Respiratory System Practice Questions

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: What is the O2 content of saturated arterial blood?

19.4 ml/100 ml. **Explanation:** The oxygen content of arterial blood is determined by the sum of oxygen bound to hemoglobin and oxygen dissolved in plasma. To calculate this, we use the following formula: **Total $O_2$ Content = ($1.34 \times Hb \times SaO_2$) + ($0.003 \times PaO_2$)** 1. **Hemoglobin-bound $O_2$:** In a healthy adult, the average Hemoglobin (Hb) is **15 g/dL**. Each gram of Hb can carry **1.34 ml** of $O_2$ (Hüfner's constant). For saturated blood ($SaO_2$ = 100%), this equals $15 \times 1.34 = \mathbf{20.1\text{ ml/dL}}$. However, in vivo, physiological saturation is typically **97%**, yielding approximately **19.1 ml/dL**. 2. **Dissolved $O_2$:** At a normal arterial $PaO_2$ of 100 mmHg, the dissolved $O_2$ is $100 \times 0.003 = \mathbf{0.3\text{ ml/dL}}$. 3. **Total:** Adding these gives approximately **19.4 ml/100 ml** (or 19.4 vol%). **Analysis of Options:** * **A (11.2 ml) & B (14.5 ml):** These values are too low for normal arterial blood and may be seen in cases of severe anemia or significant hypoxia. * **D (21.3 ml):** This value is higher than the average physiological carrying capacity unless the individual has polycythemia (elevated Hb). **High-Yield Clinical Pearls for NEET-PG:** * **Venous $O_2$ Content:** Approximately **14.4–15 ml/dL**. * **Arteriovenous (A-V) $O_2$ Difference:** Normally **5 ml/dL**; this represents the amount of $O_2$ delivered to tissues at rest. * **Utilization Coefficient:** The fraction of $O_2$ given up to tissues, normally **25%** (5 ml out of 20 ml). This increases significantly during heavy exercise. * **Key Constant:** Remember $1.34$ ml/g for Hb; some texts use $1.39$ ml/g (theoretical maximum), but $1.34$ is the standard physiological value.

Q: A patient with carbon monoxide poisoning is treated with hyperbaric oxygen that increases the arterial PO2 to 2000 mm Hg. The amount of oxygen dissolved in the arterial blood (in ml/100 ml) is:

6. **Explanation:** The amount of oxygen dissolved in the blood is directly proportional to the partial pressure of oxygen ($PO_2$), as per **Henry’s Law**. 1. **The Calculation:** In human physiology, the solubility coefficient of oxygen in plasma is **0.003 ml/100 ml/mm Hg**. To find the dissolved oxygen content: * $\text{Dissolved } O_2 = \text{Solubility Coefficient} \times PO_2$ * $\text{Dissolved } O_2 = 0.003 \times 2000 \text{ mm Hg}$ * $\text{Dissolved } O_2 = \mathbf{6 \text{ ml/100 ml}}$ (or 6 vol%) 2. **Why other options are incorrect:** * **Option A (2) & B (3):** These values are too low for a $PO_2$ of 2000 mm Hg. At normal room air ($PO_2 \approx 100$ mm Hg), dissolved oxygen is only **0.3 ml/100 ml**. * **Option C (4):** This would correspond to a $PO_2$ of approximately 1333 mm Hg, which does not match the clinical scenario provided. **Clinical Pearls for NEET-PG:** * **Total Oxygen Content:** Sum of Hemoglobin-bound $O_2$ and Dissolved $O_2$. In CO poisoning, Hb-bound $O_2$ is severely reduced, making dissolved $O_2$ critical for survival. * **Hyperbaric Oxygen (HBO):** By increasing dissolved $O_2$ to 6 vol%, HBO can meet the body’s entire resting oxygen demand (usually ~5 ml/100 ml) even if hemoglobin is completely unavailable. * **CO Poisoning:** HBO therapy reduces the half-life of Carboxyhemoglobin (COHb) from ~4-5 hours (room air) to ~20 minutes. * **Solubility Fact:** $CO_2$ is **20-24 times** more soluble in plasma than $O_2$.

Q: Pulmonary vascular resistance is reduced by:

Acutely increasing pulmonary venous pressure. **Explanation:** The correct answer is **D. Acutely increasing pulmonary venous pressure.** **Mechanism:** Pulmonary Vascular Resistance (PVR) is unique because it decreases as pulmonary arterial or venous pressure increases. This occurs through two primary mechanisms: 1. **Recruitment:** Previously closed capillaries (especially in the lung apices) open up to accommodate increased pressure. 2. **Distension:** The thin-walled, highly compliant pulmonary capillaries widen their diameter. Both mechanisms increase the total cross-sectional area of the pulmonary bed, thereby significantly lowering resistance. **Analysis of Incorrect Options:** * **A. Removal of one lung:** This reduces the total number of parallel vascular pathways. Since resistance in parallel circuits is inversely proportional to the number of pathways, removing a lung **increases** PVR. * **B. Breathing a 10% oxygen mixture:** Hypoxia triggers **Hypoxic Pulmonary Vasoconstriction (HPV)**. This is a compensatory mechanism to shunting blood away from poorly ventilated areas, but it leads to an **increase** in PVR. * **C. Exhaling from FRC to RV:** PVR follows a **U-shaped curve** in relation to lung volume. At low lung volumes (Residual Volume), the **extra-alveolar vessels** collapse or are compressed, leading to an **increase** in PVR. (Note: PVR is lowest at Functional Residual Capacity). **High-Yield Pearls for NEET-PG:** * **Lung Volume & PVR:** PVR is lowest at **FRC**. It increases at high volumes (due to stretching/thinning of alveolar capillaries) and at low volumes (due to compression of extra-alveolar vessels). * **Passive Factors:** Increased Cardiac Output (CO) decreases PVR via recruitment and distension. * **Active Factors:** Potent vasoconstrictors (increasing PVR) include Hypoxia, Hypercapnia, and Acidosis. Nitric Oxide (NO) and Prostacyclin are potent vasodilators (decreasing PVR).

Q: Which of the following is found in the respiratory zone of the lung?

Type I epithelial cells. ### Explanation The respiratory system is divided into two functional zones: the **Conducting Zone** (nose to terminal bronchioles) and the **Respiratory Zone** (respiratory bronchioles to alveoli). **Why Option D is Correct:** The respiratory zone is the site where actual gas exchange occurs. **Type I epithelial cells** (Type I pneumocytes) are thin, squamous cells that cover approximately 95% of the alveolar surface area. Their primary function is to form the blood-air barrier, allowing for the rapid diffusion of gases. Because they are located within the alveoli, they are a hallmark component of the respiratory zone. **Why the Other Options are Incorrect:** * **A & C (Goblet cells and Mucous cells):** These are specialized secretory cells that produce mucus to trap inhaled particles. They are found throughout the **conducting zone** (trachea, bronchi, and larger bronchioles) but disappear by the level of the terminal bronchioles. They are absent in the respiratory zone to prevent mucus from obstructing gas exchange. * **B (Main bronchi):** These are large airway passages that belong to the **conducting zone**. Their primary role is to warm, humidify, and filter air as it travels toward the lungs; they do not participate in gas exchange. **High-Yield NEET-PG Pearls:** * **Transition Point:** The **terminal bronchiole** is the last part of the conducting zone, while the **respiratory bronchiole** is the first part of the respiratory zone. * **Type II Pneumocytes:** These are cuboidal cells that secrete **surfactant** and act as stem cells (progenitors) for Type I cells. * **Anatomical Dead Space:** The volume of the conducting zone (~150 mL) where no gas exchange occurs. * **Cilia:** These persist longer than goblet cells along the tracheobronchial tree to ensure that mucus is cleared upward (the "mucociliary escalator") and does not reach the alveoli.

Q: Residual volume is the volume of air remaining in the lungs after which of the following actions?

Maximal expiration. **Explanation:** **Residual Volume (RV)** is defined as the volume of air remaining in the lungs after a **maximal (forceful) expiration**. It represents the air that cannot be expelled from the lungs, ensuring that the alveoli remain patent and gas exchange continues even between breaths. * **Why Option B is Correct:** Even after the most strenuous expiratory effort, the lungs never completely collapse. The air trapped in the non-collapsible airways and alveoli constitutes the RV (approximately 1200 mL in a healthy adult male). * **Why Option A is Incorrect:** The volume of air in the lungs after maximal inspiration is the **Total Lung Capacity (TLC)**. * **Why Option C is Incorrect:** The volume of air in the lungs after a normal (tidal) inspiration is the sum of FRC and Tidal Volume. * **Why Option D is Incorrect:** The volume of air remaining in the lungs after a normal, quiet expiration is the **Functional Residual Capacity (FRC)**. **High-Yield Clinical Pearls for NEET-PG:** 1. **Measurement:** RV **cannot** be measured by simple spirometry because it cannot be exhaled. It is measured using Helium Dilution, Nitrogen Washout, or Body Plethysmography. 2. **Clinical Significance:** RV is significantly **increased** in obstructive lung diseases (e.g., Emphysema, Asthma) due to air trapping and hyperinflation. 3. **Formula:** $TLC = VC + RV$ or $FRC = ERV + RV$. 4. **Age Factor:** RV increases with age due to the loss of elastic recoil of the lung tissue.

Question 1

What is the approximate normal amount of pleural fluid?

Accepted Answer

15 ml

Answer

5 ml

Answer

50 ml

Answer

100 ml

Question 2

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 3

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 4

All of the following affect resting ventilation except?

Accepted Answer

J receptors

Answer

Stretch receptors

Answer

Oxygen

Answer

PCO2

Question 5

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%

Question 6

What is the O2 content of saturated arterial blood?

Accepted Answer

19.4 ml/100 ml

Answer

11.2 ml/100 ml

Answer

14.5 ml/100 ml

Answer

21.3 ml/100 ml

Question 7

A patient with carbon monoxide poisoning is treated with hyperbaric oxygen that increases the arterial PO2 to 2000 mm Hg. The amount of oxygen dissolved in the arterial blood (in ml/100 ml) is:

Accepted Answer

6

Answer

2

Answer

3

Answer

4

Question 8

Pulmonary vascular resistance is reduced by:

Accepted Answer

Acutely increasing pulmonary venous pressure

Answer

Removal of one lung

Answer

Breathing a 10% oxygen mixture

Answer

Exhaling from functional residual capacity to residual volume

Question 9

Which of the following is found in the respiratory zone of the lung?

Accepted Answer

Type I epithelial cells

Answer

Goblet cells

Answer

Main bronchi

Answer

Mucous cells

Question 10

Residual volume is the volume of air remaining in the lungs after which of the following actions?

Accepted Answer

Maximal expiration

Answer

Maximal inspiration

Answer

Normal inspiration

Answer

Normal expiration

Respiratory System — MCQs

Respiratory System — MCQs

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