Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 101: Of the total oxygen consumed by the body per minute, which organ consumes the greatest fraction?

A. Liver (Correct Answer)
B. Brain
C. Muscle
D. Kidney

Explanation: **Explanation:** The correct answer is **Liver (Option A)**. Oxygen consumption ($VO_2$) is determined by the metabolic activity of an organ. At rest, the liver is the most metabolically active organ in the body, accounting for approximately **20–27%** of total oxygen consumption. This high demand is due to its continuous role in protein synthesis, gluconeogenesis, urea formation, and detoxification processes. **Why other options are incorrect:** * **Brain (Option B):** The brain is highly aerobic and accounts for about **18–20%** of total $VO_2$. While it has a high metabolic rate per gram of tissue, its total consumption is slightly less than that of the liver. * **Muscle (Option C):** At rest, skeletal muscle accounts for about **15–20%** of $VO_2$. However, during heavy exercise, muscle becomes the dominant consumer, potentially accounting for over 90% of total body oxygen use. * **Kidney (Option D):** The kidneys consume about **6–7%** of total $VO_2$. Most of this energy is dedicated to the active transport of sodium ($Na^+$) in the renal tubules. **High-Yield Facts for NEET-PG:** 1. **Oxygen Consumption ($VO_2$) at Rest:** Liver (27%) > Brain (20%) > Muscle (18%) > Kidney (7%) > Heart (4%). 2. **Oxygen Extraction Ratio:** The **Heart** has the highest oxygen extraction ratio (70–80%), meaning it removes the most oxygen per unit of blood delivered. 3. **Blood Flow per 100g:** The **Carotid Body** has the highest blood flow per unit mass, while the **Kidney** receives the highest percentage of total Cardiac Output (20–25%).

Question 102: Vagal stimulation causes which of the following effects on respiration?

A. Increase in rate of respiration
B. Increase in depth of respiration
C. Bronchodilation
D. Decreased depth of respiration (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Decreased depth of respiration.** **1. Understanding the Mechanism (The Hering-Breuer Reflex)** The Vagus nerve (CN X) is the primary sensory pathway for pulmonary stretch receptors located in the smooth muscles of the airways. When the lungs inflate, these receptors are stimulated and send afferent signals via the Vagus nerve to the **Dorsal Respiratory Group (DRG)** and the **Apneustic center** in the medulla and pons. This triggers the **Hering-Breuer Inflation Reflex**, which prematurely terminates inspiration (the "switch-off" mechanism). By shortening the inspiratory phase, vagal stimulation effectively **decreases the depth of respiration (tidal volume)** to prevent over-inflation of the lungs. **2. Analysis of Incorrect Options** * **A & B: Increase in rate/depth:** Vagal stimulation inhibits inspiration. Conversely, a **vagotomy** (cutting the vagus nerves) would lead to a loss of this inhibitory signal, resulting in a breathing pattern that is characteristically **slow and deep**. * **C. Bronchodilation:** This is incorrect because the Vagus nerve provides **parasympathetic** innervation to the lungs. Parasympathetic activation causes **bronchoconstriction** and increased glandular secretion via M3 muscarinic receptors. Bronchodilation is a sympathetic (adrenergic) response. **3. High-Yield Clinical Pearls for NEET-PG** * **Hering-Breuer Reflex:** In adults, this reflex is typically inactive during normal quiet breathing and only functions when tidal volume exceeds **~1.5 liters** (e.g., during exercise). However, it is highly active in **newborns**. * **Vagotomy Effect:** A classic exam question asks about the effect of bilateral vagotomy; the answer is always **"Slow and Deep"** breathing. * **Pneumotaxic Center:** Located in the upper pons (Nucleus Parabrachialis), it works alongside the Vagus nerve to limit inspiration. If both the Vagus nerves and the Pneumotaxic center are removed, **Apneusis** (prolonged inspiratory gasps) occurs.

Question 103: An adult has a respiratory rate of 15/min, tidal volume is 400cc, and anatomic dead space is 100cc. Calculate the respiratory minute volume.

A. 4500cc
B. 4000cc
C. 6000cc (Correct Answer)
D. 3500cc

Explanation: ### Explanation **1. Why the Correct Answer (C) is Right** The **Respiratory Minute Volume (RMV)**, also known as Minute Ventilation, is the total volume of gas entering (or leaving) the lungs per minute. It is calculated using the formula: $$\text{RMV} = \text{Tidal Volume (TV)} \times \text{Respiratory Rate (RR)}$$ Given: * Tidal Volume (TV) = 400 cc * Respiratory Rate (RR) = 15/min * Calculation: $400 \times 15 = 6000 \text{ cc/min}$ (or 6 L/min). The **Anatomic Dead Space** (100 cc) is provided as a distractor. While dead space is essential for calculating *Alveolar Ventilation*, it is **not** subtracted when calculating the total Minute Volume. **2. Why the Incorrect Options are Wrong** * **Option A (4500cc):** This is the **Alveolar Ventilation**. It is calculated as $(TV - \text{Dead Space}) \times RR$, i.e., $(400 - 100) \times 15 = 4500 \text{ cc}$. This represents the actual gas exchange volume. * **Option B (4000cc):** This value is obtained if one incorrectly assumes a respiratory rate of 10/min or miscalculates the product. * **Option D (3500cc):** This value does not correlate with standard respiratory physiological formulas using the provided data. **3. NEET-PG High-Yield Pearls** * **Minute Volume vs. Alveolar Ventilation:** Always check if the question asks for "Minute Volume" (Total air) or "Alveolar Ventilation" (Air reaching respiratory units). * **Anatomic Dead Space:** In a healthy adult, it is approximately **2 ml/kg** of body weight (roughly 150 ml). * **Dead Space Types:** * *Anatomic:* Volume of conducting airways. * *Physiologic:* Anatomic + Alveolar dead space (wasted ventilation in non-perfused alveoli). In healthy individuals, Anatomic $\approx$ Physiologic dead space. * **Fowler’s Method** measures Anatomic dead space, while **Bohr’s Equation** measures Physiologic dead space.

Question 104: Apneusis occurs when?

A. A lesion is above the pons
B. A lesion is midpontine with an intact vagus nerve
C. A lesion is midpontine with a damaged vagus nerve (Correct Answer)
D. A lesion is at the pontomedullary junction

Explanation: **Explanation:** **1. Understanding the Mechanism (Why C is correct):** Apneusis is characterized by prolonged inspiratory gasps followed by brief expiratory efforts. It occurs due to the unopposed activity of the **Apneustic Center** (located in the lower pons). Normally, inspiration is terminated by two "off-switches": * **The Pneumotaxic Center:** Located in the upper pons (nucleus parabrachialis), it inhibits the apneustic center to limit inspiration. * **The Vagus Nerve:** Carries inhibitory signals from pulmonary stretch receptors (Hering-Breuer reflex). If a lesion occurs at the **midpontine level**, the connection to the Pneumotaxic center is severed. However, the Vagus nerve can still terminate inspiration. Apneusis only manifests when **both** the Pneumotaxic center and the Vagus nerves are non-functional, leaving the Apneustic center completely unchecked. **2. Analysis of Incorrect Options:** * **Option A:** Lesions above the pons (e.g., forebrain) typically result in Cheyne-Stokes respiration, not apneusis. * **Option B:** If the Vagus nerve is intact, it provides enough inhibitory input to prevent true apneusis, though breathing may become slower and deeper. * **Option D:** A lesion at the pontomedullary junction separates the pontine centers from the medulla, leading to **Ataxic breathing** (Biot’s respiration) or gasping, as the rhythmic control from the pons is lost. **3. High-Yield Clinical Pearls for NEET-PG:** * **Pneumotaxic Center:** Primarily controls the *rate* and *depth* of breathing (the "limit setter"). * **Dorsal Respiratory Group (DRG):** Located in the medulla; primarily responsible for basic rhythm and *inspiration*. * **Ventral Respiratory Group (VRG):** Responsible for both inspiration and *active expiration*. * **Pre-Bötzinger Complex:** The "Pacemaker" of respiration, located in the medulla.

Question 105: What is the partial pressure of carbon dioxide in the alveolar blood?

A. 0.3 mm Hg
B. 40 mm Hg (Correct Answer)
C. 32 mm Hg
D. 158 mm Hg

Explanation: **Explanation:** The partial pressure of carbon dioxide ($PCO_2$) in the **alveolar blood** (which refers to the blood that has equilibrated with alveolar air, i.e., pulmonary capillary blood leaving the alveoli) is **40 mm Hg**. **1. Why 40 mm Hg is Correct:** Gas exchange in the lungs occurs via passive diffusion across the respiratory membrane. Deoxygenated blood enters the pulmonary capillaries with a $PCO_2$ of 46 mm Hg. Alveolar air has a $PCO_2$ of 40 mm Hg. Because $CO_2$ is highly soluble (20 times more than $O_2$), it rapidly diffuses down its pressure gradient until the blood reaches equilibrium with the alveolar air. Therefore, the $PCO_2$ of blood leaving the alveoli (and entering the systemic circulation) is exactly **40 mm Hg**. **2. Analysis of Incorrect Options:** * **A. 0.3 mm Hg:** This is the $PCO_2$ of **atmospheric air**. It is negligible because $CO_2$ concentration in the environment is very low (approx. 0.04%). * **C. 32 mm Hg:** This value is seen in states of **hyperventilation** or increased alveolar ventilation, where $CO_2$ is "washed out" of the lungs, but it is not the normal physiological value. * **D. 158 mm Hg:** This is the partial pressure of **oxygen ($PO_2$)** in atmospheric air at sea level ($21\% \text{ of } 760 \text{ mm Hg}$). **High-Yield Facts for NEET-PG:** * **Venous Blood $PCO_2$:** 46 mm Hg. * **Alveolar/Arterial Blood $PCO_2$:** 40 mm Hg. * **Diffusion Capacity:** $CO_2$ diffuses much faster than $O_2$ despite a smaller pressure gradient (6 mm Hg for $CO_2$ vs. 60 mm Hg for $O_2$) because of its high solubility. * **Clinical Correlation:** $PaCO_2$ is the best indicator of alveolar ventilation. If $PaCO_2 > 45 \text{ mm Hg}$, it indicates hypoventilation (respiratory acidosis).

Question 106: What is the most important stimulus to peripheral chemoreceptors?

A. PO2 (Correct Answer)
B. CO2
C. pH
D. HCO3

Explanation: **Explanation:** The **peripheral chemoreceptors** (located in the carotid and aortic bodies) are primarily sensitive to changes in the arterial blood chemistry. **1. Why PO2 is the Correct Answer:** The most important and potent stimulus for peripheral chemoreceptors is a **decrease in arterial PO2 (Hypoxia)**. Specifically, they respond to the partial pressure of dissolved oxygen, not the total oxygen content. When PO2 falls below **60 mmHg**, these receptors trigger a rapid increase in ventilation. This is distinct from the central chemoreceptors, which do not respond to hypoxia at all. **2. Why the Other Options are Incorrect:** * **CO2 (Option B):** While peripheral chemoreceptors do respond to an increase in PCO2 (Hypercapnia), they are responsible for only about **20%** of the total respiratory response to CO2. The remaining 80% is mediated by central chemoreceptors. * **pH (Option C):** Peripheral chemoreceptors (specifically the carotid bodies) respond to a decrease in arterial pH (Acidosis). However, this is a secondary stimulus compared to the primary drive provided by hypoxia. * **HCO3 (Option D):** Bicarbonate levels do not directly stimulate chemoreceptors; they act as a buffer that influences pH. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Carotid bodies (at the bifurcation of common carotid) are more important than aortic bodies in humans. * **Nerve Supply:** Carotid body signals via the **Glossopharyngeal nerve (CN IX)**; Aortic body via the **Vagus nerve (CN X)**. * **Mechanism:** Hypoxia closes oxygen-sensitive **K+ channels**, leading to depolarization, Ca2+ influx, and release of neurotransmitters (likely ATP or Dopamine). * **Central vs. Peripheral:** Remember, **Central chemoreceptors** respond primarily to **H+ ions** (derived from CO2) and are the most important for the minute-to-minute control of breathing, but they **cannot** detect hypoxia.

Question 107: CO2 increases ventilation by acting mainly on which receptors?

A. Apneustic center
B. Pneumotaxic center
C. Ventral surface of medulla (Correct Answer)
D. Dorsal Respiratory Group (DRG)

Explanation: ### Explanation The regulation of respiration is primarily governed by chemical control. The correct answer is the **Ventral surface of medulla** because this is the anatomical location of the **Central Chemoreceptors**. #### 1. Why the Ventral Surface of Medulla is Correct: Central chemoreceptors are located bilaterally in the chemosensitive area on the ventral surface of the medulla oblongata. While they are highly sensitive to changes in arterial $PCO_2$, they do not respond to $CO_2$ directly. Instead, $CO_2$ diffuses across the blood-brain barrier into the cerebrospinal fluid (CSF). Here, it reacts with water to form carbonic acid, which dissociates into $H^+$ and $HCO_3^-$. The **$H^+$ ions** then directly stimulate the chemoreceptors, leading to an increase in ventilation. #### 2. Why Other Options are Incorrect: * **Apneustic Center:** Located in the lower pons, it sends stimulatory signals to the DRG to increase the duration of inspiration (causing "apneustic breathing" if damaged). It does not sense $CO_2$ directly. * **Pneumotaxic Center:** Located in the upper pons (Nucleus Parabrachialis), its primary role is to "switch off" inspiration, thereby regulating breathing rate and pattern. * **Dorsal Respiratory Group (DRG):** Located in the Nucleus Tractus Solitarius (NTS), the DRG is the primary rhythm generator for inspiration. While it receives input from chemoreceptors, it is not the primary site where $CO_2$ (via $H^+$) acts to initiate the drive. #### 3. High-Yield Clinical Pearls for NEET-PG: * **Primary Stimulus:** $CO_2$ is the most potent stimulus for respiration under normal physiological conditions. * **Blood-Brain Barrier:** $H^+$ and $HCO_3^-$ cannot cross the barrier easily, but $CO_2$ crosses rapidly. This is why hypercapnia (high $CO_2$) affects the brain faster than metabolic acidosis. * **Peripheral Chemoreceptors:** Located in the **Carotid and Aortic bodies**, these respond primarily to **Hypoxia** ($PO_2 < 60$ mmHg), though they also respond to $H^+$ and $CO_2$ to a lesser extent.

Question 108: What is the total dead space in healthy individuals?

A. 50 ml
B. 150 ml (Correct Answer)
C. 500 ml
D. 100 ml

Explanation: **Explanation:** The correct answer is **150 ml**. **1. Understanding the Concept:** Dead space refers to the volume of inspired air that does not participate in gas exchange. In a healthy individual, the **Total Dead Space** (Physiological Dead Space) is virtually equal to the **Anatomical Dead Space**. This represents the air filled in the conducting zone of the respiratory tract (from the nose/mouth down to the terminal bronchioles). A standard clinical rule of thumb is that anatomical dead space is approximately **2 ml per kg of body weight**. For an average 70 kg adult, this calculates to roughly **150 ml**. **2. Analysis of Incorrect Options:** * **A (50 ml):** This value is too low for an adult; it may be seen in pediatric populations but does not represent the standard physiological average. * **C (500 ml):** This is the average **Tidal Volume (TV)**—the total amount of air inspired or expired during a normal breath. Only about 350 ml of this reaches the alveoli for gas exchange. * **D (100 ml):** While closer, it underestimates the standard anatomical volume of the conducting airways in a healthy adult. **3. NEET-PG High-Yield Pearls:** * **Physiological Dead Space = Anatomical Dead Space + Alveolar Dead Space.** In healthy lungs, Alveolar Dead Space is negligible (zero). It increases in diseases like Pulmonary Embolism (ventilation without perfusion). * **Bohr’s Equation:** Used to measure Physiological Dead Space ($V_d/V_t = [PaCO_2 - PeCO_2] / PaCO_2$). * **Fowler’s Method:** Used to measure Anatomical Dead Space (using Single Breath Nitrogen Washout). * **Instrumental Dead Space:** Artificial increase in dead space caused by breathing through equipment (e.g., a snorkel or ventilator tubing), which reduces alveolar ventilation.

Question 109: Which of the following is NOT a stimulus for pulmonary vasoconstriction?

A. Hypoxemia
B. Hypercapnia
C. Prostacyclin (PGI2) (Correct Answer)
D. Thromboxane

Explanation: **Explanation:** The pulmonary circulation is unique because it responds to alveolar hypoxia with **Hypoxic Pulmonary Vasoconstriction (HPV)**. This mechanism shunts blood away from poorly ventilated areas of the lung to well-ventilated areas to optimize ventilation-perfusion (V/Q) matching. **Why Prostacyclin (PGI2) is the correct answer:** Prostacyclin (PGI2) is a potent **vasodilator** produced by the vascular endothelium. It acts via the IP receptor to increase intracellular cAMP, leading to smooth muscle relaxation. In clinical practice, synthetic prostacyclins (e.g., Epoprostenol) are used to treat pulmonary hypertension because they decrease pulmonary vascular resistance. **Analysis of Incorrect Options:** * **Hypoxemia (A):** Low alveolar oxygen is the primary stimulus for pulmonary vasoconstriction. Unlike systemic vessels (which dilate in response to hypoxia), pulmonary vessels constrict to prevent "wasted" perfusion to hypoxic alveoli. * **Hypercapnia (B):** High CO2 levels (and the resulting acidosis) act as a local stimulus for pulmonary vasoconstriction, further assisting in diverting blood flow from hypoventilated regions. * **Thromboxane (D):** Thromboxane A2 is a potent **vasoconstrictor** and platelet aggregator. It is often released during lung injury or inflammation, contributing to increased pulmonary artery pressure. **NEET-PG High-Yield Pearls:** * **Most potent pulmonary vasoconstrictor:** Alveolar Hypoxia. * **Most potent pulmonary vasodilator:** Nitric Oxide (NO). * **V/Q Matching:** HPV is the lung's primary method of preventing a physiological shunt. * **Other Vasoconstrictors:** Endothelin, Alpha-adrenergic agonists, Serotonin, and Angiotensin II. * **Other Vasodilators:** Bradykinin, Acetylcholine, and Beta-adrenergic agonists.

Question 110: What is the normal Vd/Vt ratio in an adult?

A. 20
B. 0.35 (Correct Answer)
C. 40
D. 50

Explanation: ### Explanation **1. Understanding the Correct Answer (B: 0.35)** The **Vd/Vt ratio** represents the fraction of the tidal volume ($V_t$) that constitutes dead space ($V_d$). Dead space is the volume of air that does not participate in gas exchange. In a healthy adult at rest, the normal physiological dead space is approximately **20% to 35%** of the tidal volume (expressed as a ratio of **0.2 to 0.35**). This ratio is calculated using the **Bohr Equation**: $$V_d/V_t = (PaCO_2 - PeCO_2) / PaCO_2$$ *(Where $PaCO_2$ is arterial $CO_2$ and $PeCO_2$ is mixed expired $CO_2$)*. Since Option B (0.35) falls within the upper limit of the normal physiological range, it is the most accurate choice. **2. Why Other Options are Incorrect** * **Options A, C, and D (20, 40, 50):** These are whole numbers. The Vd/Vt is a **ratio** (fraction), meaning it must be a value between 0 and 1. A ratio of 20 or 50 would imply the dead space is 20 to 50 times larger than the total breath, which is physiologically impossible. If these numbers were intended to represent percentages (20%, 40%, 50%), 0.35 remains the standard textbook value for a healthy adult at rest. **3. Clinical Pearls & High-Yield Facts** * **Effect of Exercise:** During exercise, the Vd/Vt ratio **decreases** (often below 0.15) because tidal volume increases significantly and pulmonary perfusion improves, reducing alveolar dead space. * **Pathology:** The ratio **increases** in conditions like Pulmonary Embolism, Emphysema, or ARDS, where wasted ventilation occurs. * **Anatomical vs. Physiological Dead Space:** In healthy individuals, anatomical and physiological dead space are nearly equal. They differ only in lung diseases where "alveolar dead space" increases. * **Equipment Dead Space:** Adding a long breathing circuit (e.g., during anesthesia) increases the Vd/Vt ratio, potentially leading to hypercapnia if not compensated.

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