Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 701: What is the partial pressure of carbon dioxide in the expired air?

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

Explanation: ### Explanation The partial pressure of carbon dioxide in expired air ($P_E\text{CO}_2$) is approximately **32 mm Hg**. This value is a result of the mixing of two different air volumes during exhalation: 1. **Alveolar Air:** Rich in $\text{CO}_2$ (approx. 40 mm Hg) due to gas exchange. 2. **Anatomic Dead Space Air:** This is inspired atmospheric air that remains in the conducting airways and contains almost no $\text{CO}_2$ (approx. 0.3 mm Hg). When these two volumes mix during expiration, the $\text{CO}_2$ is diluted, resulting in a final expired concentration of roughly **32 mm Hg**. #### Analysis of Options: * **A. 0.3 mm Hg:** This is the partial pressure of $\text{CO}_2$ in **inspired (atmospheric) air**. It is negligible. * **B. 158 mm Hg:** This is the partial pressure of **Oxygen ($P\text{O}_2$)** in inspired atmospheric air at sea level. * **C. 40 mm Hg:** This is the partial pressure of $\text{CO}_2$ in **alveolar air ($P_A\text{CO}_2$)** and **systemic arterial blood ($P_a\text{CO}_2$)**. * **D. 32 mm Hg (Correct):** The average value for mixed expired air. #### NEET-PG High-Yield Pearls: * **Bohr Equation:** Uses the difference between alveolar $\text{CO}_2$ (40) and expired $\text{CO}_2$ (32) to calculate the **Physiological Dead Space**. * **End-Tidal $\text{CO}_2$ ($Et\text{CO}_2$):** In clinical monitoring (Capnography), the air at the very end of expiration is almost pure alveolar air, so $Et\text{CO}_2$ is usually closer to 40 mm Hg, reflecting arterial $P\text{CO}_2$. * **Partial Pressure of $\text{O}_2$ in Expired Air:** Approximately **116 mm Hg** (diluted from 158 mm Hg by alveolar air).

Question 702: Hypoxic pulmonary vasoconstriction is due to what mechanism?

A. Irreversible pulmonary vasoconstriction
B. Reversible pulmonary vasoconstriction (Correct Answer)
C. Shunting of blood to poorly ventilated areas
D. Occurs hours after pulmonary vasoconstriction

Explanation: ### Explanation **Hypoxic Pulmonary Vasoconstriction (HPV)** is a unique physiological phenomenon where pulmonary arterioles constrict in response to low alveolar oxygen tension ($PAO_2$). **1. Why Option B is Correct:** HPV is a **reversible** physiological adaptation. Its primary purpose is to divert blood flow away from poorly ventilated (hypoxic) alveoli toward well-ventilated areas. This optimizes **ventilation-perfusion (V/Q) matching** and prevents systemic hypoxemia. Once the alveolar oxygen levels return to normal, the vasoconstriction reverses, and normal blood flow is restored. This is mediated by the inhibition of voltage-gated potassium channels in pulmonary vascular smooth muscle cells, leading to depolarization and calcium influx. **2. Why the Other Options are Incorrect:** * **Option A:** If the constriction were irreversible, it would lead to permanent pulmonary hypertension and localized infarction even after the underlying cause (e.g., mucus plug) is resolved. * **Option C:** HPV actually **decreases** shunting. By diverting blood *away* from poorly ventilated areas, it reduces the amount of deoxygenated blood entering the systemic circulation. * **Option D:** HPV is a rapid response, occurring within **seconds to minutes** of hypoxia, not hours. **Clinical Pearls for NEET-PG:** * **Unique Response:** In the systemic circulation, hypoxia causes *vasodilation*; in the pulmonary circulation, it causes *vasoconstriction*. * **High Altitude:** Global hypoxia at high altitudes causes generalized HPV, leading to **High Altitude Pulmonary Edema (HAPE)** due to increased pulmonary capillary hydrostatic pressure. * **Fetal Circulation:** HPV is the reason for high pulmonary vascular resistance in the fetus, keeping the lungs bypassed until the first breath. * **Inhibition:** HPV is inhibited by certain anesthetic agents (e.g., Halothane) and vasodilators (e.g., Nitric Oxide).

Question 703: Which of the following is true about the main respiratory control neurons?

A. The neurons are located in the pons.
B. They are primarily responsible for inspiratory muscle contraction.
C. They respond to changes in arterial oxygen levels.
D. They are located in the medulla oblongata. (Correct Answer)

Explanation: The rhythmic control of breathing is an involuntary process regulated by the brainstem. The **Medulla Oblongata** houses the primary respiratory control centers, making Option D the correct choice. ### **Explanation of the Correct Answer** The basic rhythm of respiration is generated in the **medulla oblongata** by two main groups of neurons: 1. **Dorsal Respiratory Group (DRG):** Located in the nucleus tractus solitarius, primarily responsible for inspiration. 2. **Ventral Respiratory Group (VRG):** Located in the nucleus ambiguus and nucleus retroambiguus, active during forceful expiration and inspiration. The **Pre-Bötzinger complex** (within the VRG) acts as the pacemaker for respiration. ### **Analysis of Incorrect Options** * **Option A:** While the **Pons** contains the Pneumotaxic and Apneustic centers, these are "fine-tuners" of the respiratory cycle (regulating rate and depth) rather than the primary generators of the rhythm. * **Option B:** While the DRG does control inspiratory muscles, the "main respiratory control neurons" encompass both inspiratory and expiratory groups. Furthermore, the question asks for a fundamental anatomical/functional truth; their location is their defining characteristic. * **Option C:** Central respiratory neurons respond primarily to **Hydrogen ions (H+) and CO2 levels** via central chemoreceptors. Arterial oxygen levels are sensed by **peripheral chemoreceptors** (Carotid and Aortic bodies), not the medullary neurons themselves. ### **High-Yield NEET-PG Pearls** * **Pacemaker of Respiration:** Pre-Bötzinger Complex. * **Pneumotaxic Center:** Located in the upper pons (nucleus parabrachialis); its primary function is to "switch off" inspiration (limiting tidal volume). * **Hering-Breuer Reflex:** A protective mechanism that prevents over-inflation of the lungs via stretch receptors and the Vagus nerve. * **Most Potent Stimulus for Breathing:** An increase in arterial PCO2 (Hypercapnia).

Question 704: Which of the following are neuroendocrine cells found in the lungs?

A. Dendritic cells
B. Type I pneumocytes
C. Type II pneumocytes
D. APUD cells (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. APUD cells**. **Why APUD cells are correct:** APUD (Amine Precursor Uptake and Decarboxylation) cells, also known as **Kulchitsky cells** or Pulmonary Neuroendocrine Cells (PNECs), are specialized cells located within the bronchial epithelium. They function as "chemoreceptors" that sense hypoxia and release bioactive amines and peptides (such as serotonin, calcitonin, and bombesin). These cells are part of the diffuse neuroendocrine system and play a crucial role in lung development and the regulation of airway tone. **Why other options are incorrect:** * **A. Dendritic cells:** These are professional antigen-presenting cells (APCs) of the immune system. They reside in the respiratory epithelium to capture pathogens but do not have neuroendocrine functions. * **B. Type I pneumocytes:** These are thin, squamous cells covering ~95% of the alveolar surface area. Their primary function is to facilitate gas exchange. * **C. Type II pneumocytes:** These are cuboidal cells that act as the "caretakers" of the alveoli. Their primary roles are the secretion of **surfactant** and acting as stem cells to regenerate Type I pneumocytes after injury. **High-Yield Clinical Pearls for NEET-PG:** * **Small Cell Carcinoma of the Lung:** This highly malignant tumor originates from the **Kulchitsky (APUD) cells**. This explains why it is frequently associated with paraneoplastic syndromes (e.g., SIADH, ectopic ACTH). * **Carcinoid Tumor:** This is another neuroendocrine tumor of the lung derived from these cells, typically presenting with serotonin secretion. * **Location:** APUD cells are most numerous in the fetal lung and decrease in number after birth.

Question 705: During normal inspiration, by how much does the diaphragm descend?

A. 1-2 cm (Correct Answer)
B. 3-5 cm
C. 5-7 cm
D. 7-9 cm

Explanation: The diaphragm is the primary muscle of inspiration, responsible for approximately 75% of the change in intrathoracic volume during quiet breathing. ### **Explanation of the Correct Answer** During **normal (quiet) inspiration**, the diaphragm contracts and moves caudally (downward) by approximately **1 to 2 cm**. This descent increases the vertical dimension of the thoracic cavity. Because the lungs are compliant, this volume increase creates a negative intrapulmonary pressure (approx. -1 mmHg), allowing roughly 500 mL of tidal air to enter the lungs. ### **Analysis of Incorrect Options** * **Options B, C, and D (3-9 cm):** These values represent excessive movement for quiet breathing. While the diaphragm can descend significantly more during **forced inspiration** (deep breathing or exercise), it typically reaches a maximum excursion of **7 to 10 cm** only under extreme respiratory effort. Therefore, any value above 2 cm does not characterize "normal" or "quiet" inspiration. ### **High-Yield Clinical Pearls for NEET-PG** * **Nerve Supply:** The diaphragm is supplied by the **Phrenic Nerve (C3, C4, C5)**. "C3, 4, 5 keep the diaphragm alive." * **Piston Movement:** The movement of the diaphragm is often compared to a piston. For every 1 cm of descent, the intrathoracic volume increases by approximately 200–300 mL. * **Paradoxical Respiration:** In cases of phrenic nerve palsy, the paralyzed side of the diaphragm moves *upward* (cranially) during inspiration due to the negative pressure generated by the healthy side. * **Quiet vs. Forced:** In quiet breathing, expiration is entirely **passive** (elastic recoil). The diaphragm only performs active work during inspiration.

Question 706: What is the expected mixed venous oxygen tension, in mm Hg, in a normal adult after breathing 100% oxygen for 10 minutes?

A. 150
B. 740
C. 45 (Correct Answer)
D. 573

Explanation: **Explanation:** The correct answer is **45 mm Hg**. **1. Underlying Concept:** Mixed venous oxygen tension ($PvO_2$) is primarily determined by the balance between oxygen delivery and tissue oxygen consumption ($VO_2$). In a normal adult, arterial blood is already nearly 100% saturated with oxygen while breathing room air ($PaO_2 \approx 100$ mm Hg). When breathing 100% oxygen, the $PaO_2$ rises significantly (up to 600+ mm Hg), but this adds very little extra oxygen content to the blood because hemoglobin is already saturated; the increase is almost entirely due to a small amount of additional dissolved oxygen. As blood passes through systemic capillaries, tissues extract a fixed amount of oxygen to meet metabolic demands. Because the total oxygen content increase is marginal and tissue extraction remains constant, the $PvO_2$ rises only slightly from its baseline of 40 mm Hg to approximately **45 mm Hg**. **2. Analysis of Incorrect Options:** * **Option A (150 mm Hg):** This is the approximate $PiO_2$ (inspired oxygen tension) at sea level on room air, not venous tension. * **Option B (740 mm Hg):** This value approaches the total atmospheric pressure at sea level. It is impossible for venous blood to reach this tension. * **Option D (573 mm Hg):** This represents a typical alveolar oxygen tension ($PAO_2$) while breathing 100% oxygen, but it does not reflect venous levels after tissue extraction. **3. High-Yield Clinical Pearls for NEET-PG:** * **Normal $PvO_2$:** 40 mm Hg (Saturation $\approx$ 75%). * **The "Venous Oxygen Paradox":** Even with massive increases in $FiO_2$, $PvO_2$ remains low because the sigmoid shape of the oxyhemoglobin dissociation curve ensures that most "extra" oxygen is consumed or remains bound, preventing a massive rise in dissolved partial pressure in the veins. * **Mixed Venous Blood:** Best sampled from the **Pulmonary Artery** using a Swan-Ganz catheter.

Question 707: Transection of the brain stem at the mid-pontine level with bilateral vagotomy causes what?

A. Cessation of spontaneous respiration
B. Continuation of regular breathing
C. Irregular but rhythmic respiration
D. Apneusis (Correct Answer)

Explanation: To understand the control of respiration, one must visualize the brainstem respiratory centers and their inhibitory/excitatory inputs. ### **Mechanism of Apneusis** The **Apneustic Center** (located in the lower pons) promotes inhalation by stimulating the Dorsal Respiratory Group (DRG). Under normal physiological conditions, this center is inhibited by two main "off-switches": 1. **Pneumotaxic Center:** Located in the upper pons (nucleus parabrachialis). 2. **Vagus Nerve:** Carries inhibitory signals from pulmonary stretch receptors (Hering-Breuer reflex). When a transection occurs at the **mid-pontine level**, the Pneumotaxic center is separated from the lower respiratory centers. If the **Vagus nerves** are also cut, both inhibitory inputs are removed. This results in unchecked stimulation of the DRG, leading to **Apneusis**—characterized by prolonged, gasping inspiratory efforts with short, inefficient expirations. ### **Analysis of Incorrect Options** * **A & B:** Spontaneous respiration continues because the Medullary Rhythmicity Centers (DRG/VRG) are still intact and below the level of transection. However, the breathing pattern becomes pathological, not regular. * **C:** Irregular/Ataxic respiration (Biot’s breathing) typically occurs with lesions involving the **medulla** itself, not the mid-pons. ### **NEET-PG High-Yield Pearls** * **Upper Pontine Transection + Vagus Intact:** Breathing remains near normal because the Vagus compensates for the loss of the Pneumotaxic center. * **Medullary Transection:** Leads to immediate cessation of spontaneous respiration (separates the spinal cord from the rhythm generators). * **Pneumotaxic Center Function:** Its primary role is to limit inspiration, thereby increasing the respiratory rate. * **Location Summary:** Pneumotaxic (Upper Pons), Apneustic (Lower Pons), Rhythmicity Centers (Medulla).

Question 708: What is the most important stimulant of the respiratory center?

A. Alkalosis
B. Decreased PCO2
C. Increased PCO2
D. Decreased PaO2 (Correct Answer)

Explanation: **Explanation** The regulation of respiration is primarily governed by chemical control via central and peripheral chemoreceptors. **Why Decreased $PaO_2$ is the Correct Answer:** In the context of this specific question, **Hypoxia (Decreased $PaO_2$)** acts as a potent stimulant for the **peripheral chemoreceptors** (located in the carotid and aortic bodies). While $CO_2$ is the primary driver under normal physiological conditions, the peripheral chemoreceptors are specifically sensitive to a drop in arterial oxygen. When $PaO_2$ falls below 60 mmHg, it becomes the dominant emergency drive for the respiratory center to prevent tissue hypoxia. **Analysis of Incorrect Options:** * **Alkalosis (A):** An increase in pH (alkalosis) actually **inhibits** the respiratory center to allow $CO_2$ to accumulate and restore acid-base balance. * **Decreased $PCO_2$ (B):** Low $CO_2$ (hypocapnia) reduces the stimulus to both central and peripheral chemoreceptors, leading to a decrease in rate and depth of breathing (hypoventilation). * **Increased $PCO_2$ (C):** While hypercapnia is the most important *daily* regulator of breathing via central chemoreceptors, in many MCQ contexts focusing on acute stimulation or peripheral triggers, hypoxia is highlighted. *(Note: If the question asks for the "most potent" or "normal" regulator, $CO_2$ is often the answer; however, based on the provided key, the focus is on the hypoxic drive).* **NEET-PG High-Yield Pearls:** 1. **Central Chemoreceptors:** Located in the medulla; respond to changes in **$H^+$ concentration** in the CSF (derived from arterial $CO_2$ crossing the blood-brain barrier). They do **not** respond to $O_2$. 2. **Peripheral Chemoreceptors:** Respond to **Decreased $PO_2$**, Increased $PCO_2$, and Decreased pH. 3. **Breaking Point:** During breath-holding, the urge to breathe is driven primarily by rising $PCO_2$, not falling $O_2$. 4. **COPD Clinical Note:** Patients with chronic hypercapnia lose their $CO_2$ drive and rely entirely on the **hypoxic drive** (decreased $PaO_2$) to breathe. Excessive oxygen therapy can suppress this drive, leading to respiratory arrest.

Question 709: The statement "Inflation of lungs induces further inflation" is explained by which reflex?

A. Hering-Breuer inflation reflex
B. Hering-Breuer deflation reflex
C. Head's paradoxical reflex (Correct Answer)
D. J-reflex

Explanation: **Explanation:** **1. Why Head’s Paradoxical Reflex is Correct:** Normally, lung inflation triggers a feedback mechanism to stop inspiration (Hering-Breuer). However, **Head’s paradoxical reflex** occurs when rapid inflation of the lungs triggers a **further increase in inspiratory effort**. It is called "paradoxical" because it opposes the standard inhibitory response. This reflex is mediated by vagal afferents and is physiologically significant in newborns to help inflate collapsed alveoli (first breath) and in adults during periodic deep sighs, which prevent atelectasis. **2. Why the Other Options are Incorrect:** * **A. Hering-Breuer Inflation Reflex:** This is a protective mechanism where lung inflation (stretch) inhibits the inspiratory center via the vagus nerve to **prevent over-inflation**. It stops inspiration rather than inducing more. * **B. Hering-Breuer Deflation Reflex:** This is triggered by lung atelectasis or deflation, leading to an **increase in respiratory rate** (hyperpnea) to prevent further collapse. It does not involve inflation inducing more inflation. * **C. J-reflex (Juxtacapillary reflex):** Triggered by receptors in the alveolar wall near capillaries in response to pulmonary congestion or edema. Stimulation leads to **apnea followed by rapid shallow breathing**, bradycardia, and hypotension. **3. High-Yield Clinical Pearls for NEET-PG:** * **Receptor Type:** Head’s paradoxical reflex is believed to be mediated by **Rapidly Adapting Receptors (RARs)**, whereas the Hering-Breuer inflation reflex is mediated by **Slowly Adapting Receptors (SARs)**. * **Newborn Physiology:** Head’s reflex is most active in neonates; it helps in the initial expansion of the lungs at birth. * **Vagus Nerve:** All these reflexes (Hering-Breuer, Head’s, and J-reflex) use the **Vagus (CN X)** as the afferent pathway.

Question 710: Which of the following statements is true concerning the contraction of the diaphragm?

A. The nerves responsible for its innervation emerge from the spinal cord at the level of the lower thorax.
B. It tends to flatten the diaphragm. (Correct Answer)
C. It reduces the lateral distance between the lower rib margins.
D. It causes the anterior abdominal wall to move inward.

Explanation: ### Explanation The diaphragm is the primary muscle of inspiration, responsible for approximately 75% of the change in intrathoracic volume during quiet breathing. **Why Option B is Correct:** The diaphragm is a dome-shaped muscular partition. When the muscle fibers contract, they pull the central tendon downward toward the abdominal cavity. This action **flattens the dome**, increasing the vertical dimension of the thoracic cavity. This expansion creates a negative intrapulmonary pressure, allowing air to flow into the lungs. **Why the Other Options are Incorrect:** * **Option A:** The diaphragm is innervated by the **phrenic nerves**, which arise from the cervical plexus, specifically spinal segments **C3, C4, and C5** ("C3, 4, 5 keep the diaphragm alive"). They do not emerge from the lower thorax. * **Option C:** During contraction, the diaphragm pushes down on the abdominal viscera, which in turn creates an outward force on the lower ribs (the "bucket-handle" and "pump-handle" mechanisms). This **increases**, rather than reduces, the lateral and transverse diameters of the lower rib cage. * **Option D:** As the diaphragm descends, it increases intra-abdominal pressure. This causes the **anterior abdominal wall to move outward** (protrude) during inspiration, not inward. **High-Yield NEET-PG Pearls:** * **Shape:** At rest, the right dome is slightly higher than the left due to the presence of the liver. * **Openings:** Remember the levels of major diaphragmatic openings: **Vena Cava (T8), Esophagus (T10), and Aorta (T12)** (Mnemonic: **I** **E**at **A**pples—IVC, Esophagus, Aorta). * **Paradoxical Respiration:** If the diaphragm is paralyzed (phrenic nerve injury), the abdominal wall moves *inward* during inspiration due to the negative intrathoracic pressure pulling the flaccid muscle upward.

Practice by Chapter

Mechanics of Breathing

Practice Questions

Pulmonary Ventilation

Practice Questions

Pulmonary Circulation

Practice Questions

Gas Exchange in the Lungs

Practice Questions

Oxygen and Carbon Dioxide Transport

Practice Questions

Control of Breathing

Practice Questions

Respiratory Adjustments in Health and Disease

Practice Questions

High Altitude Physiology

Practice Questions

Diving Physiology

Practice Questions

Respiratory Function Tests

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free