Respiratory System Practice Questions

Q: Which of the following is the main component of surfactant, secreted by alveolar epithelial cells?

Phospholipids. **Explanation:** Pulmonary surfactant is a surface-active lipoprotein complex secreted by **Type II alveolar epithelial cells (Pneumocytes)**. Its primary function is to reduce surface tension at the air-liquid interface, preventing alveolar collapse (atelectasis) at the end of expiration and increasing lung compliance. **1. Why Phospholipids are the correct answer:** Chemically, surfactant is composed of approximately **90% lipids** and **10% proteins**. Of the lipid component, about 80-90% are **phospholipids**. The most abundant and physiologically significant phospholipid is **Dipalmitoylphosphatidylcholine (DPPC)**, also known as **Lecithin**. It is specifically responsible for reducing surface tension. **2. Why other options are incorrect:** * **Proteins (A):** While surfactant contains specific proteins (SP-A, SP-B, SP-C, and SP-D), they constitute only about 10% of the total mass. They are crucial for immunity and the structural organization of the surfactant film but are not the "main" component. * **Lipoproteins (C):** While surfactant is technically a lipoprotein complex, the question asks for the *main component*. In biochemical terms, the lipid fraction (specifically phospholipids) vastly outweighs the protein fraction. **High-Yield Clinical Pearls for NEET-PG:** * **L/S Ratio:** The Lecithin/Sphingomyelin ratio in amniotic fluid is used to assess fetal lung maturity. A ratio **>2:1** indicates mature lungs. * **NRDS:** Deficiency of surfactant in premature infants leads to **Neonatal Respiratory Distress Syndrome (Hyaline Membrane Disease)**. * **Storage:** Surfactant is stored in intracellular organelles of Type II pneumocytes called **Lamellar bodies**. * **Law of Laplace:** Surfactant counteracts the Law of Laplace ($P = 2T/r$), ensuring that smaller alveoli do not empty into larger ones by reducing surface tension ($T$).

Q: Normal intrapleural pressure is negative because:

The chest wall and lungs recoil in opposite directions.. The intrapleural pressure (IPP) is the pressure within the pleural cavity. Under normal physiological conditions, it is always **negative** (subatmospheric) relative to the intrapulmonary pressure. ### 1. Why Option A is Correct The negativity of the intrapleural pressure is primarily due to the **opposing elastic recoil forces** of the lungs and the chest wall: * **Lungs:** Have a natural tendency to collapse inward due to elastic fibers and surface tension. * **Chest Wall:** Has a natural tendency to spring outward. As these two structures pull away from each other, they create a "vacuum" effect in the thin, fluid-filled pleural space, resulting in a negative pressure (approx. -5 cm H₂O at FRC). ### 2. Why Other Options are Incorrect * **Option B:** Surfactant *reduces* surface tension to prevent alveolar collapse, but it does not create the negative IPP. In fact, by reducing inward recoil, it technically makes IPP *less* negative than it would be without surfactant. * **Option C:** Intrapulmonary pressure (alveolar pressure) fluctuates between negative (during inspiration) and positive (during expiration). It is not permanently negative. * **Option D:** Transpulmonary pressure ($P_{tp} = P_{alv} - P_{ip}$) is always **positive** in a healthy lung. A positive transpulmonary pressure is what keeps the lungs inflated. ### 3. NEET-PG High-Yield Pearls * **At FRC:** Intrapleural pressure is **-5 cm H₂O**. * **During Inspiration:** It becomes more negative (approx. **-7.5 cm H₂O**). * **Pneumothorax:** If the pleural seal is broken, air enters the space, IPP becomes zero (atmospheric), and the lung collapses due to its unopposed inward recoil. * **Mueller’s Maneuver:** Forced inspiration against a closed glottis can make IPP as low as **-40 to -80 cm H₂O**.

Q: Surfactant production in the lungs starts at:

18 weeks. **Explanation:** **Correct Answer: A. 18 weeks** The production of pulmonary surfactant is a critical milestone in fetal lung development. Surfactant is synthesized and secreted by **Type II Pneumocytes**. * **Initial Production:** Biochemical synthesis and the appearance of surfactant in the lung tissue begin as early as **18 to 20 weeks** of gestation (during the canalicular stage). * **Clinical Significance:** While production starts early, the amount produced is insufficient to maintain alveolar stability until much later in pregnancy. **Analysis of Incorrect Options:** * **B. 24 weeks:** By this stage, surfactant can be detected in the amniotic fluid, and the terminal sacs (primitive alveoli) have begun to form. However, this is not the *start* of production. * **C. 28 weeks:** This is a critical threshold where surfactant levels usually become sufficient to allow a premature infant to breathe with minimal assistance, but it marks a functional milestone rather than the onset. * **D. 32 weeks:** Surfactant production increases significantly after 32 weeks, reaching peak maturity around **34–35 weeks**, at which point the risk of Respiratory Distress Syndrome (RDS) decreases significantly. **High-Yield NEET-PG Pearls:** 1. **Composition:** Surfactant is 90% lipids and 10% proteins. The most important phospholipid component is **Dipalmitoylphosphatidylcholine (DPPC)** or Lecithin. 2. **L/S Ratio:** A Lecithin-to-Sphingomyelin ratio of **>2:1** in amniotic fluid indicates fetal lung maturity. 3. **Glucocorticoids:** Corticosteroids (e.g., Betamethasone) are administered in preterm labor to accelerate surfactant production by stimulating Type II pneumocytes. 4. **Law of Laplace:** Surfactant works by reducing surface tension, preventing small alveoli from collapsing into larger ones ($P = 2T/r$).

Q: Which of the following is the best determinant of adequacy of alveolar ventilation?

Arterial pCO2 level. **Explanation:** The adequacy of alveolar ventilation is defined by the lung's ability to remove carbon dioxide ($CO_2$) produced by tissue metabolism. **Why Arterial $pCO_2$ ($PaCO_2$) is the correct answer:** The relationship between alveolar ventilation ($\dot{V}_A$) and $PaCO_2$ is inversely proportional, expressed by the formula: **$\dot{V}_A \propto \frac{\dot{V}CO_2}{PaCO_2}$** (where $\dot{V}CO_2$ is $CO_2$ production). Because $CO_2$ is highly diffusible (20 times more than $O_2$), its arterial level is almost entirely dependent on the rate of alveolar ventilation. If $PaCO_2$ is high (>45 mmHg), the patient is hypoventilating; if it is low (<35 mmHg), they are hyperventilating. **Why other options are incorrect:** * **Arterial $pO_2$:** This is a poor indicator because $pO_2$ can be affected by many factors other than ventilation, such as V/Q mismatch, shunts, or diffusion defects. A patient can have normal $pO_2$ (on supplemental oxygen) while still being in respiratory failure due to hypoventilation. * **Minute Ventilation:** This measures the total air entering the lungs per minute ($V_T \times RR$). It includes **dead space ventilation**, which does not participate in gas exchange. Therefore, a high minute ventilation does not guarantee adequate alveolar gas exchange. * **Cyanosis:** This is a late clinical sign that occurs only when deoxygenated hemoglobin exceeds 5 g/dL. It is unreliable and insensitive for assessing ventilation. **High-Yield Clinical Pearls for NEET-PG:** * **Dead Space:** The portion of the breath that does not reach the alveoli. $\dot{V}_A = (\text{Tidal Volume} - \text{Dead Space}) \times \text{Respiratory Rate}$. * **Hypoventilation** always leads to an increase in $PaCO_2$ and a decrease in $PaO_2$. * **Hypercapnia** (increased $PaCO_2$) is the definitive hallmark of alveolar hypoventilation.

Q: Type-2 pulmonary epithelial cells secrete?

Surfactant. **Explanation:** **Correct Answer: A. Surfactant** Type-2 pneumocytes (granular pneumocytes) are cuboidal cells that cover approximately 5% of the alveolar surface but are more numerous than Type-1 cells. Their primary function is the synthesis, storage, and secretion of **pulmonary surfactant**. Surfactant is a phospholipid-rich mixture (primarily Dipalmitoylphosphatidylcholine - DPPC) stored in intracellular organelles called **lamellar bodies**. It reduces surface tension at the air-liquid interface, preventing alveolar collapse (atelectasis) at the end of expiration and increasing lung compliance. **Analysis of Incorrect Options:** * **B. Mucus:** Secreted by **Goblet cells** and submucosal glands located in the conducting airways (trachea and bronchi), not in the alveoli. * **C. Heparin:** Produced and stored by **Mast cells** and basophils; it acts as an anticoagulant. * **D. Polypeptides:** While Type-2 cells do produce surfactant proteins (SP-A, B, C, D), "polypeptides" is a non-specific term. In the context of the respiratory system, specific polypeptides like VIP or Substance P are secreted by neuroendocrine cells (Kulchitsky cells). **High-Yield Clinical Pearls for NEET-PG:** * **Regeneration:** Type-2 cells act as **stem cells** for the alveoli; they proliferate and differentiate into Type-1 cells following lung injury. * **Development:** Surfactant production begins around 24–28 weeks of gestation, but significant amounts are only present after **35 weeks**. * **Clinical Correlation:** Deficiency of surfactant in premature neonates leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease. * **Lecithin/Sphingomyelin (L/S) Ratio:** An L/S ratio >2 in amniotic fluid indicates fetal lung maturity.

Question 1

Which of the following is the main component of surfactant, secreted by alveolar epithelial cells?

Accepted Answer

Phospholipids

Answer

Proteins

Answer

Lipoproteins

Answer

None of the above

Question 2

Transection at the mid-pons level results in which of the following?

Accepted Answer

Apneusis

Answer

Asphyxia

Answer

Hyperventilation

Answer

Rapid and shallow breathing

Question 3

Fatal hemoglobin has all the following characteristic features except?

Accepted Answer

Strong affinity for 2,3-DPG

Answer

Oxygen dissociation curve is shifted to the left

Answer

At low fetal PO2, gives up more oxygen to tissues than adult hemoglobin

Answer

Forms 80% of hemoglobin at birth

Question 4

Normal intrapleural pressure is negative because:

Accepted Answer

The chest wall and lungs recoil in opposite directions.

Answer

Surfactant prevents pulmonary collapse.

Answer

Intrapulmonary pressure is negative.

Answer

Transpulmonary pressure is negative.

Question 5

Surfactant production in the lungs starts at:

Accepted Answer

18 weeks

Answer

24 weeks

Answer

28 weeks

Answer

32 weeks

Question 6

Which of the following is the best determinant of adequacy of alveolar ventilation?

Accepted Answer

Arterial pCO2 level

Answer

Arterial pO2 level

Answer

Minute ventilation

Answer

Presence of cyanosis

Question 7

During starvation, what is the approximate non-protein respiratory quotient (RQ)?

Accepted Answer

1.0

Answer

0.5

Answer

0.25

Answer

0.75

Question 8

A patient has a creatinine clearance of 90 ml/min, a urine flow rate of 1 ml/min, a plasma K+ concentration of 4 mEq/L, and a urine K+ concentration of 60 mEq/L. What is the approximate rate of K+ excretion?

Accepted Answer

0.06 mEq/min

Answer

0.30 mEq/min

Answer

0.36 mEq/min

Answer

3.6 mEq/min

Question 9

Hemoglobin saturation with oxygen is mostly dependent on which of the following parameters?

Accepted Answer

Partial pressure of oxygen (pO2)

Answer

Partial pressure of carbon dioxide (pCO2)

Answer

Bicarbonate (HCO3-) levels

Answer

Hemoglobin percentage

Question 10

Type-2 pulmonary epithelial cells secrete?

Accepted Answer

Surfactant

Answer

Mucus

Answer

Heparin

Answer

Polypeptides

Respiratory System — MCQs

Respiratory System — MCQs

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