Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 421: A patient with parotid gland cancer has damage to the glossopharyngeal nerve. As a result, which of the following respiratory reflexes will be impaired?

A. Hering-Breuer inflation reflex
B. Juxta pulmonary capillary receptor reflex
C. Irritant airway reflex
D. Carotid body chemoreceptor reflex (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Carotid body chemoreceptor reflex)** The **Glossopharyngeal nerve (CN IX)** serves as the afferent (sensory) pathway for the carotid body chemoreceptors and carotid sinus baroreceptors. Specifically, the **Hering’s nerve** (a branch of CN IX) carries impulses from the carotid body to the respiratory centers in the medulla. If the glossopharyngeal nerve is damaged, the body cannot sense or respond to changes in arterial $PO_2$, $PCO_2$, or pH via the peripheral chemoreceptors, thereby impairing this reflex. **Why other options are incorrect:** * **A, B, and C:** All three reflexes—the **Hering-Breuer inflation reflex** (triggered by stretch receptors), the **J-receptor reflex** (triggered by pulmonary capillary engorgement), and the **Irritant reflex** (triggered by noxious gases/dust)—rely on the **Vagus nerve (CN X)** as their afferent pathway. Since the question specifies damage only to the glossopharyngeal nerve, these Vagus-mediated reflexes remain intact. **High-Yield Clinical Pearls for NEET-PG:** * **Afferent Pathways:** Remember the "9-10 Rule": **Carotid** (Body/Sinus) = **CN IX**; **Aortic** (Body/Arch) = **CN X**. * **Peripheral vs. Central:** Peripheral chemoreceptors (Carotid/Aortic) are the primary responders to **Hypoxia** ($PO_2 < 60$ mmHg). Central chemoreceptors (Medulla) are primarily sensitive to changes in **$H^+$ and $PCO_2$**. * **J-Receptors:** Located in the alveolar walls near capillaries; their stimulation (e.g., in pulmonary edema) leads to rapid shallow breathing (tachypnea). * **Hering-Breuer Reflex:** In adults, this is a protective mechanism to prevent over-inflation and is typically active only when tidal volume exceeds 1.5 liters.

Question 422: What concentration of methemoglobin in the blood is typically required to cause cyanosis?

A. 5 gm/dL
B. 2 gm/dL
C. 1.5 gm/dL (Correct Answer)
D. 12 gm/dL

Explanation: **Explanation:** Cyanosis is the bluish discoloration of the skin and mucous membranes, typically occurring when a specific threshold of deoxygenated or abnormal hemoglobin is reached in the capillary blood. **1. Why 1.5 gm/dL is Correct:** While central cyanosis traditionally requires **5 gm/dL of reduced (deoxygenated) hemoglobin**, methemoglobin has a much more profound effect on skin color. Because methemoglobin is dark, chocolate-colored, and has a high molar extinction coefficient, it produces visible cyanosis at much lower concentrations. Specifically, only **1.5 gm/dL of methemoglobin** is required to produce clinical cyanosis. This is why patients with methemoglobinemia often appear "cyanotic" despite having a normal PaO2 on arterial blood gas. **2. Analysis of Incorrect Options:** * **5 gm/dL (Option A):** This is the threshold for **reduced hemoglobin** (deoxy-Hb) to cause cyanosis. It is a classic "distractor" for students who confuse general cyanosis with methemoglobin-specific cyanosis. * **2 gm/dL (Option B):** This value is sometimes cited as the threshold for sulfhemoglobinemia to cause cyanosis, but it is not the standard for methemoglobin. * **12 gm/dL (Option D):** This is a dangerously high level associated with severe hypoxia and neurological symptoms, far exceeding the initial threshold for visible cyanosis. **High-Yield Clinical Pearls for NEET-PG:** * **The "Saturation Gap":** A hallmark of methemoglobinemia is a discrepancy between low SpO2 (pulse oximetry, often hovering around 85%) and a normal PaO2 (on ABG). * **Blood Appearance:** Blood in these patients appears **chocolate-brown** and does not change color when exposed to 100% oxygen. * **Treatment:** The drug of choice is **Intravenous Methylene Blue**, which acts as an exogenous electron acceptor to reduce Fe3+ back to Fe2+. * **Common Triggers:** Nitrites, benzocaine, dapsone, and sulfonamides.

Question 423: The normal intrapleural pressure at the start of inspiration is approximately cm of H2O?

A. -7.5
B. -5.0 (Correct Answer)
C. -2.0
D. -0.5

Explanation: **Explanation:** The **intrapleural pressure (IPP)** is the pressure within the pleural cavity (the space between the visceral and parietal pleura). Under normal physiological conditions, this pressure is **subatmospheric (negative)** due to the opposing elastic recoil forces of the lungs (tending to collapse inward) and the chest wall (tending to expand outward). 1. **Why -5.0 cm H₂O is correct:** At the **start of inspiration** (which corresponds to the end of a normal expiration or Functional Residual Capacity), the inward recoil of the lungs and the outward recoil of the chest wall are in equilibrium. This creates a resting negative pressure of approximately **-5 cm H₂O**. This "suction" effect keeps the lungs inflated against the chest wall. 2. **Analysis of Incorrect Options:** * **-7.5 cm H₂O (Option A):** This is the approximate intrapleural pressure at the **end of inspiration**. As the chest wall expands during inspiration, the pleural space volume increases, making the pressure more negative (Boyle’s Law) to pull the lungs open. * **-2.0 cm H₂O (Option C):** This value is too high (less negative) for a healthy adult at rest. * **-0.5 cm H₂O (Option D):** This value typically represents the **intrapulmonary (alveolar) pressure** at the start of inspiration, not the intrapleural pressure. **High-Yield Pearls for NEET-PG:** * **Transpulmonary Pressure:** Defined as Alveolar Pressure minus Intrapleural Pressure ($P_{tp} = P_{alv} - P_{ip}$). It is always positive under normal conditions. * **Pneumothorax:** If the pleural cavity is breached, IPP becomes equal to atmospheric pressure (0 cm H₂O), leading to lung collapse. * **Basal vs. Apical IPP:** Due to gravity, IPP is **more negative at the apex** (~ -10 cm H₂O) and **less negative at the base** (~ -2.5 cm H₂O) in a standing position.

Question 424: The basal regions of the upright human lung are normally better ventilated than the upper regions because:

A. Airway resistance of the upper regions is higher than of the lower regions
B. There is less surfactant in the upper regions
C. The blood flow to the lower regions is higher
D. The lower regions have a small resting volume and a relatively large increase in volume (Correct Answer)

Explanation: ### Explanation The regional difference in ventilation is primarily due to the effect of **gravity** on intrapleural pressure ($P_{pl}$). **Why Option D is Correct:** In an upright lung, the weight of the lung causes the intrapleural pressure at the **base** to be less negative (more atmospheric) than at the apex. 1. **At FRC (Resting Volume):** Because the $P_{pl}$ is less negative at the base, the transpulmonary pressure is lower, meaning the basal alveoli are less expanded (smaller resting volume) compared to the stretched apical alveoli. 2. **During Inspiration:** Both regions experience the same change in $P_{pl}$. However, because the basal alveoli are on the steep, **compliant** portion of the pressure-volume curve, they expand significantly more for a given pressure change. Therefore, the base receives more fresh air per breath than the apex. **Analysis of Incorrect Options:** * **Option A:** Airway resistance is generally uniform or slightly lower in the lower regions due to larger lung volumes during inspiration; it is not the primary driver of ventilation distribution. * **Option B:** Surfactant concentration is uniform throughout the lung; it does not vary by region to affect ventilation. * **Option C:** While blood flow *is* higher at the base (due to gravity), this is a result of perfusion dynamics, not the cause of better ventilation. Ventilation is determined by regional compliance. **High-Yield Pearls for NEET-PG:** * **V/Q Ratio:** Both ventilation (V) and perfusion (Q) increase from apex to base, but **perfusion increases more steeply**. * **Apex:** High V/Q ratio (~3.3), higher $P_{O2}$, lower $P_{CO2}$. (Favors *M. tuberculosis* growth). * **Base:** Low V/Q ratio (~0.6), lower $P_{O2}$, higher $P_{CO2}$. * **Compliance:** The base of the lung is more compliant than the apex in an upright position.

Question 425: What percentage of oxygen does mouth-to-mouth breathing deliver into the subject's respiratory system?

A. 8%
B. 10%
C. 16% (Correct Answer)
D. 20%

Explanation: ### Explanation **Correct Option: C (16%)** The correct answer is **16%** because mouth-to-mouth resuscitation utilizes the rescuer’s **expired air** to ventilate the victim. * **The Underlying Concept:** Atmospheric air contains approximately **21% oxygen**. During normal respiration, a healthy individual extracts only about 4–5% of that oxygen for cellular metabolism. Consequently, the air exhaled by the rescuer still contains a significant amount of oxygen—roughly **16% to 17%**. This concentration is sufficient to maintain tissue viability and oxygenate the victim’s hemoglobin (achieving an arterial oxygen saturation of about 80–90%) until advanced life support arrives. **Analysis of Incorrect Options:** * **A & B (8% and 10%):** These values are too low. If exhaled air contained only 8–10% oxygen, it would be insufficient to create a partial pressure gradient high enough to oxygenate the victim’s blood effectively, leading to rapid hypoxia. * **D (20%):** This value is nearly equivalent to atmospheric air (21%). It is incorrect because it fails to account for the oxygen consumed by the rescuer’s own body during the respiratory cycle. **High-Yield Clinical Pearls for NEET-PG:** * **CO2 Content:** Exhaled air contains approximately **4% Carbon Dioxide**. While this sounds counterintuitive, this CO2 can actually help stimulate the victim's respiratory center in certain clinical scenarios. * **Expired Air Composition:** O2 ≈ 16%, N2 ≈ 79%, CO2 ≈ 4%. * **Tidal Volume in CPR:** In mouth-to-mouth breathing, the rescuer should provide a breath over 1 second with enough volume to see a visible **chest rise**. * **FiO2 Comparison:** While mouth-to-mouth provides ~16% O2, a simple face mask at 6–10 L/min provides 35–50%, and a non-rebreather mask (NRBM) can provide up to 90–100%.

Question 426: Which of the following is NOT a feature of chronic mountain sickness?

A. Hyperventilation
B. Increased erythropoietin
C. Increased HCO3 excretion
D. Decreased mitochondria (Correct Answer)

Explanation: **Explanation:** Chronic Mountain Sickness (Monge’s disease) occurs due to long-term exposure to high altitudes, leading to chronic alveolar hypoxia and subsequent physiological maladaptations. **Why "Decreased mitochondria" is the correct answer:** In response to chronic hypoxia, the body undergoes cellular adaptations to maximize energy efficiency. One such adaptation is an **increase in mitochondrial density** and myoglobin content in skeletal muscles. This enhances the cell's ability to utilize limited oxygen for oxidative phosphorylation. Therefore, "decreased mitochondria" is physiologically incorrect and is not a feature of the condition. **Analysis of Incorrect Options:** * **Hyperventilation:** Hypoxia stimulates peripheral chemoreceptors (carotid bodies), leading to a persistent increase in the rate and depth of breathing to improve arterial $PO_2$. * **Increased Erythropoietin:** Chronic hypoxia triggers the kidneys to release erythropoietin (EPO), stimulating the bone marrow to produce more RBCs (Polycythemia). This increases the oxygen-carrying capacity of the blood but also increases blood viscosity. * **Increased $HCO_3^-$ excretion:** Hyperventilation causes respiratory alkalosis (low $PCO_2$). To compensate and normalize pH, the kidneys increase the excretion of bicarbonate ($HCO_3^-$). **Clinical Pearls for NEET-PG:** * **Monge’s Disease:** Characterized by extreme polycythemia (Hematocrit >65%), cyanosis, and pulmonary hypertension leading to right heart failure. * **Acute Mountain Sickness (AMS):** Occurs within hours; primary symptom is headache. * **High Altitude Pulmonary Edema (HAPE):** Caused by uneven hypoxic pulmonary vasoconstriction. * **High Altitude Cerebral Edema (HACE):** Result of hypoxia-induced vasodilation and increased capillary permeability.

Question 427: Hypoxia due to slowing of circulation is termed as?

A. Anemic hypoxia
B. Histotoxic hypoxia
C. Stagnant hypoxia (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Stagnant Hypoxia** (also known as ischemic or circulatory hypoxia) occurs when there is a **decrease in blood flow velocity** or a slowing of circulation. Even though the oxygen content of the arterial blood and the oxygen-carrying capacity are normal, the tissues do not receive oxygen at a sufficient rate because the blood is moving too slowly through the capillaries. This results in increased oxygen extraction at the tissue level, leading to a significantly widened arterio-venous (A-V) oxygen difference. Common causes include heart failure, shock, or local vascular obstruction (e.g., Raynaud’s disease). **Why other options are incorrect:** * **Anemic Hypoxia:** This occurs when the arterial $PO_2$ is normal, but the **oxygen-carrying capacity** of the blood is reduced. This is seen in conditions like anemia, hemorrhage, or carbon monoxide poisoning (where Hb is unavailable for $O_2$ binding). * **Histotoxic Hypoxia:** Here, the delivery of oxygen to the tissues is normal, but the **tissues cannot utilize it** due to the inhibition of cellular oxidative enzymes (e.g., Cyanide poisoning inhibiting Cytochrome Oxidase). **High-Yield Facts for NEET-PG:** * **Hypoxic Hypoxia:** Characterized by low arterial $PO_2$. It is the only type of hypoxia where **Cyanosis** is most prominent and oxygen therapy is highly effective. * **A-V Oxygen Difference:** It is **increased** in Stagnant Hypoxia (due to slow flow) and **decreased** in Histotoxic Hypoxia (as tissues fail to take up $O_2$). * **Cyanosis:** Is typically absent in Anemic hypoxia (not enough Hb to show blue color) and Histotoxic hypoxia (blood remains oxygenated).

Question 428: Excessive tidal volume load is prevented by activation of which of the following receptors?

A. J receptor
B. Thoracic muscle spindle
C. Bronchial stretch receptors (Correct Answer)
D. Aerial baroreceptor

Explanation: **Explanation:** The correct answer is **C. Bronchial stretch receptors**. The prevention of excessive tidal volume load is mediated by the **Hering-Breuer Inflation Reflex**. This reflex is a protective mechanism that prevents over-inflation of the lungs. * **Mechanism:** When tidal volume exceeds a certain threshold (typically >1.5 liters in adults), **slowly adapting pulmonary stretch receptors** located in the smooth muscle of the bronchi and bronchioles are activated. * **Pathway:** These receptors send inhibitory impulses via the **Vagus nerve (CN X)** to the inspiratory center (Dorsal Respiratory Group) in the medulla. * **Result:** This terminates inspiration, initiates expiration, and increases respiratory frequency, thereby limiting the tidal volume. **Analysis of Incorrect Options:** * **A. J receptors (Juxtacapillary receptors):** Located in the alveolar walls near capillaries. They are stimulated by pulmonary congestion, edema, or engorgement. Activation leads to rapid, shallow breathing (tachypnea) and dyspnea, not the regulation of normal tidal volume. * **B. Thoracic muscle spindles:** These are proprioceptors in the intercostal muscles. While they help sense chest wall position and effort, they are primarily involved in the reflex control of the force of contraction rather than terminating inspiration to prevent over-inflation. * **D. Arterial baroreceptors:** Located in the carotid sinus and aortic arch, these primarily sense changes in blood pressure. While extreme hypotension can stimulate hyperpnea, they do not regulate tidal volume load. **NEET-PG High-Yield Pearls:** * **Hering-Breuer Deflation Reflex:** A separate reflex that stimulates inspiration when lungs are abnormally deflated (e.g., pneumothorax). * **Vagus Nerve:** The afferent limb for both Hering-Breuer reflexes. * **Threshold:** In normal resting humans, the Hering-Breuer inflation reflex is largely inactive; it becomes significant when tidal volume exceeds **1.5L** or during heavy exercise.

Question 429: Decompression sickness, commonly known as 'the bends', is caused by what phenomenon?

A. Escape of nitrogen bubbles from the myelin sheath of motor nerves
B. Blockage of pulmonary capillaries by nitrogen bubbles
C. Blockage of blood vessels in the brain and spinal cord by nitrogen bubbles
D. Formation of nitrogen bubbles in muscles and joints (Correct Answer)

Explanation: ### Explanation **Concept Overview:** Decompression sickness (DCS) is governed by **Henry’s Law**, which states that the solubility of a gas in a liquid is proportional to its partial pressure. When a diver descends, the high ambient pressure causes large amounts of nitrogen to dissolve into body tissues (especially fat). During a rapid ascent, the pressure drops quickly, and the dissolved nitrogen comes out of solution, forming **bubbles** in the blood and tissues. **Why Option D is Correct:** The most common clinical manifestation of Type I DCS (the "mild" form) is localized pain in the **muscles and joints**. This occurs because nitrogen bubbles form in the interstitial fluid and periarticular tissues, stretching nerve endings and causing the characteristic "deep, boring" pain known as **'the bends'**. **Analysis of Incorrect Options:** * **Option A:** While nitrogen is lipophilic and has an affinity for myelin, it does not specifically "escape" from the sheath to cause motor nerve symptoms; rather, bubbles form in the blood supplying the nerves or within the spinal cord itself. * **Option B:** Nitrogen bubbles in the pulmonary capillaries cause **'the chokes'** (shortness of breath and cough), which is a severe form of DCS (Type II), but it is not the primary definition of 'the bends'. * **Option C:** Blockage of vessels in the CNS leads to neurological deficits (paralysis or sensory loss). While serious, this is classified as Type II DCS, whereas 'the bends' specifically refers to the musculoskeletal pain. **High-Yield Facts for NEET-PG:** * **The Bends:** Pain in joints and muscles (Type I DCS). * **The Chokes:** Pulmonary embolism by nitrogen bubbles (Type II DCS). * **Staggers:** Involvement of the vestibular system (vertigo/tinnitus). * **Treatment:** 100% Oxygen and **Hyperbaric Oxygen Therapy (HBOT)** to force nitrogen back into solution. * **Prevention:** Slow ascent and decompression stops to allow "off-gassing" via the lungs.

Question 430: Which of the following neurons predominantly fire during forceful expiration?

A. Dorsal group of neurons
B. Ventral group of neurons (Correct Answer)
C. Pneumotaxic center
D. Chemoreceptors

Explanation: ### Explanation **Correct Option: B. Ventral Group of Neurons (VGN)** The respiratory center in the medulla is divided into two main functional groups: the Dorsal Respiratory Group (DRG) and the Ventral Respiratory Group (VRG). * **Normal Breathing:** The DRG is responsible for the basic rhythm of inspiration, while expiration is a passive process. During quiet breathing, the VRG remains almost totally inactive. * **Forceful Breathing:** When the need for pulmonary ventilation increases (e.g., during exercise), the VRG is activated. It contains both inspiratory and expiratory neurons. Crucially, the **expiratory neurons** of the VRG provide powerful signals to the abdominal muscles and internal intercostal muscles, making it the primary driver for **forceful expiration**. **Why Incorrect Options are Wrong:** * **A. Dorsal Group of Neurons:** These are located in the Nucleus Tractus Solitarius (NTS) and are primarily responsible for **inspiration**. They provide the "ramp signal" for quiet breathing. * **C. Pneumotaxic Center:** Located in the upper pons (Nucleus Parabrachialis), its primary role is to limit the duration of inspiration (the "off-switch"), thereby increasing the respiratory rate. It does not directly control forceful expiration. * **D. Chemoreceptors:** These are sensors (Central in medulla; Peripheral in Carotid/Aortic bodies) that detect changes in $PCO_2$, $pH$, and $PO_2$. They modulate the respiratory centers but are not the neurons that fire to execute the motor act of expiration. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** DRG is in the **Nucleus Tractus Solitarius (NTS)**; VRG is in the **Nucleus Ambiguus** and **Nucleus Retroambiguus**. * **Pre-Bötzinger Complex:** Located in the VRG, it is considered the **pacemaker** of respiration. * **Hering-Breuer Reflex:** A protective mechanism where stretch receptors in the lungs prevent over-inflation by inhibiting the DRG via the Vagus nerve.

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