor and determinant of the blood's capacity to carry oxygen. **Why other options are incorrect:** * **Amount of CO2 (B) and Acidosis (C):** These factors influence the **Oxygen-Hemoglobin Dissociation Curve** (Bohr Effect). They affect the *affinity* of hemoglobin for oxygen (how easily it is released), but they do not change the total *capacity* of the blood to carry oxygen. * **Plasma concentration (D):** Oxygen is poorly soluble in plasma. At normal physiological pressure, only 0.003 mL of O₂ dissolves in 100 mL of plasma per mmHg of PO₂. This contribution is negligible compared to the role of hemoglobin. **High-Yield NEET-PG Pearls:** * **Normal Oxygen Capacity:** In a healthy adult with 15g/dL of Hb, the capacity is approximately **20.1 mL O₂/100 mL blood**. * **Anemia vs. CO Poisoning:** In anemia, the O₂ capacity is decreased because Hb levels are low. In Carbon Monoxide (CO) poisoning, the O₂ capacity is decreased because CO occupies the binding sites, even if the Hb level is numerically normal. * **P50 Value:** The partial pressure of oxygen at which hemoglobin is 50% saturated (normally 26.7 mmHg). A "Right Shift" (caused by ↑CO₂, ↑H+, ↑Temp, ↑2,3-BPG) decreases affinity but does not change total capacity.

Question 638: Oxygen therapy is most useful in which of the following types of hypoxia?

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

Explanation: **Explanation:** The primary goal of oxygen therapy is to increase the partial pressure of oxygen in the alveoli ($PAO_2$), which in turn increases the arterial partial pressure of oxygen ($PaO_2$). **Why Hypoxic Hypoxia is the correct answer:** Hypoxic hypoxia is characterized by a low $PaO_2$ due to factors like high altitude, hypoventilation, or V/Q mismatch. Since the underlying problem is a lack of oxygen tension in the arterial blood, increasing the inspired oxygen concentration ($FiO_2$) directly corrects the gradient, significantly improving oxygen saturation. It is the only type of hypoxia where oxygen therapy is highly effective. **Analysis of Incorrect Options:** * **Anemic Hypoxia:** The $PaO_2$ is normal, but the oxygen-carrying capacity is reduced due to low hemoglobin or CO poisoning. Oxygen therapy adds only a negligible amount of dissolved oxygen in the plasma; it does not fix the hemoglobin deficit. * **Stagnant (Ischemic) Hypoxia:** The $PaO_2$ and content are normal, but blood flow to tissues is inadequate (e.g., heart failure, shock). The solution is improving cardiac output or local perfusion, not increasing inspired oxygen. * **Histotoxic Hypoxia:** Oxygen delivery is normal, but tissues (e.g., in cyanide poisoning) cannot utilize it because the cytochrome oxidase system is inhibited. Oxygen therapy is generally ineffective as the "machinery" is broken. **High-Yield Clinical Pearls for NEET-PG:** * **Cyanosis** is most marked in stagnant hypoxia and absent in anemic hypoxia (as 5g% of reduced Hb is required to see cyanosis). * **$PaO_2$** is normal in all types of hypoxia except **Hypoxic Hypoxia**. * In **CO poisoning** (a form of anemic hypoxia), 100% hyperbaric oxygen is used not just to increase dissolved $O_2$, but to displace CO from hemoglobin.

Question 639: During inspiration, the diaphragm:

A. Contracts (Correct Answer)
B. Relaxes
C. Does nothing
D. Expands

Explanation: **Explanation:** **1. Why Option A is Correct:** Inspiration is an **active process** primarily driven by the contraction of the diaphragm (the chief muscle of respiration). When the diaphragm **contracts**, its dome flattens and moves inferiorly (downward). This action increases the **vertical diameter** of the thoracic cavity. According to **Boyle’s Law** (Pressure ∝ 1/Volume), the increase in thoracic volume leads to a decrease in intra-alveolar pressure (becoming sub-atmospheric). This pressure gradient causes air to flow from the atmosphere into the lungs. **2. Why Other Options are Incorrect:** * **Option B (Relaxes):** Diaphragmatic relaxation occurs during **expiration**. As the muscle relaxes, it resumes its dome shape, decreasing thoracic volume and pushing air out of the lungs (a passive process during quiet breathing). * **Option C (Does nothing):** The diaphragm is the most essential muscle for ventilation; it is never static during the respiratory cycle. * **Option D (Expands):** In physiological terms, muscles either contract or relax. While the thoracic cavity "expands," the muscle itself "contracts" to facilitate that expansion. **3. NEET-PG High-Yield Clinical Pearls:** * **Innervation:** The diaphragm is supplied by the **Phrenic Nerve (C3, C4, C5)**. Remember: *"C3, 4, 5 keep the diaphragm alive."* * **Movement:** During quiet inspiration, the diaphragm moves down by ~1 cm; during deep inspiration, it can move up to 10 cm. * **Anatomical Openings:** Remember the levels of major structures passing through the diaphragm: **I8** (IVC at T8), **10E** (Esophagus at T10), and **A12** (Aorta at T12). * **Paradoxical Respiration:** If the phrenic nerve is paralyzed, the diaphragm moves *upward* during inspiration (sucked up by negative pressure), known as paradoxical movement.

Question 640: Peripheral chemoreceptors are maximally stimulated by which type of hypoxia?

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

Explanation: **Explanation:** The peripheral chemoreceptors (located in the carotid and aortic bodies) are primarily sensitive to changes in the **partial pressure of arterial oxygen ($PaO_2$)**, rather than the total oxygen content or oxygen delivery. **Why Histotoxic Hypoxia is the Correct Answer:** In **histotoxic hypoxia** (e.g., cyanide poisoning), the tissues and chemoreceptors are unable to utilize oxygen because the cytochrome oxidase system is inhibited. Even though $PaO_2$ remains normal, the chemoreceptors are "poisoned." Interestingly, in experimental and clinical models of histotoxic hypoxia, there is a profound and maximal stimulation of peripheral chemoreceptors. This occurs because the metabolic poisons interfere with the oxygen-sensing mechanism within the glomus cells, mimicking a state of extreme oxygen deprivation and triggering a massive increase in ventilation. **Analysis of Incorrect Options:** * **Hypoxic Hypoxia:** Characterized by low $PaO_2$. While this is the *classic* stimulus for peripheral chemoreceptors, the stimulation is often less "maximal" compared to histotoxic triggers because the resulting hyperventilation washes out $CO_2$, causing hypocapnia which subsequently inhibits the respiratory center. * **Anemic Hypoxia:** $PaO_2$ is normal, but total oxygen content is low (low Hb). Since peripheral chemoreceptors sense dissolved $O_2$ ($PaO_2$), they are **not stimulated** in anemic hypoxia. * **Stagnant Hypoxia:** $PaO_2$ is normal, but blood flow is slow. While local $PO_2$ at the receptor might drop slightly, it does not produce the maximal stimulation seen in histotoxic states. **High-Yield Clinical Pearls for NEET-PG:** * **Glomus Cells (Type I):** The actual chemoreceptors that release dopamine/acetylcholine to stimulate the glossopharyngeal nerve. * **Threshold:** Peripheral chemoreceptors typically begin to respond strongly only when $PaO_2$ drops below **60 mmHg**. * **Cyanide Poisoning:** Classically presents with "cherry-red" venous blood because oxygen is not being unloaded/utilized by the tissues.

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