Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 661: Activation of which receptor causes pulmonary vasoconstriction?

A. Alpha 2 adrenergic
B. H2 histamine
C. M3 cholinergic
D. ETA epithelial receptor (Correct Answer)

Explanation: **Explanation:** The pulmonary circulation is a low-pressure system where vascular tone is regulated by various neural, humoral, and local factors. **Why Option D is Correct:** **Endothelin-1 (ET-1)** is one of the most potent endogenous vasoconstrictors known. It acts via two main receptors: **ETA** and **ETB**. * **ETA receptors** are located primarily on vascular smooth muscle cells. Their activation leads to profound and sustained **pulmonary vasoconstriction** and cellular proliferation. * In contrast, ETB receptors on the endothelium can sometimes trigger vasodilation via Nitric Oxide (NO) release, but the predominant effect of ET-1 in pathological states is ETA-mediated constriction. **Why Other Options are Incorrect:** * **Alpha-2 Adrenergic:** These are primarily presynaptic receptors that inhibit norepinephrine release (sympatholytic effect). While Alpha-1 receptors cause vasoconstriction, Alpha-2 activation does not play a primary role in pulmonary vasoconstriction. * **H2 Histamine:** Activation of H2 receptors typically leads to **vasodilation** and gastric acid secretion. It is the H1 receptor that is more commonly associated with bronchoconstriction and variable vascular effects. * **M3 Cholinergic:** In the lungs, M3 activation causes bronchoconstriction and increased mucus secretion. In the vasculature, M3 activation typically causes **vasodilation** via the release of endothelium-derived relaxing factor (EDRF/Nitric Oxide). **High-Yield Clinical Pearls for NEET-PG:** 1. **Hypoxic Pulmonary Vasoconstriction (HPV):** The most important local regulator of pulmonary blood flow; it shunts blood away from poorly ventilated alveoli to optimize V/Q matching. 2. **Bosentan:** A dual ETA and ETB receptor antagonist used in the treatment of **Pulmonary Arterial Hypertension (PAH)**. 3. **Ambrisentan:** A selective **ETA receptor antagonist**, highlighting the clinical importance of this specific receptor in managing pulmonary hypertension.

Question 662: What causes effort during normal respiration?

A. Lung elasticity (Correct Answer)
B. Respiratory air passages
C. Alveolar air spaces
D. Creating negative pleural pressure

Explanation: **Explanation:** In normal, quiet breathing (eupnea), inspiration is an active process, while expiration is passive. The "effort" or work of breathing is primarily required to overcome the **elastic recoil** of the lungs and the chest wall. **1. Why Lung Elasticity is correct:** During inspiration, the respiratory muscles (mainly the diaphragm) must perform work to stretch the elastic fibers of the lung parenchyma and overcome surface tension in the alveoli. This stored elastic energy is then released during expiration, allowing the lungs to recoil to their original volume without further muscular effort. Therefore, the primary resistance to expansion in a healthy individual is the lung's inherent elasticity. **2. Analysis of Incorrect Options:** * **B. Respiratory air passages:** While airway resistance exists, it accounts for only a small fraction of the work in normal respiration. It becomes a major factor only in obstructive pathologies like asthma or COPD. * **C. Alveolar air spaces:** The spaces themselves do not cause effort; rather, it is the *surface tension* at the air-liquid interface within the alveoli that contributes to elastic work. * **D. Creating negative pleural pressure:** This is the *mechanism* by which inspiration occurs, not the cause of the effort. The effort is expended *to* create this negative pressure against the resistance of lung elasticity. **NEET-PG High-Yield Pearls:** * **Compliance:** Defined as $\Delta V / \Delta P$. High elasticity means low compliance (stiff lungs), which significantly increases the work of breathing (e.g., Pulmonary Fibrosis). * **Surfactant:** Reduces the work of breathing by lowering alveolar surface tension, thereby increasing lung compliance. * **Active Expiration:** Becomes necessary during exercise or in diseases like emphysema, where elastic recoil is lost.

Question 663: The oxygen-hemoglobin dissociation curve is shifted to the left by:

A. Acidosis
B. Alkalosis (Correct Answer)
C. Hyperthermia
D. Anemia

Explanation: The oxygen-hemoglobin (O2-Hb) dissociation curve represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. ### **Why Alkalosis is Correct** A **left shift** indicates an **increased affinity** of hemoglobin for oxygen. This means hemoglobin binds oxygen more tightly in the lungs but releases it less readily to the tissues. **Alkalosis** (increased pH/decreased $H^+$ concentration) stabilizes the "R" (Relaxed) state of hemoglobin, which has a higher affinity for oxygen, thus shifting the curve to the left. ### **Why Other Options are Incorrect** * **Acidosis (A):** Increased $H^+$ ions (decreased pH) shift the curve to the **right** (Bohr Effect). This decreases Hb-O2 affinity, facilitating oxygen unloading to tissues. * **Hyperthermia (C):** Increased temperature increases the kinetic energy of the molecules, weakening the bond between Hb and $O_2$, shifting the curve to the **right**. * **Anemia (D):** While anemia reduces the total oxygen-carrying capacity, the chronic compensatory increase in **2,3-BPG** levels in anemic patients typically shifts the curve to the **right** to improve tissue oxygenation. ### **High-Yield Clinical Pearls for NEET-PG** To remember the factors shifting the curve, use the mnemonic **"CADET, face Right!"** Factors that shift the curve to the **Right** (Decreased affinity): * **C** – $CO_2$ (Hypercapnia) * **A** – Acidosis * **D** – 2,3-DPG (increased) * **E** – Exercise * **T** – Temperature (Hyperthermia) **Note:** Fetal Hemoglobin (HbF) and Carbon Monoxide (CO) poisoning both shift the curve to the **Left**, though CO poisoning also decreases the maximum oxygen-carrying capacity (plateau).

Question 664: Anemic hypoxia is seen in which of the following conditions?

A. Carbon monoxide poisoning (Correct Answer)
B. Carbon dioxide poisoning
C. Hydrogen cyanide poisoning
D. Nerve gas exposure

Explanation: **Explanation:** **Anemic hypoxia** occurs when the arterial $PO_2$ is normal, but the total oxygen-carrying capacity of the blood is reduced. This can be due to a decrease in hemoglobin concentration or an alteration in the hemoglobin molecule that prevents it from binding or releasing oxygen effectively. **Why Carbon Monoxide (CO) Poisoning is correct:** In CO poisoning, carbon monoxide binds to hemoglobin with an affinity **200–250 times greater** than oxygen, forming **carboxyhemoglobin**. This results in: 1. A reduction in the number of binding sites available for oxygen. 2. A **leftward shift** of the Oxygen-Hemoglobin Dissociation Curve (OHDC), which prevents the unloading of oxygen to tissues. Despite normal dissolved oxygen ($PO_2$), the total oxygen content is severely reduced, fitting the definition of anemic hypoxia. **Analysis of Incorrect Options:** * **Carbon dioxide poisoning:** High levels of $CO_2$ (hypercapnia) lead to respiratory acidosis but do not primarily cause anemic hypoxia. * **Hydrogen cyanide poisoning:** This causes **histotoxic hypoxia**. Cyanide inhibits the enzyme **cytochrome oxidase** in the mitochondria, preventing tissues from utilizing oxygen despite adequate delivery. * **Nerve gas exposure:** These agents (e.g., Sarin) inhibit acetylcholinesterase, leading to a cholinergic crisis. Death occurs due to respiratory failure (paralysis of respiratory muscles), which leads to **hypoxic hypoxia**. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Sign:** CO poisoning presents with "cherry-red" skin/mucosa (though rare in clinical practice). * **Methemoglobinemia:** Another classic cause of anemic hypoxia (presents with "chocolate-colored" blood and cyanosis). * **Key Distinction:** In anemic hypoxia, **Arterial $PO_2$ is normal**, but **Arterial $O_2$ content is decreased**.

Question 665: Least arteriovenous oxygen difference is seen in which of the following conditions?

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

Explanation: The arteriovenous (A-V) oxygen difference represents the amount of oxygen extracted by tissues from the blood. **Correct Answer: D. Cyanide Poisoning (Histotoxic Hypoxia)** In cyanide poisoning, the cyanide ion binds to the ferric ($Fe^{3+}$) iron in **cytochrome oxidase a3** within the mitochondrial electron transport chain. This inhibits cellular respiration, rendering tissues unable to utilize the oxygen delivered to them. Since oxygen is not extracted, the venous blood remains highly oxygenated, nearly matching the arterial oxygen content. This results in the **least (minimal) A-V oxygen difference** and classically causes the venous blood to appear "cherry red." **Explanation of Incorrect Options:** * **A. Hypoxic Hypoxia:** Caused by low atmospheric $PO_2$ or hypoventilation. While arterial oxygen is low, tissues still extract oxygen to survive, maintaining a significant A-V difference. * **B. Anemic Hypoxia:** Arterial $PO_2$ is normal, but hemoglobin concentration is low. Tissues extract a larger percentage of the available oxygen, often leading to a normal or slightly increased A-V difference relative to the total oxygen carrying capacity. * **C. Stagnant Hypoxia:** Occurs due to reduced blood flow (e.g., heart failure or shock). Because blood moves slowly through capillaries, tissues have more time to extract oxygen, leading to a **maximal (increased) A-V oxygen difference.** **High-Yield Pearls for NEET-PG:** 1. **Stagnant Hypoxia:** Characterized by the *highest* A-V oxygen difference. 2. **Histotoxic Hypoxia:** The only type where venous $PO_2$ is elevated. 3. **Cyanosis:** Not seen in Anemic hypoxia (low Hb) or Histotoxic hypoxia (high venous $O_2$). 4. **Treatment for Cyanide:** Amyl nitrite/Sodium nitrite (creates methemoglobin to sequester cyanide) and Sodium thiosulfate.

Question 666: Death due to cyanide poisoning results from which of the following types of anoxia?

A. Anoxic anoxia
B. Anaemic anoxia
C. Stagnant anoxia
D. Histotoxic anoxia (Correct Answer)

Explanation: **Explanation:** **Histotoxic anoxia** is the correct answer because cyanide poisoning interferes with the utilization of oxygen at the cellular level, rather than its delivery. Cyanide binds to the **ferric (Fe³⁺) iron** in the **cytochrome c oxidase** enzyme (Complex IV) of the mitochondrial electron transport chain. This inhibits aerobic respiration, preventing cells from using the oxygen delivered to them. Consequently, the venous blood remains highly oxygenated, often giving the patient a characteristic "cherry-red" skin appearance. **Analysis of Incorrect Options:** * **Anoxic Anoxia:** Occurs when there is a decrease in the arterial partial pressure of oxygen ($PaO_2$). Common causes include high altitude, airway obstruction, or alveolar hypoventilation. * **Anaemic Anoxia:** Occurs when the $PaO_2$ is normal, but the oxygen-carrying capacity of the blood is reduced. Causes include anemia, hemorrhage, or carbon monoxide (CO) poisoning (where Hb is unavailable). * **Stagnant (Ischemic) Anoxia:** Occurs when blood flow to the tissues is slowed or stopped despite normal oxygen content. Causes include heart failure, shock, or local thrombosis. **Clinical Pearls for NEET-PG:** * **Key Finding:** In histotoxic hypoxia, the **Arterio-Venous (A-V) oxygen difference is decreased** because tissues cannot extract oxygen from the blood. * **Antidote for Cyanide:** Amyl nitrite/Sodium nitrite (to create methemoglobin, which sequesters cyanide) followed by Sodium thiosulfate (to convert cyanide to non-toxic thiocyanate). Hydroxocobalamin is also a first-line treatment. * **Classic Sign:** "Cherry-red" discoloration of skin and mucous membranes (due to high venous $O_2$ saturation).

Question 667: The main controlling agent for respiratory drive is which of the following?

A. CO2 (Correct Answer)
B. Oxygen
C. NO
D. HCO3

Explanation: **Explanation:** The primary drive for respiration in a healthy individual is the arterial concentration of **Carbon Dioxide (CO2)**. This is mediated through two main pathways: 1. **Central Chemoreceptors (Primary):** Located in the medulla oblongata, these are exquisitely sensitive to changes in the pH of the cerebrospinal fluid (CSF). While H+ ions cannot cross the blood-brain barrier, CO2 diffuses readily. Once in the CSF, CO2 hydrates to form carbonic acid, which dissociates into H+ and HCO3-. The resulting rise in H+ directly stimulates the chemosensitive area, increasing the respiratory rate. 2. **Peripheral Chemoreceptors:** Located in the carotid and aortic bodies, these respond to increases in PCO2 and decreases in pH, though they are less influential than the central receptors for CO2 regulation. **Analysis of Incorrect Options:** * **B. Oxygen:** Under normal physiological conditions, oxygen plays a secondary role. The "hypoxic drive" only becomes the primary stimulus when arterial PO2 falls below **60 mmHg**. * **C. NO (Nitric Oxide):** NO is a potent vasodilator and neurotransmitter but does not act as a primary regulator of the respiratory center. * **D. HCO3 (Bicarbonate):** While bicarbonate acts as a buffer, it does not cross the blood-brain barrier easily and is a product of CO2 metabolism rather than the primary trigger for the drive. **High-Yield NEET-PG Pearls:** * **Most potent stimulus** for central chemoreceptors: **H+ ions** (derived from CO2). * **Most potent stimulus** for peripheral chemoreceptors: **Decreased PO2** (<60 mmHg). * **Clinical Correlation:** In patients with chronic hypercapnia (e.g., severe COPD), the central receptors become desensitized to CO2, and the respiratory drive becomes dependent on low Oxygen (Hypoxic Drive). Giving high-flow oxygen to these patients can suppress their drive to breathe.

Question 668: A 62-year-old patient has been diagnosed with a restrictive pulmonary disease. Which of the following lung measurements is likely to be normal?

A. FEV1
B. FVC
C. FEV1/FVC (Correct Answer)
D. FRC

Explanation: In **Restrictive Lung Diseases** (e.g., Idiopathic Pulmonary Fibrosis, Sarcoidosis, or Chest wall deformities), the primary pathology is reduced lung compliance or "stiffness." This leads to a global reduction in all lung volumes and capacities. ### 1. Why FEV1/FVC is the Correct Answer In restrictive disease, both the **FEV1** (Forced Expiratory Volume in 1 second) and the **FVC** (Forced Vital Capacity) decrease proportionately. Because both the numerator and denominator decrease, the **FEV1/FVC ratio remains normal (typically >0.7 or 70%) or may even be slightly increased** due to increased radial traction on the airways, which keeps them open during expiration. ### 2. Why Other Options are Incorrect * **FEV1 (Option A):** This is decreased. Although there is no airway obstruction, the total volume of air the lungs can hold is reduced, so the amount exhaled in the first second is lower than normal. * **FVC (Option B):** This is the hallmark of restriction. FVC is significantly decreased because the stiff lungs cannot expand fully. * **FRC (Option D):** Functional Residual Capacity is decreased in restrictive disease. Since the inward elastic recoil of the lungs is increased, the equilibrium point between the lungs and chest wall (FRC) shifts to a lower volume. ### 3. High-Yield Clinical Pearls for NEET-PG * **Obstructive Disease (e.g., Asthma/COPD):** FEV1 decreases significantly more than FVC, leading to a **decreased FEV1/FVC ratio (<0.7).** * **Flow-Volume Loop:** In restrictive disease, the loop is shifted to the **right**, appearing tall and narrow (the "Witch’s Hat" appearance). * **TLC (Total Lung Capacity):** This is the gold standard for diagnosing restriction; a TLC <80% of predicted confirms the diagnosis.

Question 669: Identify the process indicated by '1' in the illustration depicting lung compliance.

A. Inspiration (Correct Answer)
B. Expiration
C. Inspiratory air
D. Expiratory air

Explanation: ***Inspiration*** - Curve '1' in the **pressure-volume diagram** represents the **inflation phase** of the lung during inspiration, where volume increases with increasing transpulmonary pressure. - During inspiration, **surfactant** reduces surface tension progressively, allowing easier lung expansion as alveoli recruit and inflate. *Expiration* - This would be represented by curve '2' in the **hysteresis loop**, showing the **deflation phase** where volume decreases with decreasing pressure. - The **expiratory curve** follows a different path due to **surface tension hysteresis** and surfactant properties during deflation. *Inspiratory air* - This refers to the **actual air volume** being inhaled, not the process or curve shown in the compliance diagram. - The curve '1' represents the **mechanical relationship** between pressure and volume, not the air itself. *Expiratory air* - This refers to the **actual air volume** being exhaled, not the process or curve depicted. - The **P-V loop** demonstrates lung mechanics and compliance, not the composition or volume of expired air.

Question 670: The alveoli are normally kept dry by which of the following mechanisms?

A. Macrophages
B. Surfactant (Correct Answer)
C. Negative intrapleural pressure
D. High pCO2 in the alveoli

Explanation: **Explanation:** The dryness of the alveoli is primarily maintained by **Surfactant**, which reduces surface tension at the air-liquid interface. According to the **Law of Laplace ($P = 2T/r$)**, surface tension ($T$) creates an inward collapsing pressure ($P$) that tends to pull fluid from the pulmonary capillaries into the alveolar space (pulmonary edema). By significantly lowering surface tension, surfactant minimizes this inward hydrostatic pressure, preventing fluid transudation and keeping the alveoli dry for efficient gas exchange. **Analysis of Incorrect Options:** * **Macrophages (A):** These are immune cells responsible for phagocytosing debris and pathogens; they do not regulate fluid dynamics or surface tension. * **Negative intrapleural pressure (C):** This pressure keeps the lungs expanded against the chest wall. If anything, excessive negative pressure (as seen in upper airway obstruction) can actually promote pulmonary edema by increasing the pressure gradient that pulls fluid into the alveoli. * **High pCO2 (D):** High alveolar $pCO_2$ typically causes localized vasodilation but does not play a role in maintaining alveolar dryness. **Clinical Pearls for NEET-PG:** * **Composition:** Surfactant is 90% lipids (mainly **Dipalmitoylphosphatidylcholine - DPPC**) and 10% proteins (SP-A, B, C, D). * **Source:** Secreted by **Type II Pneumocytes** (lamellar bodies). * **Clinical Correlation:** Deficiency of surfactant in premature infants leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease, characterized by alveolar collapse and pulmonary edema. * **Other factors keeping alveoli dry:** Low pulmonary capillary hydrostatic pressure (~10 mmHg) and efficient lymphatic drainage.

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