Respiratory System Practice Questions

Q: Which of the following is not a consequence of stimulation of lung C fiber endings?

Systemic vasoconstriction. **Explanation:** Lung C-fiber endings (Juxtacapillary or **J-receptors**) are sensory nerve endings located in the alveolar walls, near the pulmonary capillaries. They are innervated by the vagus nerve and are primarily stimulated by pulmonary congestion, edema, or chemical irritants (e.g., capsaicin). **Why Systemic Vasoconstriction is the Correct Answer:** Stimulation of J-receptors triggers the **J-reflex** (Pulmonary Chemoreflex). This reflex is characterized by a triad of **bradycardia, hypotension, and apnea**. The hypotension occurs due to **systemic vasodilation** (not vasoconstriction) and a decrease in heart rate. Therefore, systemic vasoconstriction is not a consequence of J-receptor stimulation. **Analysis of Incorrect Options:** * **Bronchoconstriction (A):** Stimulation of C-fibers leads to a reflex increase in parasympathetic outflow, causing narrowing of the airways. * **Apnea (B):** The immediate response to J-receptor activation is a brief period of apnea (cessation of breathing). * **Rapid Shallow Breathing (C):** Following the initial apnea, the respiratory pattern shifts to tachypnea (rapid) and shallow breathing to minimize the work of breathing in the presence of interstitial irritation. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** J-receptors are located in the alveolar interstitium, while bronchial C-fibers are in the bronchial walls. * **Classic Stimulus:** Pulmonary edema (interstitial fluid accumulation) is the most common physiological trigger. * **The Triad:** Remember the "Pulmonary Chemoreflex" triad: **Apnea, Bradycardia, and Hypotension.** * **Sensation:** J-receptor stimulation is believed to contribute to the sensation of **dyspnea** (shortness of breath) in patients with left heart failure.

Q: In the following diagram, the curve "X" represents the normal relationship of alveolar ventilation with PaCO2 when PaO2 is 100 mmHg. If pH changed from 7.4 to 7.3, by which curve will the change in alveolar ventilation shift?

Curve B. ***Curve B*** - A decrease in **pH from 7.4 to 7.3** represents **acidosis** (increased H+ ions), which stimulates the **respiratory center** and shifts the CO2 response curve **leftward/upward**. - This leftward shift means **greater alveolar ventilation** occurs at any given PaCO2 level, as the body compensates for acidosis by increasing ventilation to blow off CO2. *Curve A* - Represents a **rightward/downward shift**, which occurs during **alkalosis** (increased pH) when respiratory drive is **decreased**. - This would be inappropriate for acidosis, as it indicates **reduced ventilatory response** to CO2, opposite to what occurs with decreased pH. *Curve C* - Appears to represent either **no shift** from baseline or a different type of physiological change not related to **acid-base status**. - Does not account for the **increased respiratory drive** that occurs with acidosis and the need for compensatory hyperventilation. *Curve D* - Likely represents a **rightward shift** similar to alkalosis or potentially **hypoxia-related changes** in ventilatory response. - Fails to represent the **leftward shift** that characterizes the body's compensatory response to **metabolic acidosis** through increased alveolar ventilation.

Q: What is true regarding the alveolar-arterial oxygen gradient (A-a gradient)?

All of the above.. The **Alveolar-arterial (A-a) oxygen gradient** is a clinical measure used to differentiate between causes of hypoxemia. It represents the difference between the oxygen concentration in the alveoli ($P_AO_2$) and the arterial blood ($P_aO_2$). ### **Explanation of Options** * **Option A (Normal Gradient):** In a healthy young adult, the normal A-a gradient is typically **5–15 mmHg**. It increases naturally with age (estimated as $[Age/4] + 4$). A value of 15 mmHg is considered the upper limit of normal. * **Option B (Pulmonary Collapse):** Pulmonary collapse (atelectasis) leads to a **Right-to-Left Shunt**. Blood perfuses non-ventilated alveoli, entering the arterial system without being oxygenated. This significantly widens (increases) the A-a gradient. * **Option C (Venous Admixture):** This refers to the mixing of deoxygenated blood with oxygenated blood. Physiologically, this occurs via the **Thebesian veins** (draining the myocardium) and **bronchial veins**. This "physiological shunt" is the primary reason why a small A-a gradient exists even in healthy individuals. ### **High-Yield NEET-PG Pearls** 1. **Normal A-a Gradient Hypoxemia:** Occurs in **Alveolar Hypoventilation** (e.g., opioid overdose, neuromuscular weakness) and **High Altitude**. 2. **Increased A-a Gradient Hypoxemia:** Occurs in **V/Q Mismatch** (e.g., PE, pneumonia), **Diffusion defects** (e.g., ILD), and **Shunts** (e.g., Atelectasis, ASD/VSD). 3. **Formula:** $P_AO_2 = FiO_2(P_{atm} - P_{H2O}) - (P_aCO_2 / R)$. 4. **Key Distinction:** Hypoxemia due to shunting is the only type that **does not** fully correct with 100% supplemental oxygen.

Q: Under resting conditions, what is the total body oxygen consumption?

250 ml/min. **Explanation:** **1. Why Option D is Correct:** Under basal resting conditions, an average adult (70 kg) consumes approximately **250 ml of oxygen per minute ($VO_2$)**. This value represents the metabolic demand required to maintain vital organ functions. It is calculated using the **Fick Principle**, which states that oxygen consumption is the product of Cardiac Output ($CO$) and the Arterio-venous oxygen difference ($A-V O_2$ difference). * Calculation: $CO$ (5000 ml/min) × $[$Arterial $O_2$ (20 ml/dL) – Venous $O_2$ (15 ml/dL)$]$ = $5000 \times 0.05 = 250 \text{ ml/min}$. **2. Why Other Options are Incorrect:** * **Option A (100 ml/min):** This is significantly lower than the basal metabolic rate of a healthy adult. * **Option B (150 ml/min):** This value is insufficient for a 70 kg adult but may be seen in individuals with a very small body mass or severe hypometabolic states. * **Option C (200 ml/min):** While closer, the standard physiological reference for a resting adult is 250 ml/min. Carbon dioxide production ($VCO_2$) is approximately **200 ml/min**, which is often confused with $O_2$ consumption. **3. NEET-PG High-Yield Pearls:** * **Respiratory Quotient (RQ):** The ratio of $CO_2$ produced to $O_2$ consumed ($200/250$) is **0.8** on a mixed diet. * **MET (Metabolic Equivalent):** 1 MET is defined as $3.5 \text{ ml/kg/min}$ of $O_2$ consumption. For a 70 kg man: $70 \times 3.5 \approx 250 \text{ ml/min}$. * **Exercise:** During strenuous exercise, $O_2$ consumption can increase up to 20-fold (approx. 5000 ml/min) in elite athletes. * **Utilization Coefficient:** At rest, tissues extract about **25%** of the oxygen delivered by arterial blood.

Q: Which of the following is a typical manifestation of chronic hyperventilation?

Tetany. **Explanation:** The correct answer is **Tetany**. **Mechanism:** Chronic hyperventilation leads to excessive "washing out" of carbon dioxide ($CO_2$), resulting in **respiratory alkalosis**. In an alkaline state, the concentration of hydrogen ions ($H^+$) in the blood decreases. To compensate, $H^+$ ions dissociate from plasma proteins (like albumin). This frees up binding sites on albumin, which then bind to **ionized calcium ($Ca^{2+}$)**. Consequently, the level of free ionized calcium in the plasma drops (**hypocalcemia**), even though total body calcium remains normal. Since ionized calcium is crucial for stabilizing neuronal membranes, its deficiency increases membrane permeability to sodium, leading to nerve hyperexcitability and involuntary muscle contractions known as **tetany**. **Analysis of Incorrect Options:** * **A. Hypoxemia:** Hyperventilation actually increases alveolar $PO_2$ and typically results in normal or elevated arterial oxygen levels, not hypoxemia. * **B. Hyperphosphatemia:** Respiratory alkalosis causes an intracellular shift of phosphate to support increased glycolysis, typically leading to **hypophosphatemia**. * **D. Clubbing:** This is a sign of chronic hypoxia or underlying suppurative lung diseases (like bronchiectasis) and is not a physiological result of hyperventilation. **High-Yield Clinical Pearls for NEET-PG:** * **Trousseau’s sign** (carpal spasm with BP cuff inflation) and **Chvostek’s sign** (facial twitching) are classic bedside tests for latent tetany. * **Acute Management:** Breathing into a paper bag helps by increasing the fraction of inspired $CO_2$, reversing the alkalosis. * **Henderson-Hasselbalch Link:** Remember that $pH$ is inversely proportional to $pCO_2$. A decrease in $pCO_2$ always shifts the equation toward alkalemia.

Q: One gram of hemoglobin, when fully saturated in arterial blood, carries what volume of oxygen?

1.34 mL of oxygen. ### Explanation **1. Why Option B is Correct:** The oxygen-carrying capacity of hemoglobin (Hb) is a fundamental physiological constant. Theoretically, one molecule of hemoglobin can bind four molecules of oxygen. Based on its molecular weight, the maximum theoretical capacity is **1.39 mL** of oxygen per gram of Hb. However, in vivo (within the human body), a small fraction of hemoglobin exists as inactive forms such as methemoglobin or carboxyhemoglobin, which do not bind oxygen. Therefore, the **physiological/actual oxygen-carrying capacity** is measured at **1.34 mL of oxygen per gram of Hb**. This value (Hüfner's constant) is used in clinical calculations for oxygen content. **2. Why Other Options are Incorrect:** * **Option A (1.24 mL):** This value is too low and does not correspond to any standard physiological measurement of Hb binding. * **Option C (1.39 mL):** This is the **theoretical maximum** capacity of pure, 100% functional hemoglobin. While chemically accurate in a lab setting, it is not the standard value used for arterial blood in human physiology due to the presence of non-functional Hb. * **Option D (1.43 mL):** This is an incorrect figure and exceeds even the theoretical binding capacity of hemoglobin. **3. NEET-PG High-Yield Pearls:** * **Total Oxygen Content Equation:** $CaO_2 = (1.34 \times Hb \times SaO_2) + (0.003 \times PaO_2)$. * **Dissolved Oxygen:** Only **0.003 mL** of $O_2$ is dissolved per 100 mL of blood per mmHg of $PO_2$. This is why Hb is essential for life. * **Normal Hb values:** For calculation purposes, assume 15g/dL. Thus, 100 mL of blood carries approximately **20.1 mL** of oxygen ($15 \times 1.34$). * **P50 Value:** The $PO_2$ at which Hb is 50% saturated is **26-27 mmHg**. A shift to the right (increased P50) indicates decreased affinity (e.g., increased $CO_2$, Temp, or 2,3-BPG).

Question 1

Which of the following is not a consequence of stimulation of lung C fiber endings?

Accepted Answer

Systemic vasoconstriction

Answer

Bronchoconstriction

Answer

Apnea

Answer

Rapid shallow breathing

Question 2

What is the reason for the sigmoid shape of the hemoglobin-oxygen dissociation curve?

Accepted Answer

The binding of one oxygen molecule increases the affinity for the subsequent oxygen molecule.

Answer

The alpha chain has a higher affinity for oxygen than the beta chain.

Answer

The beta chain has a higher affinity for oxygen than the alpha chain.

Answer

Hemoglobin is acidic in nature.

Question 3

An anesthetized patient is mechanically ventilated at her normal tidal volume but at twice her normal frequency. When mechanical ventilation is stopped, the patient fails to breathe spontaneously for 1 minute. This temporary cessation of breathing occurs because:

Accepted Answer

The lowered arterial carbon dioxide tension (PaCO2) reduced central chemoreceptor activity.

Answer

The elevated arterial oxygen tension (PaO2) reduced peripheral chemoreceptor activity.

Answer

The elevated arterial oxygen tension (PaO2) reduced central chemoreceptor activity.

Answer

Low levels of nitrogen are known to inhibit ventilation.

Question 4

In the following diagram, the curve "X" represents the normal relationship of alveolar ventilation with PaCO2 when PaO2 is 100 mmHg. If pH changed from 7.4 to 7.3, by which curve will the change in alveolar ventilation shift?

Accepted Answer

Curve B

Answer

Curve A

Answer

Curve C

Answer

Curve D

Question 5

What is true regarding the alveolar-arterial oxygen gradient (A-a gradient)?

Accepted Answer

All of the above.

Answer

A normal gradient is approximately 15 mmHg (2 kPa).

Answer

An increased gradient can be caused by pulmonary collapse.

Answer

Venous admixture affects the A-a oxygen gradient.

Question 6

Under resting conditions, what is the total body oxygen consumption?

Accepted Answer

250 ml/min

Answer

100 ml/min

Answer

150 ml/min

Answer

200 ml/min

Question 7

Apneusis occurs when there is damage to:

Accepted Answer

The pneumotaxic center with bilateral vagotomy

Answer

The apneustic center with intact vagi

Answer

The apneustic center with bilateral vagotomy

Answer

The pneumotaxic center with intact vagi

Question 8

Which of the following is a typical manifestation of chronic hyperventilation?

Accepted Answer

Tetany

Answer

Hypoxemia

Answer

Hyperphosphatemia

Answer

Clubbing

Question 9

One gram of hemoglobin, when fully saturated in arterial blood, carries what volume of oxygen?

Accepted Answer

1.34 mL of oxygen

Answer

1.24 mL of oxygen

Answer

1.39 mL of oxygen

Answer

1.43 mL of oxygen

Question 10

Cyanosis may be seen in which of the following types of hypoxia?

Accepted Answer

Stagnant hypoxia

Answer

Histotoxic hypoxia

Answer

Anemic hypoxia

Answer

All of the above

Respiratory System — MCQs

Respiratory System — MCQs

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