Respiratory System Practice Questions

Q: A premature infant born at 28 weeks gestation develops respiratory distress syndrome. Arterial blood gas shows pH 7.25, PaCO2 55 mmHg, PaO2 50 mmHg on 60% FiO2. Chest X-ray reveals ground-glass opacities. Surfactant therapy is administered. Which mechanism best explains the improvement in lung mechanics following treatment?

Decreased alveolar surface tension increasing compliance. ***Decreased alveolar surface tension increasing compliance*** - Surfactant contains **dipalmitoylphosphatidylcholine**, which breaks up the cohesive forces between water molecules at the air-fluid interface, effectively **decreasing surface tension**. - By lowering surface tension, surfactant prevents **alveolar collapse** (atelectasis) and significantly **increases lung compliance**, making it easier for the lungs to inflate. *Increased pulmonary blood flow improving V/Q matching* - While surfactant indirectly improves **ventilation-perfusion (V/Q) matching** by opening collapsed alveoli, it is not the primary mechanical effect of the therapy. - Pulmonary blood flow changes are secondary to improved **oxygenation** and decreased hypoxic pulmonary vasoconstriction rather than a direct mechanical action. *Increased elastic recoil decreasing compliance* - Increased **elastic recoil** actually makes the lungs stiffer and harder to expand, which would **decrease compliance** and worsen respiratory distress. - Surfactant therapy aims to **decrease excessive recoil** caused by surface tension forces, thereby increasing the ease of lung expansion. *Bronchodilation decreasing airway resistance* - Surfactant acts specifically within the **alveoli** to maintain stability and does not have a direct physiological effect on **bronchial smooth muscle**. - **Airway resistance** is primarily a function of the radius of the conducting airways, whereas RDS is a disease of alveolar **compliance**. *Strengthened alveolar walls increasing elastic fibers* - Surfactant is a **biochemical surface-active agent**, not a structural protein that reinforces the physical anatomy of the **alveolar walls**. - The therapy provides immediate mechanical relief via surface tension reduction and does not involve the synthesis or modification of **elastic fibers**.

Q: A 45-year-old woman presents with progressive dyspnea and dry cough over 6 months. Chest CT shows bilateral interstitial infiltrates. Pulmonary function tests reveal FEV1/FVC ratio of 0.85, reduced total lung capacity, and a steep pressure-volume curve shifted downward and to the right. What is the primary mechanical change in her lungs?

Decreased lung compliance with normal chest wall compliance. ***Decreased lung compliance with normal chest wall compliance*** - The patient presents with **Interstitial Lung Disease (ILD)**, where fibrosis results in stiff lungs that require greater pressure to inflate, signifying **decreased lung compliance**. - The **FEV1/FVC ratio >0.7** and the **downward/right shift** of the pressure-volume curve are classic indicators of a **restrictive lung pattern** with high elastic recoil. *Increased lung compliance with normal chest wall compliance* - **Increased compliance** is characteristic of **emphysema**, where alveolar wall destruction leads to a loss of elastic recoil. - In obstructive diseases like emphysema, the pressure-volume curve shifts **upward and to the left**, opposite to what is described here. *Increased lung and chest wall compliance* - This physiological state is not typically seen in clinical disease; most pathologies that affect compliance involve a **decrease** in flexibility. - **Increased chest wall compliance** only occurs in specific scenarios like a **flail chest**, which does not present with chronic dry cough or interstitial infiltrates. *Decreased lung and increased chest wall compliance* - While **decreased lung compliance** fits the ILD profile, there is no clinical evidence in this case to suggest an abnormality in the **chest wall** structure. - Diseases causing **restrictive patterns** usually affect either the lung parenchyma or the chest wall/neuromuscular system, but they do not typically increase chest wall flexibility. *Normal lung compliance with decreased chest wall compliance* - This pattern is seen in **extrapulmonary restrictive diseases** such as **obesity**, **kyphoscoliosis**, or **ankylosing spondylitis**. - The presence of **bilateral interstitial infiltrates** on CT specifically points to intrinsic lung parenchyma pathology rather than a primary chest wall issue.

Q: A 65-year-old man with COPD undergoes pulmonary function testing. His FEV1 is 65% predicted, and spirometry shows an obstructive pattern. A pressure-volume loop demonstrates reduced elastic recoil with increased total lung capacity. When comparing his lungs to a healthy individual, what physiological change best explains his altered compliance?

Destruction of elastic tissue in alveolar walls. ***Destruction of elastic tissue in alveolar walls*** - In **emphysema**, a component of **COPD**, the destruction of **elastin** by proteases increases lung **compliance**, leading to a floppy, easily distensible lung. - This loss of **elastic recoil** causes the pressure-volume loop to shift upward and to the left, resulting in **increased total lung capacity (TLC)** and air trapping. *Increased alveolar surface tension from surfactant deficiency* - **Surfactant deficiency** increases surface tension, which significantly **decreases lung compliance**, making the lungs difficult to inflate. - This is a hallmark of **Neonatal Respiratory Distress Syndrome**, not the obstructive pattern seen in COPD patients. *Increased collagen deposition in interstitial spaces* - **Collagen deposition** is characteristic of **restrictive lung diseases** like pulmonary fibrosis, which leads to **decreased compliance**. - This would result in a **reduced total lung capacity (TLC)** and a pressure-volume loop shifted downward and to the right. *Pulmonary vascular remodeling* - This refers to thickening of vessel walls and is associated with **pulmonary hypertension**, which primarily affects blood flow and pressures. - While it can occur in late-stage COPD, it does not explain the primary change in **lung compliance** or the increase in TLC. *Bronchial smooth muscle hypertrophy* - This is a feature of **chronic bronchitis** and asthma that contributes to **airway resistance** and the obstructive FEV1/FVC ratio. - While it limits airflow, it does not directly alter the **elasticity** of the lung parenchyma or the lung compliance seen on pressure-volume loops.

Q: Lungs do not collapse during expiration because of the presence of?

Dipalmitoyl phosphatidyl choline. **Explanation:** The correct answer is **Dipalmitoyl phosphatidyl choline (DPPC)**, which is the primary phospholipid component of **Pulmonary Surfactant**. **1. Why DPPC is correct:** According to the **Law of Laplace ($P = 2T/r$)**, smaller alveoli have a higher collapsing pressure ($P$) due to surface tension ($T$). During expiration, as the radius ($r$) of the alveoli decreases, the risk of collapse (atelectasis) increases. Surfactant, secreted by **Type II Pneumocytes**, reduces surface tension. DPPC molecules are amphipathic; they crowd together as the alveolus shrinks, significantly lowering surface tension and preventing the lungs from collapsing at low lung volumes. **2. Why other options are incorrect:** * **Hyaline membrane:** This is a pathological finding (not a physiological one) seen in Neonatal Respiratory Distress Syndrome (NRDS) or ARDS, where protein-rich fluid leaks into alveoli, actually *hindering* gas exchange. * **Macrophages:** Alveolar macrophages (Dust cells) are part of the immune system and are responsible for clearing debris and pathogens; they do not influence surface tension. * **Interstitial fluid:** Excessive fluid in the interstitium leads to pulmonary edema, which decreases lung compliance and impairs respiration. **Clinical Pearls for NEET-PG:** * **L/S Ratio:** A Lecithin (DPPC) to Sphingomyelin ratio of **>2:1** in amniotic fluid indicates fetal lung maturity. * **Glucocorticoids:** These are administered to mothers in preterm labor to accelerate surfactant production by stimulating Type II pneumocytes. * **Surfactant Proteins:** SP-A and SP-D are involved in innate immunity, while SP-B and SP-C are essential for the film-forming properties of surfactant.

Q: What is the partial pressure of O2, which constitutes approximately 21% of inspired air?

160 mmHg. **Explanation:** The partial pressure of a gas in a mixture is determined by **Dalton’s Law**, which states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the individual gases. To calculate the partial pressure of Oxygen ($PO_2$) in dry inspired air at sea level: 1. **Total Atmospheric Pressure ($P_{atm}$):** 760 mmHg. 2. **Fraction of Oxygen ($FiO_2$):** 21% or 0.21. 3. **Calculation:** $PO_2 = P_{atm} \times FiO_2 = 760 \times 0.21 \approx \mathbf{159.6 \text{ mmHg}}$ (rounded to **160 mmHg**). **Analysis of Incorrect Options:** * **B (240 mmHg):** This value does not correspond to any physiological gas pressure at sea level under normal atmospheric conditions. * **C (580 mmHg):** This is the approximate partial pressure of **Nitrogen** ($PN_2$) in the atmosphere ($760 \times 0.79$). * **D (760 mmHg):** This represents the **total atmospheric pressure** at sea level, not the pressure of an individual gas component. **NEET-PG High-Yield Pearls:** * **Humidification Effect:** As air enters the upper airways, it is saturated with water vapor ($PH_2O = 47 \text{ mmHg}$). The $PO_2$ in **humidified tracheal air** drops to $\approx 149 \text{ mmHg}$ $[(760 - 47) \times 0.21]$. * **Alveolar Air:** In the alveoli, $PAO_2$ is further reduced to $\approx \mathbf{100 \text{ mmHg}}$ due to the continuous diffusion of $O_2$ into the blood and the addition of $CO_2$. * **Fractional Concentration:** Note that while partial pressure changes with altitude, the *percentage* of $O_2$ (21%) remains constant.

Q: What is the most important form of carbon dioxide transport in the blood?

Bicarbonates. **Explanation:** Carbon dioxide (CO₂) is transported from the tissues to the lungs in three primary forms. Understanding the distribution of these forms is a high-yield concept for NEET-PG: 1. **Bicarbonate Ions (70%):** This is the **most important and predominant form**. CO₂ enters RBCs and reacts with water to form carbonic acid ($H_2CO_3$), catalyzed by the enzyme **Carbonic Anhydrase**. This acid dissociates into $H^+$ and $HCO_3^-$. The bicarbonate then diffuses into the plasma in exchange for chloride ions (the **Chloride Shift** or Hamburger phenomenon). 2. **Carbamino Compounds (23%):** CO₂ binds directly to the amino groups of hemoglobin (forming **carbaminohemoglobin**) and plasma proteins. 3. **Dissolved Form (7%):** A small fraction is carried physically dissolved in the plasma. **Analysis of Incorrect Options:** * **A. Carboxyhemoglobin:** This is a trap. Carboxyhemoglobin refers to **Carbon Monoxide (CO)** bound to hemoglobin, which is a toxic state, not a physiological CO₂ transport mechanism. * **B. Dissolved CO₂:** While CO₂ is 20 times more soluble than oxygen, this form only accounts for ~7% of total transport. * **D. CO₂ molecules attached to hemoglobin:** This refers to carbaminohemoglobin. While significant (23%), it is not the "most important" or majority form. **High-Yield Clinical Pearls:** * **Haldane Effect:** Deoxygenation of blood increases its ability to carry CO₂. In the lungs, when $O_2$ binds to Hb, it promotes the release of CO₂. * **Carbonic Anhydrase:** It is one of the fastest enzymes in the body; its absence would make CO₂ transport insufficient to sustain life. * **Chloride Shift:** Remember that $Cl^-$ moves **into** the RBC in systemic tissues and **out** of the RBC in pulmonary capillaries.

Q: Type 1 glomus cells secrete neurotransmitters in response to oxygen levels due to the function of which channel?

K+ channel. **Explanation:** The **Type 1 Glomus cells** (chief cells) of the carotid and aortic bodies act as peripheral chemosensors. The primary mechanism of oxygen sensing involves **Oxygen-sensitive Potassium (K+) channels**. **Mechanism of Action:** 1. **Hypoxia:** When arterial $PO_2$ falls, these oxygen-sensitive K+ channels **close**. 2. **Depolarization:** The reduction in K+ efflux leads to the accumulation of positive charge inside the cell, causing membrane depolarization. 3. **Neurotransmitter Release:** Depolarization opens voltage-gated **Calcium (Ca²⁺) channels**, leading to an influx of Calcium and subsequent exocytosis of neurotransmitters (primarily **Dopamine**, but also ACh and ATP). 4. **Afferent Signaling:** These transmitters stimulate the glossopharyngeal (CN IX) and vagus (CN X) nerves to increase the respiratory rate. **Analysis of Options:** * **D (Correct):** K+ channels are the "sensors" that initiate the electrical response to hypoxia. * **A (Incorrect):** While Na+ channels are involved in action potential propagation, they are not the primary oxygen-sensing mechanism. * **B (Incorrect):** Cl- channels do not play a significant role in the initial depolarization phase of glomus cells. * **C (Incorrect):** Ca²⁺ channels are the *effectors* that lead to neurotransmitter release, but they open only *after* the K+ channels have closed and the cell has depolarized. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Carotid bodies (bifurcation of common carotid) and Aortic bodies (arch of aorta). * **Stimuli:** Peripheral chemoreceptors respond to **decreased $PO_2$** (<60 mmHg), **increased $PCO_2$**, and **decreased pH**. * **Nerve Supply:** Carotid body via **Hering’s nerve** (branch of CN IX); Aortic body via CN X. * **Key Difference:** Central chemoreceptors (medulla) do **not** respond to hypoxia; they respond primarily to changes in $H^+$ concentration in the CSF derived from arterial $CO_2$.

Question 1

A premature infant born at 28 weeks gestation develops respiratory distress syndrome. Arterial blood gas shows pH 7.25, PaCO2 55 mmHg, PaO2 50 mmHg on 60% FiO2. Chest X-ray reveals ground-glass opacities. Surfactant therapy is administered. Which mechanism best explains the improvement in lung mechanics following treatment?

Accepted Answer

Decreased alveolar surface tension increasing compliance

Answer

Increased pulmonary blood flow improving V/Q matching

Answer

Increased elastic recoil decreasing compliance

Answer

Bronchodilation decreasing airway resistance

Answer

Strengthened alveolar walls increasing elastic fibers

Question 2

A 45-year-old woman presents with progressive dyspnea and dry cough over 6 months. Chest CT shows bilateral interstitial infiltrates. Pulmonary function tests reveal FEV1/FVC ratio of 0.85, reduced total lung capacity, and a steep pressure-volume curve shifted downward and to the right. What is the primary mechanical change in her lungs?

Accepted Answer

Decreased lung compliance with normal chest wall compliance

Answer

Increased lung compliance with normal chest wall compliance

Answer

Increased lung and chest wall compliance

Answer

Decreased lung and increased chest wall compliance

Answer

Normal lung compliance with decreased chest wall compliance

Question 3

A 65-year-old man with COPD undergoes pulmonary function testing. His FEV1 is 65% predicted, and spirometry shows an obstructive pattern. A pressure-volume loop demonstrates reduced elastic recoil with increased total lung capacity. When comparing his lungs to a healthy individual, what physiological change best explains his altered compliance?

Accepted Answer

Destruction of elastic tissue in alveolar walls

Answer

Increased alveolar surface tension from surfactant deficiency

Answer

Increased collagen deposition in interstitial spaces

Answer

Pulmonary vascular remodeling

Answer

Bronchial smooth muscle hypertrophy

Question 4

Lungs do not collapse during expiration because of the presence of?

Accepted Answer

Dipalmitoyl phosphatidyl choline

Answer

Hyaline membrane

Answer

Macrophages

Answer

Interstitial fluid

Question 5

Given an arterial blood oxygen content of 20 mL per 100 mL of blood and a venous blood oxygen content of 15 mL per 100 mL of blood, calculate the amount of oxygen transferred from the blood to the tissues if the blood flow is 200 mL/min?

Accepted Answer

10 mL/min

Answer

5 mL/min

Answer

15 mL/min

Answer

20 mL/min

Question 6

What is the partial pressure of O2, which constitutes approximately 21% of inspired air?

Accepted Answer

160 mmHg

Answer

240 mmHg

Answer

580 mmHg

Answer

760 mmHg

Question 7

What is the most important form of carbon dioxide transport in the blood?

Accepted Answer

Bicarbonates

Answer

Carboxyhemoglobin

Answer

Dissolved CO2

Answer

CO2 molecules attached to hemoglobin

Question 8

100% oxygen is not effective in which of the following conditions?

Accepted Answer

Tetralogy of Fallot

Answer

Pulmonary edema

Answer

Myocardial infarction

Answer

COPD

Question 9

Which of the following statements is true concerning normal expiration during resting conditions?

Accepted Answer

The flow velocity of gas in the large airways exceeds that in the terminal bronchioles.

Answer

Expiration is generated by the contraction of expiratory muscles.

Answer

Alveolar pressure is less than atmospheric pressure.

Answer

Intrapleural pressure gradually falls (becomes more negative) during expiration.

Question 10

Type 1 glomus cells secrete neurotransmitters in response to oxygen levels due to the function of which channel?

Accepted Answer

K+ channel

Answer

Na+ channel

Answer

Cl- channel

Answer

Ca+2 channel

Respiratory System — MCQs

Respiratory System — MCQs

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