Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 601: What are the properties of Helium?

A. Atomic number 2
B. Viscosity is zero
C. Used in COPD
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. All of the above**, based on the physical properties of Helium and its clinical application in respiratory medicine. 1. **Atomic Number 2:** Helium is the second element in the periodic table. It is a noble gas, characterized by its low density and chemical inertness. 2. **Viscosity is Zero (Relative Concept):** While no gas has an absolute viscosity of zero, in the context of respiratory physiology, Helium has a **very low density** (about 1/7th that of air). According to **Graham’s Law**, the rate of diffusion is inversely proportional to the square root of density. Furthermore, in the airways, flow is often turbulent. Helium reduces the **Reynolds number**, converting turbulent flow into **laminar flow**, which significantly reduces the work of breathing. 3. **Used in COPD:** Helium is clinically administered as **Heliox** (usually a mixture of 79% Helium and 21% Oxygen). In obstructive conditions like COPD or severe asthma, Heliox decreases airway resistance and improves gas delivery to the alveoli by promoting laminar flow through narrowed airways. **Clinical Pearls for NEET-PG:** * **Heliox Ratio:** Most commonly used as 80:20 or 70:30 (Helium:Oxygen). * **Reynolds Number ($Re$):** $Re = (\text{Density} \times \text{Velocity} \times \text{Diameter}) / \text{Viscosity}$. Helium’s low density is the primary factor that lowers $Re$, preventing turbulence. * **Indications:** Acute severe asthma, COPD exacerbations, and upper airway obstruction (e.g., stridor, post-extubation croup). * **Diagnostic Use:** Helium dilution method is used to measure **Functional Residual Capacity (FRC)** and Residual Volume (RV).

Question 602: The carotid body contains islands of type I and type II cells surrounded by fenestrated sinusoidal capillaries. Type I cells are excited by hypoxia. What is the principal transmitter released by type I cells?

A. Serotonin
B. Adrenaline
C. Dopamine (Correct Answer)
D. Potassium

Explanation: **Explanation:** The **carotid body** is a peripheral chemoreceptor located at the bifurcation of the common carotid artery. It consists of two cell types: **Type I (Glomus) cells**, which are the primary oxygen sensors, and Type II (Sustentacular) cells, which provide structural support. **Why Dopamine is correct:** When arterial $PO_2$ decreases (hypoxia), $K^+$ channels in the Type I cell membrane close, leading to depolarization. This opens voltage-gated $Ca^{2+}$ channels, triggering the exocytosis of neurotransmitters. **Dopamine** is the principal and most abundant neurotransmitter stored in the dense-core vesicles of Type I cells. It acts on the sensory nerve endings of the **glossopharyngeal nerve (CN IX)** to increase the firing rate to the respiratory centers in the medulla. **Analysis of Incorrect Options:** * **A. Serotonin:** While trace amounts of serotonin and acetylcholine are found in glomus cells, they are not the primary transmitters responsible for the initial hypoxic response. * **B. Adrenaline:** This is the primary hormone of the adrenal medulla, not the carotid body. * **D. Potassium:** Potassium is an ion, not a neurotransmitter. While the *closure* of $K^+$ channels initiates the response, it is not the substance released to signal the afferent nerve. **High-Yield Facts for NEET-PG:** * **Innervation:** Carotid body signals via the **Hering’s nerve** (branch of CN IX); Aortic body signals via the **Vagus nerve** (CN X). * **Sensitivity:** Peripheral chemoreceptors are primarily sensitive to **low $PO_2$** (<60 mmHg), but also respond to high $PCO_2$ and low pH (H+). * **Blood Flow:** The carotid body has the highest blood flow per unit weight in the body (approx. 2000 ml/100g/min), allowing it to monitor arterial blood gases instantaneously.

Question 603: Which of the following ions is involved in peripheral oxygen sensing chemoreceptors?

A. Potassium (Correct Answer)
B. Calcium
C. Sodium
D. Chlorine

Explanation: The peripheral chemoreceptors (located in the **Carotid and Aortic bodies**) are primarily responsible for sensing arterial hypoxia ($PaO_2 < 60$ mmHg). The key cell type involved is the **Glomus cell (Type I cell)**. ### Why Potassium (A) is Correct: The physiological mechanism of oxygen sensing follows a specific sequence: 1. **Hypoxia** leads to the inhibition (closure) of **Oxygen-sensitive Potassium ($K^+$) channels** on the glomus cell membrane. 2. The resulting decrease in $K^+$ efflux causes **depolarization** of the cell. 3. This depolarization opens voltage-gated **Calcium channels**, leading to $Ca^{2+}$ influx and the release of neurotransmitters (mainly **ATP** and Acetylcholine). 4. These transmitters stimulate the afferent nerves (Glossopharyngeal and Vagus) to signal the respiratory centers in the medulla. *Note: While Calcium is involved in the later stage of neurotransmitter release, the primary "sensing" ion that initiates the electrical response to hypoxia is Potassium.* ### Why Other Options are Incorrect: * **B. Calcium:** Calcium influx is a *downstream* effect of depolarization; it is the trigger for exocytosis, not the primary sensor of oxygen levels. * **C. Sodium:** Sodium channels are involved in propagating action potentials but do not serve as the initial chemical sensor for hypoxia. * **D. Chlorine:** Chloride ions are primarily involved in inhibitory neurotransmission (GABA) or the "Chloride Shift" in RBCs, but play no role in peripheral chemoreception. ### High-Yield Clinical Pearls for NEET-PG: * **Primary Stimulus:** Hypoxia ($ \downarrow PaO_2$) is the strongest stimulus for peripheral chemoreceptors. They also respond to $\uparrow PaCO_2$ and $\downarrow pH$. * **Central Chemoreceptors:** Located in the medulla; they respond **only** to $\uparrow H^+$ (via $\uparrow PaCO_2$) and do **not** respond to hypoxia. * **Nerve Supply:** Carotid body $\rightarrow$ Hering’s nerve (branch of CN IX); Aortic body $\rightarrow$ Vagus nerve (CN X).

Question 604: Moderate exercise is one of the most powerful stimulators of ventilation. By what mechanism does this occur?

A. Low partial pressure of oxygen in inspired air
B. Low partial pressure of oxygen in mixed venous blood
C. Low pH in inspired air
D. None of the above (Correct Answer)

Explanation: ### Explanation The correct answer is **None of the above** because the primary drive for increased ventilation during moderate exercise is not related to changes in blood gases or pH in the inspired air. **1. Why "None of the above" is correct:** During moderate exercise, arterial $PO_2$, $PCO_2$, and pH remain remarkably **constant** and within normal limits. The dramatic increase in ventilation (hyperpnea) is primarily driven by **neural mechanisms** rather than chemical changes. These include: * **Feed-forward signals:** The cerebral cortex sends parallel impulses to the respiratory muscles and the skeletal muscles (central command). * **Proprioceptive feedback:** Joint and muscle mechanoreceptors stimulate the respiratory center as soon as movement begins. * **Body temperature:** A rise in core temperature further stimulates the respiratory drive. **2. Why the other options are incorrect:** * **Option A & C:** The composition of **inspired air** (ambient air) does not change during exercise. Changes in ventilation are driven by internal metabolic demands, not the quality of the air being inhaled. * **Option B:** While $PO_2$ in **mixed venous blood** does decrease during exercise (due to increased oxygen extraction by muscles), the peripheral and central chemoreceptors respond to **arterial** blood gas levels, not venous levels. Since arterial $PO_2$ remains normal, this is not the trigger for increased ventilation. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Oscillatory Hypothesis:** Some experts suggest that while *mean* arterial values are constant, rapid *oscillations* in $PCO_2$ during exercise might stimulate chemoreceptors. * **Anaerobic Threshold:** In **severe** (not moderate) exercise, lactic acid accumulates, causing arterial pH to drop. This stimulates peripheral chemoreceptors, leading to a further compensatory increase in ventilation. * **Phase 1 of Exercise:** The initial, immediate rise in ventilation is entirely neural. The subsequent gradual increase (Phase 2) involves humoral factors.

Question 605: The surfactant is produced by which of the following cells?

A. Type I Pneumocytes
B. Type II Pneumocytes (Correct Answer)
C. Alveolar macrophages
D. Clara cells

Explanation: **Explanation:** The correct answer is **Type II Pneumocytes**. **1. Why Type II Pneumocytes are correct:** Type II pneumocytes (granular pneumocytes) are cuboidal cells located in the alveolar walls, covering about 5% of the alveolar surface area. Their primary function is the synthesis, storage, and secretion of **pulmonary surfactant**, a lipoprotein complex (mainly Dipalmitoylphosphatidylcholine - DPPC). Surfactant reduces surface tension at the air-liquid interface, preventing alveolar collapse (atelectasis) during expiration and increasing lung compliance. These cells also act as "progenitor cells"; they can proliferate and differentiate into Type I pneumocytes following lung injury. **2. Why the other options are incorrect:** * **Type I Pneumocytes:** These are thin, squamous cells covering 95% of the alveolar surface. Their primary role is facilitating gas exchange due to their minimal thickness. They do not produce surfactant. * **Alveolar Macrophages (Dust Cells):** These are immune cells that phagocytose debris, dust, and pathogens within the alveoli. * **Clara Cells (Club Cells):** Found in the bronchioles, these cells secrete a surfactant-like substance (surface-active agent) and uteroglobin, but they are not the primary source of pulmonary alveolar surfactant. **High-Yield Clinical Pearls for NEET-PG:** * **Lamellar Bodies:** Surfactant is stored in these characteristic intracellular organelles within Type II pneumocytes. * **Development:** Surfactant production begins between **24–28 weeks** of gestation, but adequate levels are usually reached only after **35 weeks**. * **L/S Ratio:** A Lecithin/Sphingomyelin ratio of **>2** in amniotic fluid indicates fetal lung maturity. * **NRDS:** Deficiency of surfactant in premature infants leads to Neonatal Respiratory Distress Syndrome (Hyaline Membrane Disease).

Question 606: A 54-year-old man sustains third degree burns in a house fire. His respiratory rate is 30/min, Hb = 17 g/dL, arterial PO2 is 95 mm Hg, and arterial O2 saturation is 50%. What is the most likely cause of his low oxygen saturation?

A. Airway obstruction from smoke inhalation
B. Carbon monoxide poisoning (Correct Answer)
C. Pulmonary edema
D. Fever

Explanation: ### Explanation **Correct Option: B. Carbon Monoxide (CO) Poisoning** The hallmark of carbon monoxide poisoning is a **normal arterial $PO_2$** (dissolved oxygen) but a **critically low arterial $O_2$ saturation ($SaO_2$)**. * **Mechanism:** CO has an affinity for hemoglobin that is 200–250 times greater than oxygen. It binds to hemoglobin to form **carboxyhemoglobin**, physically displacing oxygen and reducing the $SaO_2$. * **The "Normal $PO_2$" Trap:** $PO_2$ measures oxygen dissolved in plasma, which is unaffected by CO. Therefore, the patient is not technically "hypoxemic" in terms of $PO_2$, but suffers from severe **anemic hypoxia** because the oxygen-carrying capacity is decimated. * **Left Shift:** CO also causes a leftward shift of the Oxyhemoglobin Dissociation Curve (OHDC), making it harder for the remaining oxygen to be released to tissues. **Why Incorrect Options are Wrong:** * **A & C (Airway Obstruction/Pulmonary Edema):** Both conditions interfere with gas exchange at the alveolar-capillary membrane. This would lead to a **low arterial $PO_2$** (hypoxemia), which is not present in this patient ($PO_2$ = 95 mmHg is normal). * **D (Fever):** Fever causes a **Right shift** of the OHDC (decreasing affinity), but it does not cause a massive drop in $SaO_2$ to 50% in the presence of normal $PO_2$. **NEET-PG High-Yield Pearls:** 1. **Pulse Oximetry Gap:** Standard pulse oximeters cannot distinguish between oxyhemoglobin and carboxyhemoglobin; they may show a falsely normal $SaO_2$. Co-oximetry is required for diagnosis. 2. **Cherry Red Skin:** A classic but rare clinical sign of CO poisoning. 3. **Treatment:** 100% Hyperbaric Oxygen (HBO) to reduce the half-life of carboxyhemoglobin. 4. **OHDC Shift:** CO poisoning causes a **Left shift**; Anemia (low Hb) does not shift the curve, but CO poisoning (functional anemia) does.

Question 607: What is the respiratory quotient of protein in the body?

A. 0.5
B. 0.8 (Correct Answer)
C. 0.75
D. 1

Explanation: **Explanation:** The **Respiratory Quotient (RQ)** is the ratio of the volume of carbon dioxide ($CO_2$) produced to the volume of oxygen ($O_2$) consumed per unit of time ($RQ = \frac{CO_2 \text{ produced}}{O_2 \text{ consumed}}$). This value varies depending on the type of substrate being oxidized for energy. **Why Option B is Correct:** Proteins have an average RQ of **0.8**. This is because proteins are not completely oxidized in the body (the nitrogenous component is excreted as urea), and their chemical structure requires more oxygen for oxidation relative to the amount of $CO_2$ they produce compared to carbohydrates. **Analysis of Incorrect Options:** * **Option A (0.5):** This value is lower than any standard physiological substrate. An RQ this low is generally not seen under normal metabolic conditions. * **Option C (0.75):** This is close to the RQ of **Fats (0.7)**. Fats have a lower RQ because they are "oxygen-poor" molecules, requiring significantly more external oxygen to oxidize their long hydrocarbon chains. * **Option D (1):** This is the RQ for **Carbohydrates**. Since carbohydrates contain enough internal oxygen to react with their own hydrogen atoms to form water, the oxygen consumed from the air is used solely to form $CO_2$ in a 1:1 ratio. **High-Yield Clinical Pearls for NEET-PG:** * **Mixed Diet:** The average RQ for an individual on a standard mixed diet is approximately **0.82–0.85**. * **Overfeeding/Lipogenesis:** If the RQ exceeds **1.0**, it indicates lipogenesis (conversion of carbohydrates to fats), often seen in overfed patients on TPN (Total Parenteral Nutrition). * **Starvation/Diabetes:** In states of starvation or uncontrolled Diabetes Mellitus, the RQ drops toward **0.7** as the body shifts to fat utilization. * **Non-protein RQ:** Calculated to determine the relative combustion of carbohydrates and fats by subtracting the $CO_2$ and $O_2$ associated with protein metabolism (measured via urinary nitrogen).

Question 608: Which of the following findings demonstrates a difference between pulmonary circulation and systemic circulation?

A. Pulmonary vasodilation in response to hypoxia
B. Pulmonary vasoconstriction in response to hypoxia (Correct Answer)
C. Decreased blood volume during systole
D. Increased basal vasoconstrictor tone

Explanation: **Explanation:** The fundamental difference between pulmonary and systemic circulation lies in their response to low oxygen levels (hypoxia). **1. Why Option B is Correct:** In the **systemic circulation**, hypoxia causes **vasodilation** to increase blood flow and oxygen delivery to tissues. However, in the **pulmonary circulation**, hypoxia triggers **Hypoxic Pulmonary Vasoconstriction (HPV)**. This is a protective mechanism where pulmonary arterioles constrict in poorly ventilated areas of the lung. This shunts blood away from hypoxic alveoli toward well-ventilated alveoli, optimizing **ventilation-perfusion (V/Q) matching** and preventing arterial hypoxemia. **2. Why the other options are incorrect:** * **Option A:** This is the opposite of the physiological reality in the lungs; vasodilation in response to hypoxia occurs in systemic vessels (e.g., skeletal muscle), not pulmonary ones. * **Option C:** Blood volume changes during the cardiac cycle do not define the functional difference between these two circuits in the context of vascular regulation. * **Option D:** The pulmonary circulation is a **low-pressure, low-resistance** system with very little basal vasoconstrictor tone compared to the systemic circulation, which maintains high tone to regulate blood pressure. **High-Yield NEET-PG Pearls:** * **Mechanism of HPV:** Hypoxia inhibits voltage-gated potassium channels in pulmonary artery smooth muscle cells, leading to depolarization and calcium influx, causing contraction. * **Clinical Correlation:** Global alveolar hypoxia (e.g., at high altitudes) causes generalized pulmonary vasoconstriction, leading to **High Altitude Pulmonary Edema (HAPE)** and pulmonary hypertension. * **V/Q Ratio:** The main goal of HPV is to minimize "shunting" by ensuring blood only goes where oxygen is available.

Question 609: What is the most significant change in maternal lung volume that occurs in the third trimester of pregnancy?

A. Decrease in vital capacity
B. Increase in residual volume
C. Decrease in functional residual capacity (Correct Answer)
D. Decrease in closing capacity

Explanation: ### Explanation The correct answer is **C. Decrease in functional residual capacity (FRC).** #### 1. Why is the correct answer right? During the third trimester, the enlarging uterus displaces the diaphragm cephalad (upward) by approximately 4 cm. This mechanical elevation reduces the resting volume of the lungs at the end of a normal expiration. * **Functional Residual Capacity (FRC)** is the sum of Expiratory Reserve Volume (ERV) and Residual Volume (RV). * In late pregnancy, FRC decreases by approximately **20–30%**. This is the most significant and clinically relevant change in lung volumes during pregnancy. #### 2. Why the other options are wrong: * **A. Decrease in vital capacity:** Vital capacity (VC) remains **unchanged** or may slightly increase. Although the diaphragm is elevated, there is a compensatory increase in the anteroposterior and transverse diameters of the thoracic cage due to the relaxation of ligamentous attachments (mediated by the hormone Relaxin). * **B. Increase in residual volume:** Residual volume (RV) actually **decreases** (by about 15–20%) due to the upward pressure on the diaphragm. * **D. Decrease in closing capacity:** Closing capacity (CC) remains **unchanged** during pregnancy. However, because the FRC decreases, the FRC may fall below the CC in some pregnant women when supine, leading to small airway closure and atelectasis. #### 3. High-Yield Facts for NEET-PG: * **Tidal Volume (TV):** Increases by ~40% (due to progesterone stimulating the respiratory center). * **Minute Ventilation:** Increases by ~40–50% (primarily due to increased TV, not respiratory rate). * **Inspiratory Capacity (IC):** Increases by ~10% to compensate for the decreased FRC. * **Clinical Pearl:** The combination of **decreased FRC** (lower oxygen reserve) and **increased metabolic rate** (higher oxygen demand) makes pregnant patients desaturate very rapidly during periods of apnea or induction of anesthesia.

Question 610: The provided pressure-volume curves represent a normal subject and a patient with a pulmonary disease. Based on these curves, what is the likely pulmonary condition of the patient?

A. Pulmonary fibrosis
B. Asbestosis
C. Emphysema (Correct Answer)
D. Pulmonary congestion

Explanation: ***Emphysema*** - **Increased lung compliance** causes the pressure-volume curve to shift **left and upward**, allowing greater volume at lower pressures due to **alveolar wall destruction**. - Loss of **elastic recoil** from damaged alveolar septa results in easier lung expansion but impaired expiration. *Pulmonary fibrosis* - **Decreased lung compliance** shifts the pressure-volume curve **right and downward**, requiring higher pressures to achieve the same volume. - **Collagen deposition** and **fibrotic tissue** make the lungs stiffer and harder to expand, opposite to the pattern seen in emphysema. *Asbestosis* - A **restrictive lung disease** that decreases compliance, shifting the curve **right and downward** like other fibrotic conditions. - **Asbestos fiber deposition** causes **pulmonary fibrosis** and scarring, making lungs less distensible, not more compliant. *Pulmonary congestion* - **Fluid accumulation** in pulmonary interstitium and alveoli decreases compliance, shifting the curve **right and downward**. - **Increased pulmonary capillary pressure** from heart failure causes **interstitial edema**, making lungs stiffer and harder to ventilate.

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