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: 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: 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).

Q: The carotid body contains islands of type I and type II cells surrounded by fenestrated sinusoidal capillaries. Type I cells are excited by hypoxia. What is the principal transmitter released by type I cells?

Dopamine. **Explanation:** The **carotid body** is a peripheral chemoreceptor located at the bifurcation of the common carotid artery. It consists of two cell types: **Type I (Glomus) cells**, which are the primary oxygen sensors, and Type II (Sustentacular) cells, which provide structural support. **Why Dopamine is correct:** When arterial $PO_2$ decreases (hypoxia), $K^+$ channels in the Type I cell membrane close, leading to depolarization. This opens voltage-gated $Ca^{2+}$ channels, triggering the exocytosis of neurotransmitters. **Dopamine** is the principal and most abundant neurotransmitter stored in the dense-core vesicles of Type I cells. It acts on the sensory nerve endings of the **glossopharyngeal nerve (CN IX)** to increase the firing rate to the respiratory centers in the medulla. **Analysis of Incorrect Options:** * **A. Serotonin:** While trace amounts of serotonin and acetylcholine are found in glomus cells, they are not the primary transmitters responsible for the initial hypoxic response. * **B. Adrenaline:** This is the primary hormone of the adrenal medulla, not the carotid body. * **D. Potassium:** Potassium is an ion, not a neurotransmitter. While the *closure* of $K^+$ channels initiates the response, it is not the substance released to signal the afferent nerve. **High-Yield Facts for NEET-PG:** * **Innervation:** Carotid body signals via the **Hering’s nerve** (branch of CN IX); Aortic body signals via the **Vagus nerve** (CN X). * **Sensitivity:** Peripheral chemoreceptors are primarily sensitive to **low $PO_2$** (<60 mmHg), but also respond to high $PCO_2$ and low pH (H+). * **Blood Flow:** The carotid body has the highest blood flow per unit weight in the body (approx. 2000 ml/100g/min), allowing it to monitor arterial blood gases instantaneously.

Question 1

Holding one's breath after hyperventilating for some time is dangerous because:

Accepted Answer

Alkalosis can lead to tetany.

Answer

It can lead to CO2 necrosis.

Answer

Lack of CO2 stimulation can lead to dangerous levels of anoxia.

Answer

Decreased CO2 shifts the oxygen dissociation curve to the left.

Question 2

If the concentration of inhaled O2 is doubled in a normal person, what will be the resultant effect?

Accepted Answer

O2 dissolved in plasma increases, but O2 bound to Hb remains relatively constant

Answer

O2 dissolved in plasma increases, but O2 bound to Hb decreases

Answer

O2 dissolved in plasma remains constant, but O2 bound to Hb increases

Answer

O2 dissolved in plasma remains constant, but O2 bound to Hb remains constant

Question 3

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 4

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

Accepted Answer

70%

Answer

85%

Answer

60%

Answer

40%

Question 5

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 6

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 7

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 8

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 9

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

Accepted Answer

GBS

Answer

COPD

Answer

Pulmonary Edema

Answer

Interstitial Lung Disease

Question 10

The carotid body contains islands of type I and type II cells surrounded by fenestrated sinusoidal capillaries. Type I cells are excited by hypoxia. What is the principal transmitter released by type I cells?

Accepted Answer

Dopamine

Answer

Serotonin

Answer

Adrenaline

Answer

Potassium

Respiratory System — MCQs

Respiratory System — MCQs

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