NEET-PG 2025 — Physiology

Q: Effect of pulmonary embolism on graph of V/Q?

Increased V/Q ratio approaching infinity (dead space). ***Increased V/Q ratio approaching infinity (dead space)*** - In pulmonary embolism, an embolus obstructs a pulmonary artery, leading to a significant reduction or complete cessation of **perfusion (Q)** to the downstream lung tissue, while **ventilation (V)** remains unaffected. - This creates a V/Q mismatch where the ratio V/Q becomes very high, approaching infinity. This ventilated but unperfused lung region is known as **physiological dead space**. *Decreased V/Q ratio approaching zero* - A decreased V/Q ratio occurs when ventilation is impaired relative to perfusion (V<Q), a condition known as a **shunt**. - This is characteristic of conditions like **pneumonia**, **pulmonary edema**, or **atelectasis**, where alveoli are filled with fluid or collapsed but still receive blood flow. *Normal V/Q ratio* - Pulmonary embolism is a classic cause of a severe **V/Q mismatch**, specifically creating high V/Q units; therefore, the ratio in the affected area is not normal. - A normal V/Q ratio (approximately 0.8) implies well-matched ventilation and perfusion, which is the state PE directly disrupts. *Increased perfusion (Q) and decreased ventilation (V)* - This describes the opposite of what occurs in pulmonary embolism, where the primary problem is a **decrease in perfusion (Q)** due to the arterial blockage. - Increased perfusion with decreased ventilation would result in a **low V/Q ratio (shunt)**, not the high V/Q dead space seen in PE. [Practice more Physiology PYQs on OnCourse]

Q: A man is climbing a mountain for trekking. Based on his physiological response to the high altitude, what is the most likely primary acid-base abnormality in his blood?

Respiratory alkalosis. ***Respiratory alkalosis*** - Exposure to **high altitude** causes decreased ambient partial pressure of oxygen (PO₂), leading to **hypoxemia**. - The physiological response to hypoxemia is reflex **hyperventilation** mediated by peripheral chemoreceptors, which blows off excessive **carbon dioxide (CO₂)**, causing decreased PaCO₂ and consequently elevated blood pH (alkalosis). - This is the **primary and immediate** acid-base abnormality at high altitude. *Metabolic acidosis* - This condition occurs due to the accumulation of **non-volatile acids** (e.g., lactic acid, ketoacids) or loss of bicarbonate. - While the kidney eventually compensates for the respiratory alkalosis by excreting bicarbonate (leading to compensatory **metabolic acidosis**), this is the **secondary, not the primary**, abnormality. *Metabolic alkalosis* - This abnormality is typically caused by loss of acid (e.g., severe vomiting, gastric suction) or excessive administration of alkali. - It is not related to the immediate respiratory compensatory response to **high-altitude hypoxemia**. *Respiratory acidosis* - This is caused by **hypoventilation** or impaired alveolar ventilation, leading to retention of **CO₂** and decreased pH. - At high altitude, the body actively **hyperventilates** to improve oxygen uptake, making respiratory acidosis the opposite of the expected primary response. [Practice more Physiology PYQs on OnCourse]

2 Previous Year Questions with Answers & Explanations

Questions

Effect of pulmonary embolism on graph of V/Q?

A man is climbing a mountain for trekking. Based on his physiological response to the high altitude, what is the most likely primary acid-base abnormality in his blood?

NEET-PG 2025 - Physiology NEET-PG Practice Questions and MCQs

Question 1: Effect of pulmonary embolism on graph of V/Q?

A. Decreased V/Q ratio approaching zero
B. Increased V/Q ratio approaching infinity (dead space) (Correct Answer)
C. Normal V/Q ratio
D. Increased perfusion (Q) and decreased ventilation (V)

Explanation: ***Increased V/Q ratio approaching infinity (dead space)*** - In pulmonary embolism, an embolus obstructs a pulmonary artery, leading to a significant reduction or complete cessation of **perfusion (Q)** to the downstream lung tissue, while **ventilation (V)** remains unaffected. - This creates a V/Q mismatch where the ratio V/Q becomes very high, approaching infinity. This ventilated but unperfused lung region is known as **physiological dead space**. *Decreased V/Q ratio approaching zero* - A decreased V/Q ratio occurs when ventilation is impaired relative to perfusion (V<Q), a condition known as a **shunt**. - This is characteristic of conditions like **pneumonia**, **pulmonary edema**, or **atelectasis**, where alveoli are filled with fluid or collapsed but still receive blood flow. *Normal V/Q ratio* - Pulmonary embolism is a classic cause of a severe **V/Q mismatch**, specifically creating high V/Q units; therefore, the ratio in the affected area is not normal. - A normal V/Q ratio (approximately 0.8) implies well-matched ventilation and perfusion, which is the state PE directly disrupts. *Increased perfusion (Q) and decreased ventilation (V)* - This describes the opposite of what occurs in pulmonary embolism, where the primary problem is a **decrease in perfusion (Q)** due to the arterial blockage. - Increased perfusion with decreased ventilation would result in a **low V/Q ratio (shunt)**, not the high V/Q dead space seen in PE.

Question 2: A man is climbing a mountain for trekking. Based on his physiological response to the high altitude, what is the most likely primary acid-base abnormality in his blood?

A. Metabolic acidosis
B. Respiratory alkalosis (Correct Answer)
C. Respiratory acidosis
D. Metabolic alkalosis

Explanation: ***Respiratory alkalosis*** - Exposure to **high altitude** causes decreased ambient partial pressure of oxygen (PO₂), leading to **hypoxemia**. - The physiological response to hypoxemia is reflex **hyperventilation** mediated by peripheral chemoreceptors, which blows off excessive **carbon dioxide (CO₂)**, causing decreased PaCO₂ and consequently elevated blood pH (alkalosis). - This is the **primary and immediate** acid-base abnormality at high altitude. *Metabolic acidosis* - This condition occurs due to the accumulation of **non-volatile acids** (e.g., lactic acid, ketoacids) or loss of bicarbonate. - While the kidney eventually compensates for the respiratory alkalosis by excreting bicarbonate (leading to compensatory **metabolic acidosis**), this is the **secondary, not the primary**, abnormality. *Metabolic alkalosis* - This abnormality is typically caused by loss of acid (e.g., severe vomiting, gastric suction) or excessive administration of alkali. - It is not related to the immediate respiratory compensatory response to **high-altitude hypoxemia**. *Respiratory acidosis* - This is caused by **hypoventilation** or impaired alveolar ventilation, leading to retention of **CO₂** and decreased pH. - At high altitude, the body actively **hyperventilates** to improve oxygen uptake, making respiratory acidosis the opposite of the expected primary response.