Respiratory System Practice Questions

Q: What is true about primary pulmonary hypoventilation?

It does not respond to chemical stimuli.. **Explanation:** **Primary Alveolar Hypoventilation (PAH)**, also known as **Ondine’s Curse**, is a rare neurological disorder characterized by the failure of autonomic control of breathing. 1. **Why Option A is Correct:** The hallmark of PAH is a **decreased or absent sensitivity of the central chemoreceptors** to hypercapnia (high $CO_2$) and hypoxia (low $O_2$). While voluntary breathing (controlled by the motor cortex) remains intact, the brainstem fails to trigger involuntary breaths in response to chemical stimuli, especially during sleep. 2. **Why the other options are incorrect:** * **Option B:** Since the patient is hypoventilating, they retain $CO_2$ and fail to take in enough $O_2$. Therefore, it is characterized by **hypercapnia** (high $PaCO_2$) and **hypoxemia** (low $PaO_2$), not hypocapnia. * **Option C:** While a congenital form exists (CCHS), primary pulmonary hypoventilation is classically described as a rare condition that typically presents in **males in their third or fourth decades** of life. * **Option D:** Hypoventilation leads to $CO_2$ retention, which results in **respiratory acidosis**, not alkalosis. **High-Yield Clinical Pearls for NEET-PG:** * **Ondine’s Curse:** Named after a myth where a nymph cursed a mortal to forget to breathe if he fell asleep. * **PFT Findings:** Lung function tests (spirometry) and respiratory muscle strength are typically **normal**; the defect is purely in the central respiratory drive. * **Diagnosis:** Confirmed by demonstrating hypercapnia during sleep that disappears or improves during wakefulness/voluntary hyperventilation. * **Association:** Congenital cases are often linked to mutations in the **PHOX2B gene**.

Q: Fowler's method is employed for the measurement of:

Anatomic dead space. **Explanation:** **Fowler’s Method** (Single-breath Nitrogen Washout) is the gold standard for measuring **Anatomic Dead Space**. **Mechanism:** The patient takes a single breath of 100% oxygen, which fills the entire respiratory tract. During expiration, the concentration of nitrogen in the exhaled air is measured. 1. Initially, the exhaled air comes from the conducting zones (anatomic dead space), which contains **0% nitrogen** (only the inhaled O2). 2. As expiration continues, nitrogen levels rise as alveolar air (which contains nitrogen) mixes with the dead space air. 3. The volume of air expired until the nitrogen concentration reaches a plateau represents the anatomic dead space. **Analysis of Incorrect Options:** * **A. Residual Volume:** Measured using **Helium Dilution** or **Body Plethysmography**, as it cannot be measured by simple spirometry or single-breath tests. * **B. Alveolar PO2:** Calculated using the **Alveolar Gas Equation** [$PAO2 = FiO2(Pb – PH2O) – (PaCO2/R)$]. * **D. Physiologic Dead Space:** Measured using **Bohr’s Method**, which utilizes arterial and expired CO2 levels. Note: In healthy individuals, anatomic and physiologic dead space are nearly equal. **High-Yield Clinical Pearls for NEET-PG:** * **Bohr’s Method = CO2** (Physiologic Dead Space). * **Fowler’s Method = N2** (Anatomic Dead Space). * **Anatomic Dead Space** is roughly **2 ml/kg** of body weight (approx. 150 ml in a 70kg adult). * Dead space increases with upright posture, large inspirations, and drugs like atropine (bronchodilation).

Q: The presence of hemoglobin in normal arterial blood increases its oxygen-carrying capacity approximately how many times?

70. **Explanation:** The oxygen-carrying capacity of blood is determined by two factors: oxygen dissolved in plasma and oxygen bound to hemoglobin (Hb). 1. **Dissolved Oxygen:** According to Henry’s Law, the amount of dissolved $O_2$ is proportional to the partial pressure ($PaO_2$). In normal arterial blood ($PaO_2 \approx 100\text{ mmHg}$), only **$0.3\text{ mL}$ of $O_2$** is dissolved in every $100\text{ mL}$ of blood. 2. **Hemoglobin-Bound Oxygen:** Each gram of Hb can carry $1.34\text{ mL}$ of $O_2$. In a normal individual ($Hb \approx 15\text{ g/dL}$), the oxygen bound to Hb is approximately $15 \times 1.34 = \mathbf{20.1\text{ mL}}$ **of $O_2$** per $100\text{ mL}$ of blood. **The Calculation:** To find the increase in capacity, we divide the total oxygen content by the dissolved oxygen: $$\text{Ratio} = \frac{20.1\text{ mL (Bound)}}{0.3\text{ mL (Dissolved)}} \approx \mathbf{67\text{ times}}$$ Rounding to the nearest clinical estimate provided in standard textbooks (like Guyton), the presence of hemoglobin increases the $O_2$ carrying capacity by approximately **70 times**. **Analysis of Incorrect Options:** * **A (10) & B (30):** These values significantly underestimate the efficiency of hemoglobin. Without Hb, the heart would have to pump blood at an impossible rate to meet tissue demands. * **C (50):** While closer, it does not account for the full physiological capacity of $15\text{ g/dL}$ of hemoglobin. **High-Yield Clinical Pearls for NEET-PG:** * **Hüfner's Constant:** $1.34\text{ mL}$ (the amount of $O_2$ bound per gram of Hb). * **Solubility Coefficient of $O_2$:** $0.003\text{ mL/dL/mmHg}$. * **Clinical Significance:** In cases of severe anemia, the dissolved $O_2$ remains the same ($0.3\text{ mL}$), but the total $O_2$ content drops drastically, leading to hemic hypoxia. * **CO2 Comparison:** $CO_2$ is about **20 times** more soluble in plasma than $O_2$.

Q: In which of the following conditions does a reduction in aerial oxygen tension occur?

Hypoventilation. ### Explanation The question asks for a condition characterized by a reduction in **aerial oxygen tension**, which refers to the partial pressure of oxygen within the alveoli ($PAO_2$). **Why Hypoventilation is Correct:** According to the **Alveolar Gas Equation**: $$PAO_2 = FiO_2(P_{atm} - PH_2O) - \frac{PaCO_2}{R}$$ In **hypoventilation**, there is a failure to adequately exchange air, leading to the retention of carbon dioxide ($CO_2$). As the partial pressure of arterial $CO_2$ ($PaCO_2$) rises, it displaces oxygen within the limited space of the alveoli. Consequently, the alveolar oxygen tension ($PAO_2$) decreases, leading to hypoxemia. This is a classic cause of a "normal A-a gradient" hypoxia. **Analysis of Incorrect Options:** * **Anemia (A):** In anemia, the $PAO_2$ and the amount of oxygen dissolved in plasma ($PaO_2$) are normal. The pathology lies in reduced hemoglobin concentration, leading to decreased total **oxygen content**, not tension. * **Carbon Monoxide (CO) Poisoning (B):** CO competes with $O_2$ for hemoglobin binding sites. While it severely reduces oxygen saturation ($SaO_2$) and content, the $PAO_2$ and $PaO_2$ remain normal because the lungs' ability to transfer gas is unaffected. * **Moderate Exercise (C):** During moderate exercise, increased ventilation and cardiac output typically maintain or even slightly increase $PAO_2$ to meet metabolic demands. **Clinical Pearls for NEET-PG:** * **A-a Gradient:** Hypoventilation and High Altitude are the only two causes of hypoxia where the **A-a gradient remains normal**. * **Oxygen Tension vs. Content:** Tension refers to partial pressure (dissolved gas), while content includes oxygen bound to hemoglobin. * **High-Yield Rule:** If $PaCO_2$ is elevated, think hypoventilation; if $PaCO_2$ is normal/low but $PaO_2$ is low, think V/Q mismatch or diffusion defect.

Q: All of the following are true about obstructive lung disease except?

Increased diffusion capacity. **Explanation:** In obstructive lung diseases (e.g., Asthma, COPD, Bronchiectasis), the primary pathology is **increased airway resistance**, making it difficult to exhale air rapidly. **Why Option D is the Correct Answer:** Diffusion capacity (DLCO) measures the ability of gases to transfer from the alveoli to the pulmonary capillaries. In obstructive diseases, DLCO is either **decreased** (as in Emphysema, due to destruction of the alveolar-capillary membrane) or **normal** (as in Asthma). It is **never increased** as a hallmark of obstructive pathology. Therefore, "Increased diffusion capacity" is the false statement. **Analysis of Incorrect Options:** * **A. Decreased FEV1:** This is the hallmark of obstruction. Narrowed airways increase resistance, significantly reducing the volume of air exhaled in the first second. * **B. Decreased MEFR:** Maximum Expiratory Flow Rate (MEFR) represents the flow during the middle portion of expiration. It is highly sensitive to airway obstruction and is characteristically reduced. * **C. Increased RV:** Due to premature airway closure (air trapping), air remains stuck in the lungs at the end of expiration, leading to an increase in Residual Volume (RV) and Total Lung Capacity (TLC). **High-Yield Clinical Pearls for NEET-PG:** * **FEV1/FVC Ratio:** The most important diagnostic parameter for obstruction is a **decreased FEV1/FVC ratio (<0.7)**. In restrictive disease, this ratio is normal or increased. * **Flow-Volume Loop:** Obstructive disease shows a characteristic **"scooped-out"** appearance on the expiratory limb. * **DLCO Differentiation:** DLCO is the key to distinguishing types of COPD; it is **decreased in Emphysema** but typically **normal in Chronic Bronchitis**.

Q: Which of the following physiological changes occurs during pregnancy?

Tidal volume increases. **Explanation:** During pregnancy, the respiratory system undergoes significant physiological adaptations to meet the increased metabolic demands of the fetus and the mother. **1. Why Tidal Volume (TV) Increases:** Progesterone acts as a direct respiratory stimulant, increasing the sensitivity of the central respiratory center to CO2. This leads to an increase in **Tidal Volume (by ~40%)**, while the respiratory rate remains relatively constant. This increase in TV enhances minute ventilation, ensuring efficient gas exchange. **2. Analysis of Incorrect Options:** * **Arterial pO2 decreases:** Incorrect. Due to hyperventilation (increased TV), the arterial pO2 actually **increases** slightly (typically 100–105 mmHg) to facilitate oxygen transfer across the placenta. * **Cardiac output decreases:** Incorrect. Cardiac output **increases** significantly (by 30–50%) during pregnancy to support fetal growth and increased maternal blood volume. * **Respiratory acidosis:** Incorrect. The progesterone-driven hyperventilation causes a "washout" of CO2, leading to a decrease in arterial pCO2 (hypocapnia). This results in a state of **compensated respiratory alkalosis**, not acidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Most common change:** Increase in Tidal Volume. * **Lung Volumes:** Functional Residual Capacity (FRC), Residual Volume (RV), and Expiratory Reserve Volume (ERV) all **decrease** due to the upward displacement of the diaphragm by the gravid uterus. * **Vital Capacity (VC):** Remains **unchanged** because the decrease in RV is compensated by the increase in TV. * **Oxygen Dissociation Curve:** Shifts to the **right** (due to increased 2,3-DPG), favoring oxygen unloading to the fetus.

Q: The affinity of hemoglobin for oxygen increases with a fall in pH. What is this phenomenon called?

Bohr effect. ### Explanation **1. The Correct Answer: Bohr Effect** The **Bohr effect** describes the phenomenon where hemoglobin’s affinity for oxygen is inversely related to acidity (H⁺ concentration) and the concentration of carbon dioxide (PCO₂). * **Mechanism:** When pH falls (becomes more acidic) or PCO₂ rises, H⁺ ions bind to specific amino acid residues on hemoglobin. This stabilizes the **T-state (Tense state)** or deoxygenated form of hemoglobin, causing the Oxygen-Dissociation Curve (ODC) to **shift to the right**. * **Physiological Significance:** This occurs primarily in metabolically active tissues, facilitating the unloading of oxygen where it is needed most. **2. Analysis of Incorrect Options** * **A. Bainbridge Effect:** This is a cardiovascular reflex. An increase in venous return stretches the right atrium, leading to an increase in heart rate to prevent blood pooling in the veins. * **C. Haldane Effect:** Often confused with the Bohr effect, this occurs in the **lungs**. It describes how the binding of oxygen to hemoglobin promotes the release of CO₂. Deoxygenated hemoglobin has a higher affinity for CO₂ than oxygenated hemoglobin. * **D. Hering-Breuer Reflex:** This is a pulmonary stretch reflex. It prevents over-inflation of the lungs; when stretch receptors in the bronchi are activated, they send signals via the Vagus nerve to inhibit the inspiratory center. **3. High-Yield Clinical Pearls for NEET-PG** * **Right Shift of ODC (Decreased Affinity):** Remember the mnemonic **"CADET, face Right!"** (CO₂, Acidosis, DPG [2,3-BPG], Exercise, Temperature). * **Left Shift of ODC (Increased Affinity):** Occurs with Alkalosis, decreased 2,3-BPG, Hypothermia, and **Fetal Hemoglobin (HbF)**. * **Key Distinction:** Bohr effect = CO₂/H⁺ affecting O₂ binding (Tissues). Haldane effect = O₂ affecting CO₂ binding (Lungs).

Q: Increased alveolar-arterial difference in PaO2 is seen in all except?

GBS. The **Alveolar-arterial (A-a) gradient** measures the difference between the oxygen concentration in the alveoli ($P_AO_2$) and the arterial blood ($PaO_2$). It is a crucial tool for differentiating causes of hypoxemia. ### **Why GBS is the Correct Answer** **Guillain-Barré Syndrome (GBS)** causes hypoxemia through **Alveolar Hypoventilation** due to respiratory muscle weakness. In pure hypoventilation, the lungs themselves are healthy; therefore, oxygen transfers normally across the alveolar-capillary membrane. Since both alveolar and arterial oxygen levels decrease proportionately, the **A-a gradient remains normal**. * *Note:* Other causes of hypoxemia with a normal A-a gradient include high altitude (low $FiO_2$) and CNS depression (opioid overdose). ### **Why the Other Options are Incorrect** In these conditions, the A-a gradient is **increased** because there is a primary defect in gas exchange: * **COPD:** Causes **Ventilation-Perfusion (V/Q) mismatch** due to airway obstruction and alveolar destruction. * **Pulmonary Edema:** Increases the diffusion distance and causes V/Q mismatch/shunting as fluid fills the alveoli. * **Interstitial Lung Disease (ILD):** Causes **Diffusion impairment** due to thickening and fibrosis of the alveolar-capillary membrane. ### **High-Yield Clinical Pearls for NEET-PG** 1. **Formula:** $A-a\text{ Gradient} = P_AO_2 - PaO_2$. 2. **Normal Range:** Approximately 5–15 mmHg (increases with age: $\text{Age}/4 + 4$). 3. **Rule of Thumb:** * **Normal A-a Gradient:** Extrapulmonary causes (Hypoventilation, Low $FiO_2$). * **Increased A-a Gradient:** Intrapulmonary causes (V/Q mismatch, Shunt, Diffusion defect). 4. Hypoventilation is always associated with **increased $PaCO_2$** (Hypercapnia).

Question 1

What is true about primary pulmonary hypoventilation?

Accepted Answer

It does not respond to chemical stimuli.

Answer

It is characterized by hypocapnoea and normal PaO2.

Answer

It is common in children.

Answer

Respiratory alkalosis is diagnostic.

Question 2

Fowler's method is employed for the measurement of:

Accepted Answer

Anatomic dead space

Answer

Residual volume

Answer

Alveolar PO2

Answer

Physiologic dead space

Question 3

During heavy exercise, at what dyspneic index value does dyspnea typically appear?

Accepted Answer

70%

Answer

85%

Answer

60%

Answer

40%

Question 4

The presence of hemoglobin in normal arterial blood increases its oxygen-carrying capacity approximately how many times?

Accepted Answer

70

Answer

10

Answer

30

Answer

50

Question 5

In which of the following conditions does a reduction in aerial oxygen tension occur?

Accepted Answer

Hypoventilation

Answer

Anemia

Answer

Carbon monoxide poisoning

Answer

Moderate exercise

Question 6

All of the following are true about obstructive lung disease except?

Accepted Answer

Increased diffusion capacity

Answer

Decreased FEV1

Answer

Decreased MEFR

Answer

Increased RV

Question 7

Peripheral chemoreceptors respond to hypoxia by activating oxygen-sensitive channels. Which type of channel is primarily involved in this response?

Accepted Answer

K+ channel

Answer

Na+ channel

Answer

Ca+2 channel

Answer

Cl- channel

Question 8

Which of the following physiological changes occurs during pregnancy?

Accepted Answer

Tidal volume increases

Answer

Arterial pO2 decreases

Answer

Cardiac output decreases

Answer

Respiratory acidosis

Question 9

The affinity of hemoglobin for oxygen increases with a fall in pH. What is this phenomenon called?

Accepted Answer

Bohr effect

Answer

Bain-Bridge effect

Answer

Haldane effect

Answer

Herring effect

Question 10

Increased alveolar-arterial difference in PaO2 is seen in all except?

Accepted Answer

GBS

Answer

COPD

Answer

Pulmonary Edema

Answer

Interstitial Lung Disease

Respiratory System — MCQs

Respiratory System — MCQs

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