Respiratory System Practice Questions

Q: What is the normal value of arterial blood oxygen in mL of O2 per dL?

19.8. The oxygen content of arterial blood ($CaO_2$) is the sum of oxygen bound to hemoglobin and oxygen dissolved in plasma. This is a high-yield calculation for NEET-PG. ### **The Calculation** 1. **Bound to Hemoglobin:** 1 gram of pure $Hb$ can carry **1.34 mL** of $O_2$. Assuming a normal $Hb$ of **15 g/dL** and an arterial saturation ($SaO_2$) of **98%**: * $15 \times 1.34 \times 0.98 = \mathbf{19.7 \text{ mL/dL}}$ 2. **Dissolved in Plasma:** Oxygen is poorly soluble. At a normal $PaO_2$ of 100 mmHg, only **0.3 mL/dL** is dissolved ($0.003 \times 100$). 3. **Total Content:** $19.7 + 0.3 = \mathbf{20 \text{ mL/dL}}$ (approximately). Option B (**19.8**) is the most accurate value representing this physiological total. ### **Analysis of Incorrect Options** * **A (12.1):** This value is too low for arterial blood; it may be seen in severe anemia or significant hypoxia. * **C (15.6):** This represents the typical oxygen content of **mixed venous blood** ($CvO_2$), where saturation is approximately 75%. * **D (27.8):** This value is physiologically impossible under normal atmospheric conditions and standard hemoglobin levels. ### **NEET-PG High-Yield Pearls** * **Utilization Coefficient:** The fraction of oxygen given up to tissues (normally ~25%). * **Arterio-venous $O_2$ difference:** Normally **5 mL/dL** ($20 - 15$). This increases during exercise as tissues extract more oxygen. * **Hüfner's Constant:** While 1.34 is used clinically, the theoretical maximum is **1.39 mL/g**. * **Dissolved $O_2$:** Though small (0.3 mL), this is the only form of oxygen that exerts partial pressure ($PaO_2$) and determines hemoglobin saturation.

Q: Maximum voluntary ventilation is measured for what duration?

60 seconds. ### Explanation **Maximum Voluntary Ventilation (MVV)** is defined as the maximum volume of air that can be inhaled and exhaled from the lungs per minute by maximum conscious effort. **1. Why 60 seconds is the correct answer:** By definition, MVV is a measure of the **ventilatory capacity per minute**. Therefore, the standard unit of measurement is liters per minute (L/min). While the actual clinical test is performed for a shorter duration (usually 12–15 seconds) to prevent fatigue and hypocapnia, the value is mathematically extrapolated to **60 seconds** to represent the total breathing capacity over a full minute. **2. Why other options are incorrect:** * **30 seconds:** This duration is too short for a standard physiological "per minute" definition and is not used as the reference timeframe for MVV. * **75 and 120 seconds:** These durations are excessively long. Performing maximal respiratory effort for this long would lead to severe respiratory muscle fatigue and significant respiratory alkalosis due to the "washout" of CO₂, potentially causing syncope or tetany. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Normal Values:** In a healthy young adult male, the average MVV is approximately **140–180 L/min**. * **Clinical Significance:** MVV assesses the entire respiratory pump, including airway resistance, lung compliance, and the integrity of respiratory muscles. * **The 15-second Rule:** In clinical practice, the patient breathes as deeply and rapidly as possible for only **12 to 15 seconds**. This result is then multiplied by 4 (or 5) to calculate the 60-second MVV. * **Relationship with FEV1:** MVV can be indirectly estimated using the formula: **MVV = FEV1 × 35 (or 40)**. This is a common numerical MCQ in physiology.

Q: What is asthma?

Obstructive lung disease. **Explanation:** **1. Why Option A is Correct:** Asthma is a chronic inflammatory disorder of the airways characterized by **reversible airway obstruction**. The underlying pathophysiology involves bronchial hyperresponsiveness and inflammation, leading to bronchoconstriction, mucosal edema, and excessive mucus production. In obstructive lung diseases, the primary physiological defect is **increased resistance to airflow**, particularly during expiration. This is reflected in pulmonary function tests (PFTs) by a **decreased FEV1/FVC ratio (<0.7)**. **2. Why Other Options are Incorrect:** * **Option B (Restrictive Lung Disease):** These conditions (e.g., Pulmonary Fibrosis, Sarcoidosis) are characterized by reduced lung compliance and "stiff" lungs, leading to decreased lung volumes (TLC). In restriction, the FEV1/FVC ratio is typically normal or increased, unlike in asthma. * **Option C & D:** These are incorrect as asthma fits the specific physiological profile of obstruction. **3. NEET-PG High-Yield Clinical Pearls:** * **Hallmark PFT Finding:** Reversibility is key. An increase in FEV1 of **>12% and >200 ml** after inhaling a bronchodilator (e.g., Salbutamol) confirms the diagnosis. * **Airway Remodeling:** Chronic untreated inflammation can lead to permanent structural changes (subepithelial fibrosis), which may eventually cause irreversible obstruction. * **Pathology:** Look for **Curschmann spirals** (mucus plugs) and **Charcot-Leyden crystals** (eosinophil breakdown products) in sputum samples. * **Gold Standard for Diagnosis:** Spirometry is the preferred method to demonstrate variable expiratory airflow limitation.

Q: What is the approximate normal amount of pleural fluid?

15 ml. **Explanation:** The pleural space is a potential space between the visceral and parietal pleura. In a healthy adult, it contains a small amount of serous fluid that acts as a lubricant, reducing friction between the lungs and the chest wall during respiration. **Why Option B is Correct:** The normal volume of pleural fluid is approximately **0.1 to 0.2 ml/kg body weight**, which translates to roughly **7 to 15 ml** in a 70 kg adult. While some textbooks provide a range (e.g., 10–20 ml), **15 ml** is the most widely accepted "average" value in standard medical physiology (such as Guyton and Ganong). This fluid is constantly produced by the parietal pleura and drained by the lymphatic system. **Why Other Options are Incorrect:** * **Option A (5 ml):** This is slightly below the physiological average for a standard adult, though it may be seen in very small individuals. * **Option C (50 ml) & Option D (100 ml):** These volumes are pathologically high. Any accumulation of fluid in the pleural space exceeding 20–25 ml is generally considered a **pleural effusion**, which can be detected on imaging (e.g., blunting of the costophrenic angle on a chest X-ray usually requires at least 150–200 ml of fluid). **High-Yield Clinical Pearls for NEET-PG:** * **Protein Content:** Pleural fluid is a transudate with low protein content (< 1.5 g/dL). * **Drainage:** The primary route for pleural fluid clearance is through the **lymphatic stomata** in the parietal pleura. * **Pressure:** The intrapleural pressure is normally **negative** (approx. -5 cm H₂O) due to the opposing elastic recoil of the lungs and the chest wall. * **Light’s Criteria:** Used to differentiate between transudative and exudative pleural effusions (High yield for Medicine/Surgery).

Q: Normal pulmonary capillary wedge pressure (PCWP) with pulmonary edema is seen in which of the following conditions?

High altitude. **Explanation:** The **Pulmonary Capillary Wedge Pressure (PCWP)** is a clinical surrogate for left atrial pressure and left ventricular end-diastolic pressure. Pulmonary edema occurs when the hydrostatic pressure in the pulmonary capillaries exceeds the oncotic pressure, causing fluid to leak into the interstitium and alveoli. **Why High Altitude is correct:** At high altitudes, the low partial pressure of oxygen ($FiO_2$) triggers **Hypoxic Pulmonary Vasoconstriction (HPV)**. This constriction occurs at the level of the **pulmonary arterioles** (pre-capillary). Because the obstruction/constriction is *proximal* to the capillaries, the pressure measured downstream (PCWP) remains **normal** (usually <12 mmHg). However, the uneven nature of this vasoconstriction leads to high pressure in non-constricted vessels, causing "stress failure" of the capillary membrane and resulting in **High-Altitude Pulmonary Edema (HAPE)**. **Analysis of Incorrect Options:** * **Left Atrial Myxoma:** This creates a mechanical obstruction at the mitral valve. This increases left atrial pressure, which is transmitted backwards, leading to an **elevated PCWP**. * **Pulmonary Vein Obstruction:** Since PCWP measures the pressure distal to the pulmonary capillaries (reflecting the veins and left atrium), any obstruction in the pulmonary veins will result in an **elevated PCWP**. * **Pulmonary Artery Obstruction (e.g., PE):** While this also features a normal PCWP, it typically causes decreased blood flow to the lungs and does **not** cause pulmonary edema. **Clinical Pearls for NEET-PG:** * **Normal PCWP:** 6–12 mmHg. * **Cardiogenic Edema:** PCWP >18 mmHg (e.g., CHF, Mitral Stenosis). * **Non-Cardiogenic Edema (Normal PCWP):** Includes HAPE, ARDS (due to increased permeability), and Neurogenic pulmonary edema. * **HAPE Treatment:** Rapid descent, supplemental oxygen, and **Nifedipine** (to reduce pulmonary artery pressure).

Question 1

What is the normal value of arterial blood oxygen in mL of O2 per dL?

Accepted Answer

19.8

Answer

12.1

Answer

15.6

Answer

27.8

Question 2

A 49-year-old man has a pulmonary embolism that completely blocks blood flow to his left lung. As a result, which of the following will occur?

Accepted Answer

Alveolar PO2 in the left lung will be approximately equal to the PO2 in inspired air

Answer

Ventilation/perfusion (V/Q) ratio in the left lung will be zero

Answer

Systemic arterial PO2 will be elevated

Answer

V/Q ratio in the left lung will be lower than in the right lung

Question 3

Maximum voluntary ventilation is measured for what duration?

Accepted Answer

60 seconds

Answer

30 seconds

Answer

75 seconds

Answer

120 seconds

Question 4

What is asthma?

Accepted Answer

Obstructive lung disease

Answer

Restrictive lung disease

Answer

Both

Answer

None

Question 5

What is the approximate normal amount of pleural fluid?

Accepted Answer

15 ml

Answer

5 ml

Answer

50 ml

Answer

100 ml

Question 6

Normal pulmonary capillary wedge pressure (PCWP) with pulmonary edema is seen in which of the following conditions?

Accepted Answer

High altitude

Answer

Left atrial myxoma

Answer

Pulmonary vein obstruction

Answer

Pulmonary artery obstruction

Question 7

In a healthy subject, under what condition is the partial pressure of oxygen (PO2) in the blood leaving a pulmonary capillary lower than the PO2 in the alveolus served by that capillary?

Accepted Answer

None of the above

Answer

When breathing a low oxygen gas mixture

Answer

When performing an exercise that doubles cardiac output

Answer

At the base of the lungs, where perfusion exceeds ventilation

Question 8

Given the Haldane effect, carbon dioxide uptake is 2 ml/100 ml of blood in arteries. What will be the carbon dioxide uptake in veins in the absence of the Haldane effect?

Accepted Answer

4 ml/100 ml of blood

Answer

2 ml/100 ml of blood

Answer

6 ml/100 ml of blood

Answer

8 ml/100 ml of blood

Question 9

All of the following affect resting ventilation except?

Accepted Answer

J receptors

Answer

Stretch receptors

Answer

Oxygen

Answer

PCO2

Question 10

What is the cut-off limit for deoxygenated hemoglobin for the appearance of cyanosis?

Accepted Answer

>5 gm%

Answer

>4 gm%

Answer

< 2 gm%

Answer

<4 gm%

Respiratory System — MCQs

Respiratory System — MCQs

On this page

Practice by Chapter

Want unlimited practice?