Respiratory System Practice Questions

Q: According to Poiseuille's law, reducing the radius of an airway to one-third will increase its resistance how many fold?

81. ### Explanation **1. The Underlying Concept: Poiseuille’s Law** In respiratory physiology, the resistance to airflow ($R$) in the airways is governed by **Poiseuille’s Law**. The formula states that resistance is inversely proportional to the fourth power of the radius ($r$): $$R \propto \frac{1}{r^4}$$ When the radius of an airway is reduced to **one-third ($1/3$)** of its original size, the new resistance ($R_{new}$) is calculated as: $$R_{new} \propto \frac{1}{(1/3)^4} = \frac{1}{1/81} = 81$$ Thus, the resistance increases by **81-fold**. This demonstrates that even minor changes in airway caliber (due to bronchoconstriction or edema) lead to massive increases in the work of breathing. **2. Analysis of Incorrect Options** * **Option A & B (3):** This assumes a linear relationship ($R \propto 1/r$), which is incorrect for fluid/air flow dynamics. * **Option C (9):** This assumes resistance is inversely proportional to the square of the radius ($R \propto 1/r^2$), which describes the relationship with cross-sectional area, not resistance. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Site of Maximum Resistance:** Although individual small airways have high resistance, the **medium-sized bronchi (generations 2-5)** are the site of *maximum* total airway resistance. * **Small Airway Paradox:** The terminal bronchioles (small airways) contribute very little to total resistance because they are arranged in **parallel**, greatly increasing the total cross-sectional area. * **Autonomic Control:** Parasympathetic stimulation (ACh) causes bronchoconstriction (decreasing radius, increasing resistance), while Sympathetic stimulation ($\beta_2$ receptors) causes bronchodilation. * **Density vs. Viscosity:** According to Poiseuille's law, resistance is also proportional to gas viscosity. However, in **turbulent flow** (high Reynolds number), density becomes more important than viscosity.

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: Which of the following causes a right shift of the oxygen-hemoglobin dissociation curve?

Hypoxia. The oxygen-hemoglobin (O₂-Hb) dissociation curve represents the relationship between the partial pressure of oxygen (PO₂) and the percentage saturation of hemoglobin. A **right shift** indicates a decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to the tissues. ### Why Hypoxia is Correct **Hypoxia** (low oxygen levels) leads to a right shift primarily through the production of **2,3-Bisphosphoglycerate (2,3-BPG)**. When tissues are hypoxic, RBCs increase anaerobic glycolysis, producing more 2,3-BPG. This molecule binds to the beta chains of deoxyhemoglobin, stabilizing the "T" (Tense) state and promoting the release of oxygen to oxygen-starved tissues. ### Why Other Options are Incorrect * **Hypothermia (A):** Low temperature stabilizes the "R" (Relaxed) state of hemoglobin, increasing its affinity for oxygen and causing a **left shift**. * **Alkalosis (C):** An increase in pH (decreased H⁺ concentration) causes a **left shift** (the Bohr effect). Conversely, acidosis causes a right shift. * **Hemoglobin Fetal (HbF) (D):** HbF has a higher affinity for oxygen than adult hemoglobin (HbA) because it binds 2,3-BPG poorly. This results in a **left shift**, allowing the fetus to extract oxygen from maternal blood. ### NEET-PG High-Yield Pearls: "CADET, face Right!" To remember the factors that cause a **Right Shift** (decreased affinity), use the mnemonic **CADET**: * **C** – CO₂ increase (Hypercapnia) * **A** – Acidosis (Low pH) * **D** – 2,3-DPG (or 2,3-BPG) increase * **E** – Exercise * **T** – Temperature increase (Fever) **Note:** While acute hypoxia causes a right shift via 2,3-BPG, chronic high-altitude exposure also utilizes this mechanism to improve peripheral oxygen delivery.

Q: What is a characteristic finding in restrictive lung disease?

Normal FEV1/VC. **Explanation:** In **Restrictive Lung Diseases** (e.g., Pulmonary Fibrosis, Kyphoscoliosis, Sarcoidosis), the primary pathology is reduced lung compliance or chest wall expansion, leading to a decrease in all lung volumes. **1. Why the Correct Answer (C) is Right:** In restrictive disease, both the **FEV1** (Forced Expiratory Volume in 1 second) and the **FVC** (Forced Vital Capacity) decrease proportionately. Because both the numerator and denominator decrease, the **FEV1/FVC ratio remains normal (typically >0.7) or may even be increased** due to increased radial traction on the airways, which keeps them open during expiration. **2. Why the Other Options are Wrong:** * **Option A (Decreased FEV1):** While FEV1 is indeed decreased in restrictive disease, it is not a *distinguishing* characteristic because FEV1 is also significantly decreased in obstructive diseases (like Asthma/COPD). The hallmark of restriction is the preserved ratio, not just the drop in FEV1. * **Option B (Increased TLC):** Total Lung Capacity (TLC) is **decreased** in restrictive disease. An increased TLC is characteristic of obstructive diseases due to air trapping and hyperinflation. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Diagnosis:** A decrease in **TLC** (<80% of predicted) is the definitive diagnostic criterion for restrictive lung disease. * **Flow-Volume Loop:** Shows a characteristic **"Witch’s Hat"** appearance (narrower loop shifted to the right with preserved peak flow). * **DLCO:** Usually decreased in intrinsic restrictive diseases (interstitial lung disease) but normal in extrinsic/extrapulmonary causes (obesity, neuromuscular weakness).

Question 1

According to Poiseuille's law, reducing the radius of an airway to one-third will increase its resistance how many fold?

Accepted Answer

81

Answer

3

Answer

3

Answer

9

Question 2

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 3

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 4

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 5

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 6

Which of the following causes a right shift of the oxygen-hemoglobin dissociation curve?

Accepted Answer

Hypoxia

Answer

Hypothermia

Answer

Alkalosis

Answer

Hemoglobin Fetal (HbF)

Question 7

What is a characteristic finding in restrictive lung disease?

Accepted Answer

Normal FEV1/VC

Answer

Decreased FEV1

Answer

Increased TLC

Answer

All of the above

Question 8

In response to ascent to high altitude, the blood pH normalizes. What is the mechanism behind this normalization?

Accepted Answer

Retention of bicarbonate by the kidneys.

Answer

Increased erythropoiesis leads to increased buffering by hemoglobin.

Answer

Increased excretion of HCO3- by the kidneys.

Answer

Increased levels of 2,3-DPG.

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

What is the pacemaker that regulates the rate of respiration, being a component of the respiratory control pattern generator responsible for automatic respiration?

Accepted Answer

Pre-Botzinger Complex

Answer

Pneumotaxic centre

Answer

Dorsal group of nucleus

Answer

Apneustic centre

Respiratory System — MCQs

Respiratory System — MCQs

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