Respiratory System Practice Questions

Q: The gradient of alveolar-arterial oxygen tension increases in all of the following conditions except?

Hypoventilation. **Explanation:** The **Alveolar-arterial (A-a) gradient** is a measure of the difference between the oxygen concentration in the alveoli ($P_AO_2$) and the arterial blood ($PaO_2$). It is a critical tool for localizing the cause of hypoxemia. **Why Hypoventilation is the Correct Answer:** In **Hypoventilation** (and high altitude), the hypoxemia is caused by a primary decrease in alveolar oxygen ($P_AO_2$). Because the lungs themselves are healthy, the oxygen that *is* present in the alveoli diffuses perfectly into the blood. Therefore, both $P_AO_2$ and $PaO_2$ decrease proportionately, keeping the **A-a gradient within the normal range** (usually <15 mmHg). **Analysis of Incorrect Options:** * **Diffusion Defect (A):** Conditions like pulmonary fibrosis thicken the blood-gas barrier, preventing oxygen from equilibrating. This leads to a significant drop in $PaO_2$ relative to $P_AO_2$, **increasing** the gradient. * **Right-to-Left Shunt (B):** Deoxygenated blood bypasses ventilated alveoli and mixes with oxygenated blood. This is the most "refractory" cause of an **increased** A-a gradient. * **V/Q Mismatch (D):** Seen in pneumonia or pulmonary embolism, where there is a disharmony between airflow and blood flow. This impairs gas exchange efficiency, **increasing** the gradient. **High-Yield NEET-PG Pearls:** 1. **Normal A-a Gradient Hypoxemia:** Only two causes—**Hypoventilation** (e.g., opioid overdose, neuromuscular weakness) and **High Altitude**. 2. **Increased A-a Gradient Hypoxemia:** Caused by V/Q mismatch, Shunt, or Diffusion limitation. 3. **Formula:** $P_AO_2 = FiO_2(P_{atm} - P_{H2O}) - (PaCO_2 / R)$. 4. **Age Adjustment:** A normal gradient increases with age. A quick rule of thumb: $(Age / 4) + 4$.

Q: Which of the following statements is TRUE regarding the partial pressures of alveolar gases?

Mixed venous PCO2 is greater than alveolar PCO2.. **Explanation** The movement of gases across the alveolar-capillary membrane is governed by **simple diffusion**, which occurs down a partial pressure gradient. **1. Why Option D is Correct:** For carbon dioxide ($CO_2$) to be excreted from the body, it must move from the blood into the alveoli. The **Mixed Venous $PCO_2$** (blood returning to the lungs) is approximately **46 mmHg**, while the **Alveolar $PCO_2$ ($PACO_2$)** is approximately **40 mmHg**. This gradient of 6 mmHg allows $CO_2$ to diffuse into the alveoli to be exhaled. **2. Analysis of Incorrect Options:** * **Option A:** If Alveolar $PCO_2$ were greater than mixed venous $PCO_2$, $CO_2$ would move into the blood rather than being cleared, leading to respiratory acidosis. * **Option B:** Alveolar $PO_2$ (~104 mmHg) is actually **greater** than the $PO_2$ of expired air (~120 mmHg). This is because expired air is a mixture of alveolar air and "dead space" air (which has a $PO_2$ closer to atmospheric levels, ~159 mmHg). * **Option C:** Room air contains negligible $CO_2$ (~0.3 mmHg). Alveolar $PCO_2$ (40 mmHg) is more than 100 times greater than room air, not just twice. **High-Yield NEET-PG Pearls:** * **Diffusion Capacity:** $CO_2$ is 20-25 times more soluble than $O_2$; therefore, it requires a much smaller pressure gradient (6 mmHg) compared to $O_2$ (~60 mmHg) to diffuse effectively. * **Alveolar Gas Equation:** $PAO_2 = FiO_2(P_{atm} - PH_2O) - (PACO_2 / R)$. * **Normal Values:** * **Alveolar:** $PO_2 = 104$ mmHg; $PCO_2 = 40$ mmHg. * **Mixed Venous:** $PO_2 = 40$ mmHg; $PCO_2 = 46$ mmHg.

Q: Vital capacity is decreased, while timed vital capacity (FEV1/VC %) is normal in which of the following conditions?

Scoliosis. ### Explanation The question tests the ability to differentiate between **Restrictive** and **Obstructive** lung diseases based on Pulmonary Function Tests (PFTs). **1. Why Scoliosis is Correct (Restrictive Pattern):** Scoliosis is a chest wall deformity that limits the expansion of the thoracic cage. This leads to a **Restrictive Lung Disease** pattern. In restrictive diseases: * **Vital Capacity (VC)** and Total Lung Capacity (TLC) are **decreased** because the lungs cannot expand fully. * **FEV1** is decreased proportionately with VC. * Therefore, the **FEV1/VC ratio (Timed Vital Capacity) remains normal** (usually >70-80%) or may even be slightly increased, as there is no airway obstruction to slow down expiration. **2. Why Other Options are Incorrect (Obstructive Pattern):** * **Bronchial Asthma, Chronic Bronchitis, and Acute Bronchitis** are all examples of **Obstructive Lung Diseases**. * In these conditions, the primary pathology is increased airway resistance. While the Vital Capacity may be slightly reduced or normal, the **FEV1 is significantly decreased** because the patient cannot exhale rapidly. * This results in a **decreased FEV1/VC ratio** (<70%), which is the hallmark of obstruction. **Clinical Pearls for NEET-PG:** * **Restrictive Pattern (Normal/High Ratio):** Think of "PAINT" — **P**hreatic nerve palsy, **A**lveolar (Edema/Pus), **I**nterstitial Lung Disease (Fibrosis), **N**euromuscular (Myasthenia Gravis), and **T**horacic/Extrathoracic (Scoliosis, Obesity). * **Obstructive Pattern (Low Ratio):** Think of "CBABE" — **C**ystic Fibrosis, **B**ronchitis (Chronic), **A**sthma, **B**ronchiectasis, and **E**mphysema. * **Gold Standard:** FEV1/FVC ratio is the most reliable parameter to differentiate between obstructive and restrictive patterns on spirometry.

Q: CO2 is carried in the blood mainly by which form?

Bicarbonate form. **Explanation:** Carbon dioxide (CO₂) is transported from the tissues to the lungs in three primary forms. The distribution is as follows: 1. **Bicarbonate Form (70%):** This is the **major form** of CO₂ transport. CO₂ enters the RBCs and reacts with water to form carbonic acid ($H_2CO_3$), a reaction catalyzed by the enzyme **Carbonic Anhydrase**. This acid dissociates into $H^+$ and $HCO_3^-$. The bicarbonate then diffuses out into the plasma in exchange for chloride ions (the **Chloride Shift** or **Hamburger Phenomenon**). 2. **Carbamino-compounds (23%):** CO₂ binds directly to the amine groups of hemoglobin to form **Carbaminohemoglobin**. Note: CO₂ does not bind to the iron (heme) site, but to the globin chain. 3. **Dissolved Form (7%):** A small fraction is carried physically dissolved in the plasma. Despite its low percentage, this dissolved CO₂ is what exerts the partial pressure ($PCO_2$). **Analysis of Incorrect Options:** * **Dissolved form:** Only accounts for ~7% of transport. * **Carboxyhemoglobin:** This is a trap. Carboxyhemoglobin is formed when **Carbon Monoxide (CO)** binds to hemoglobin, not $CO_2$. * **Carbamino-compounds:** While significant (23%), it is not the "main" or majority form. **High-Yield Clinical Pearls for NEET-PG:** * **Haldane Effect:** Deoxygenation of the blood increases its ability to carry $CO_2$. In the lungs, when $O_2$ binds to Hb, it promotes the release of $CO_2$. * **Carbonic Anhydrase:** It is one of the fastest enzymes known; it is absent in plasma but present in high concentrations in RBCs. * **Chloride Shift:** Occurs at the tissue level (Chloride moves into RBCs); **Reverse Chloride Shift** occurs in the lungs (Chloride moves out).

Q: What is the ventilation-perfusion ratio?

0.8. **Explanation:** The **Ventilation-Perfusion (V/Q) ratio** is the ratio of the amount of air reaching the alveoli (Alveolar Ventilation) to the amount of blood reaching the alveoli (Pulmonary Capillary Blood Flow). 1. **Why 0.8 is Correct:** In a healthy adult at rest, normal alveolar ventilation ($\dot{V}_A$) is approximately **4.2 L/min**, and normal pulmonary cardiac output ($\dot{Q}$) is approximately **5.0 L/min**. $$\text{V/Q Ratio} = \frac{4.2}{5.0} = 0.84 \approx 0.8$$ This value represents the "ideal" average for the entire lung, ensuring optimal gas exchange where oxygenation of blood and removal of $CO_2$ are balanced. 2. **Analysis of Incorrect Options:** * **Option A (0.5):** This indicates a "shunting" effect where perfusion exceeds ventilation, often seen in basal lung segments or pathological states like atelectasis. * **Option C (1.0):** While theoretically "perfect" matching, it is not the physiological average in humans due to the slight excess of perfusion over ventilation. * **Option D (1.2):** This indicates ventilation exceeds perfusion, characteristic of the lung **apices** due to gravity, or "dead space" ventilation. 3. **High-Yield NEET-PG Pearls:** * **Regional Variation:** V/Q is highest at the **Apex (~3.3)** and lowest at the **Base (~0.6)**. * **Gravity Effect:** Both ventilation and perfusion increase from the top to the bottom of the lung, but **perfusion increases more steeply** than ventilation. * **Exercise:** During exercise, the V/Q ratio becomes more uniform across the lung and the overall average increases toward 1.0 or higher. * **Extreme V/Q:** A ratio of **0** (Ventilation = 0) is called a **Shunt**; a ratio of **$\infty$** (Perfusion = 0) is called **Dead Space**.

Q: J-receptors, which are responsible for rapid shallow breathing, are located in which of the following structures?

Alveoli-capillary junction. ### Explanation **Correct Answer: C. Alveoli-capillary junction** **Concept Overview:** Juxtacapillary receptors, commonly known as **J-receptors**, are sensory nerve endings located in the interstitial space of the alveolar wall, specifically near the **alveoli-capillary junction**. They are innervated by non-myelinated **vagal C-fibers**. These receptors are primarily stimulated by an increase in interstitial fluid volume (pulmonary edema) or engorgement of pulmonary capillaries. When activated, they trigger the **"J-reflex,"** which results in a characteristic triad: **apnea followed by rapid shallow breathing (tachypnea), bradycardia, and hypotension.** **Analysis of Incorrect Options:** * **A. Thoracic cage and lung:** While the lungs contain various receptors (like irritant receptors in the airway epithelium), J-receptors are specifically localized to the alveolar-capillary interface, not the thoracic cage. * **B. Carotid artery:** This is the site of **peripheral chemoreceptors** (Carotid bodies), which respond to changes in $PaO_2$, $PaCO_2$, and pH, rather than mechanical or fluid changes in the lung parenchyma. * **D. Respiratory muscles:** These contain **muscle spindles** and Golgi tendon organs that sense stretch and tension to coordinate the work of breathing, but they do not mediate the J-reflex. **High-Yield Clinical Pearls for NEET-PG:** * **Stimulus:** The most common clinical trigger for J-receptor activation is **Left Heart Failure** leading to pulmonary congestion/edema. * **Chemical Triggers:** J-receptors are also stimulated by chemicals injected into the pulmonary circulation, such as **capsaicin** or phenyl diguanide. * **Dyspnea:** Activation of these receptors contributes to the sensation of dyspnea (breathlessness) in patients with pulmonary edema or pneumonia. * **Reflex Pathway:** Sensory limb = Vagus nerve (C-fibers) $\rightarrow$ Medullary respiratory centers.

Question 1

Small airway resistance is best measured by?

Accepted Answer

FEV25-75%

Answer

FEV1

Answer

Closing volume

Answer

Total lung capacity

Question 2

The gradient of alveolar-arterial oxygen tension increases in all of the following conditions except?

Accepted Answer

Hypoventilation

Answer

Diffusion defect

Answer

Right-left shunt

Answer

Ventilation-perfusion abnormality

Question 3

Which of the following statements is TRUE regarding the partial pressures of alveolar gases?

Accepted Answer

Mixed venous PCO2 is greater than alveolar PCO2.

Answer

Alveolar PCO2 is greater than mixed venous PCO2.

Answer

Alveolar PO2 is less than the PO2 of expired air.

Answer

Alveolar PCO2 is twice that of room air.

Question 4

Vital capacity is decreased, while timed vital capacity (FEV1/VC %) is normal in which of the following conditions?

Accepted Answer

Scoliosis

Answer

Bronchial asthma

Answer

Chronic bronchitis

Answer

Acute bronchitis

Question 5

CO2 is carried in the blood mainly by which form?

Accepted Answer

Bicarbonate form

Answer

Dissolved form

Answer

Carboxyhemoglobin

Answer

Carbamino-compounds

Question 6

What is the ventilation-perfusion ratio?

Accepted Answer

0.8

Answer

0.5

Answer

1

Answer

1.2

Question 7

All of the following statements about bronchial circulation are true, except?

Accepted Answer

Contributes to gaseous exchange

Answer

Contributes 2% of systemic circulation

Answer

Causes venous admixture of blood

Answer

Provides nutritive function to the lungs

Question 8

What is true about airway resistance?

Accepted Answer

It is increased in asthma.

Answer

It is increased if the lungs are removed and inflated with saline.

Answer

It does not affect the work of breathing.

Answer

It is increased in paraplegic patients.

Question 9

J-receptors, which are responsible for rapid shallow breathing, are located in which of the following structures?

Accepted Answer

Alveoli-capillary junction

Answer

Thoracic cage and lung

Answer

Carotid artery

Answer

Respiratory muscles

Question 10

If the lungs were allowed to recoil without the chest wall during expiration, what would be the lung volume?

Accepted Answer

Minimal volume

Answer

Residual volume

Answer

Functional residual capacity (FRC)

Answer

Zero

Respiratory System — MCQs

Respiratory System — MCQs

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