Respiratory System Practice Questions

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

32 mm Hg. ### 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).

Q: Hypoxic pulmonary vasoconstriction is due to what mechanism?

Reversible pulmonary vasoconstriction. ### 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).

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

APUD cells. **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.

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

1-2 cm. 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.

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

45. **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.

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

Apneusis. 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).

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

Head's paradoxical reflex. **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.

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

It tends to flatten the diaphragm.. ### 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.

Question 1

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

Accepted Answer

32 mm Hg

Answer

0.3 mm Hg

Answer

158 mm Hg

Answer

40 mm Hg

Question 2

Hypoxic pulmonary vasoconstriction is due to what mechanism?

Accepted Answer

Reversible pulmonary vasoconstriction

Answer

Irreversible pulmonary vasoconstriction

Answer

Shunting of blood to poorly ventilated areas

Answer

Occurs hours after pulmonary vasoconstriction

Question 3

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

Accepted Answer

They are located in the medulla oblongata.

Answer

The neurons are located in the pons.

Answer

They are primarily responsible for inspiratory muscle contraction.

Answer

They respond to changes in arterial oxygen levels.

Question 4

Which of the following are neuroendocrine cells found in the lungs?

Accepted Answer

APUD cells

Answer

Dendritic cells

Answer

Type I pneumocytes

Answer

Type II pneumocytes

Question 5

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

Accepted Answer

1-2 cm

Answer

3-5 cm

Answer

5-7 cm

Answer

7-9 cm

Question 6

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

Accepted Answer

45

Answer

150

Answer

740

Answer

573

Question 7

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

Accepted Answer

Apneusis

Answer

Cessation of spontaneous respiration

Answer

Continuation of regular breathing

Answer

Irregular but rhythmic respiration

Question 8

What is the most important stimulant of the respiratory center?

Accepted Answer

Decreased PaO2

Answer

Alkalosis

Answer

Decreased PCO2

Answer

Increased PCO2

Question 9

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

Accepted Answer

Head's paradoxical reflex

Answer

Hering-Breuer inflation reflex

Answer

Hering-Breuer deflation reflex

Answer

J-reflex

Question 10

Which of the following statements is true concerning the contraction of the diaphragm?

Accepted Answer

It tends to flatten the diaphragm.

Answer

The nerves responsible for its innervation emerge from the spinal cord at the level of the lower thorax.

Answer

It reduces the lateral distance between the lower rib margins.

Answer

It causes the anterior abdominal wall to move inward.

Respiratory System — MCQs

Respiratory System — MCQs

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