Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 111: In a normal adult, what is the approximate ratio of physiological dead space to anatomical dead space?

A. 2:1
B. 1:3
C. 3:1
D. 1:1 (Correct Answer)

Explanation: **Explanation** In a healthy adult, the **Physiological Dead Space** is approximately equal to the **Anatomical Dead Space**, making the ratio **1:1**. **Understanding the Concept:** * **Anatomical Dead Space:** The volume of the conducting airways (nose to terminal bronchioles) where no gas exchange occurs. In a 70 kg adult, this is roughly **150 mL**. * **Alveolar Dead Space:** The volume of air in alveoli that are ventilated but not perfused (no gas exchange). In a healthy individual, this is **negligible (near zero)**. * **Physiological Dead Space:** The sum of Anatomical + Alveolar dead space. Since Alveolar dead space is nearly zero in healthy lungs, **Physiological Dead Space ≈ Anatomical Dead Space**, resulting in a 1:1 ratio. **Analysis of Incorrect Options:** * **A (2:1) & C (3:1):** These ratios imply that physiological dead space is significantly larger than anatomical dead space. This occurs only in **pathological states** (e.g., Pulmonary Embolism, Emphysema) where alveolar ventilation-perfusion mismatch increases. * **B (1:3):** This is physiologically impossible, as physiological dead space must always be equal to or greater than anatomical dead space. **NEET-PG High-Yield Pearls:** 1. **Measurement:** Anatomical dead space is measured by **Fowler’s Method** (Nitrogen washout), while Physiological dead space is measured by **Bohr’s Equation** (using $CO_2$ levels). 2. **Positioning:** Physiological dead space increases in the **upright position** due to increased ventilation-perfusion mismatch at the lung apices. 3. **Pathology:** In lung diseases, Physiological Dead Space > Anatomical Dead Space.

Question 112: Type I Glomus cells of peripheral chemoreceptors possess which of the following?

A. CO2 sensitive K+ channels
B. O2 sensitive Cl– channel
C. CO2 sensitive Cl– channel
D. O2 sensitive K+ channel (Correct Answer)

Explanation: **Explanation:** The **Type I Glomus cells** (chief cells) of the carotid and aortic bodies are the primary sensory receptors for detecting arterial hypoxia. The transduction mechanism involves the following steps: 1. **Mechanism of Action:** Under normal conditions, **O2-sensitive K+ channels** remain open, allowing potassium efflux and maintaining a resting membrane potential. 2. **Hypoxia Detection:** When arterial PO2 falls (hypoxia), these specific O2-sensitive K+ channels **close**. 3. **Depolarization:** The closure of these channels prevents K+ exit, leading to cell depolarization. This opens voltage-gated Ca2+ channels, causing an influx of calcium. 4. **Neurotransmitter Release:** Increased intracellular calcium triggers the exocytosis of neurotransmitters (primarily **ATP** and acetylcholine, though dopamine is also present), which stimulate the glossopharyngeal (CN IX) or vagus (CN X) nerve endings to increase the respiratory rate. **Analysis of Incorrect Options:** * **Options B & C:** Chloride (Cl–) channels do not play a primary role in the initial depolarization phase of glomus cells in response to blood gas changes. * **Option A:** While glomus cells do respond to hypercapnia (high CO2) and acidosis (low pH), the classic, most high-yield mechanism described for the initiation of the hypoxic response specifically involves **O2-sensitive K+ channels**. **High-Yield Facts for NEET-PG:** * **Location:** Carotid bodies (bifurcation of common carotid) and Aortic bodies (arch of aorta). * **Innervation:** Carotid body via **Hering’s nerve** (branch of CN IX); Aortic body via CN X. * **Primary Stimulus:** Peripheral chemoreceptors respond primarily to **low PO2** (<60 mmHg), whereas central chemoreceptors respond to **high PCO2/low pH** in the CSF. * **Type II Cells:** These are sustentacular (supportive) cells, similar to glial cells, and do not have a sensory function.

Question 113: In which of the following conditions would oxygen therapy be most effective in alleviating hypoxia?

A. Anemia due to blood loss
B. Edematous tissues
C. Emphysema (Correct Answer)
D. Localized circulatory deficiencies

Explanation: **Explanation:** The effectiveness of oxygen therapy depends on whether the underlying cause of hypoxia is a failure of oxygen to enter the blood (Hypoxic Hypoxia) or a failure of delivery/utilization. **Why Emphysema is Correct:** Emphysema is a classic example of **Hypoxic Hypoxia**. It involves the destruction of alveolar walls, which decreases the surface area for gas exchange and causes ventilation-perfusion (V/Q) mismatch. Oxygen therapy is highly effective here because increasing the alveolar $PO_2$ creates a steeper diffusion gradient, forcing more oxygen across the damaged respiratory membrane into the pulmonary capillaries, thereby significantly increasing arterial oxygen saturation. **Why the other options are incorrect:** * **Anemia (Anemic Hypoxia):** The primary problem is a lack of hemoglobin (carriers), not a lack of oxygen tension. Since the existing hemoglobin is already near 100% saturated, supplemental $O_2$ only slightly increases the dissolved oxygen in plasma, providing minimal benefit. * **Edematous Tissues (Histotoxic/Diffusion limitation):** While $O_2$ can help slightly, the increased distance for diffusion at the peripheral tissue level makes it less effective than correcting the underlying fluid balance. * **Localized Circulatory Deficiencies (Stagnant Hypoxia):** In conditions like ischemia or heart failure, the blood flow is too slow or blocked. Oxygen is present in the blood, but it cannot reach the tissues. Improving blood flow is the priority, not increasing $O_2$ concentration. **High-Yield NEET-PG Pearls:** * **Hypoxic Hypoxia** (e.g., high altitude, COPD, pneumonia) is the condition most responsive to $O_2$ therapy. * **Cyanosis** is typically absent in Anemic Hypoxia because cyanosis requires at least 5g/dL of deoxygenated hemoglobin, which is rarely reached in anemic patients. * In **CO poisoning**, $O_2$ therapy is used not just for saturation, but to reduce the half-life of Carboxyhemoglobin (especially Hyperbaric $O_2$).

Question 114: Which of the following statements about surfactant is false?

A. Produced by Type II alveolar epithelial cells.
B. Contains dipalmitoyl phosphatidylcholine (DPPC) and phosphatidyl glycerol.
C. Is a lipid-surface tension lowering agent.
D. Consists of 3 types of unique proteins in it. (Correct Answer)

Explanation: **Explanation** The correct answer is **D** because surfactant actually contains **four** unique proteins, not three. These are designated as **SP-A, SP-B, SP-C, and SP-D**. * **SP-A and SP-D** are hydrophilic and play a major role in innate immunity (opsonization of pathogens). * **SP-B and SP-C** are hydrophobic and are essential for the spreading and stability of the surfactant film. **Analysis of other options:** * **Option A:** Surfactant is synthesized, stored (in lamellar bodies), and secreted by **Type II pneumocytes**, which cover about 5% of the alveolar surface area. * **Option B:** Surfactant is approximately 90% lipids and 10% proteins. The most abundant phospholipid is **DPPC** (Lecithin), which is primarily responsible for reducing surface tension. **Phosphatidylglycerol (PG)** is the second most common lipid and serves as a marker for fetal lung maturity. * **Option C:** By definition, surfactant is a surface-active agent. It reduces the work of breathing by lowering alveolar surface tension, preventing alveolar collapse (atelectasis) at low lung volumes. **High-Yield Clinical Pearls for NEET-PG:** * **Law of Laplace:** $P = 2T/r$. Surfactant reduces tension ($T$), preventing small alveoli from collapsing into larger ones. * **Fetal Lung Maturity:** Surfactant production begins at 24–28 weeks, but matures significantly after **35 weeks**. * **L/S Ratio:** A Lecithin/Sphingomyelin ratio **> 2.0** in amniotic fluid indicates mature lungs. * **Glucocorticoids:** Administered to mothers in preterm labor to accelerate surfactant production by stimulating Type II cells.

Question 115: What is the normal pulmonary artery pressure?

A. 120/80 mm Hg
B. 25/0 mm Hg
C. 25/8 mm Hg (Correct Answer)
D. 120/0 mm Hg

Explanation: ### Explanation **1. Understanding the Correct Answer (Option C: 25/8 mm Hg)** The pulmonary circulation is a **low-pressure, low-resistance system** compared to the systemic circulation. The right ventricle (RV) only needs to pump blood through the lungs to the left atrium. * **Systolic Pulmonary Artery Pressure (sPAP):** Normally ranges from **15–25 mm Hg**. This reflects the pressure during RV contraction. * **Diastolic Pulmonary Artery Pressure (dPAP):** Normally ranges from **8–15 mm Hg**. This reflects the pressure while the pulmonary valve is closed. * **Mean Pulmonary Artery Pressure (mPAP):** Typically **15 mm Hg**. A mPAP >20 mm Hg at rest is the diagnostic threshold for Pulmonary Hypertension. **2. Analysis of Incorrect Options** * **Option A (120/80 mm Hg):** This represents normal **Systemic Arterial Pressure**. The left ventricle must generate significantly higher pressure to overcome high systemic vascular resistance and perfuse the entire body. * **Option B (25/0 mm Hg):** This represents **Right Ventricular (RV) Pressure**. While the systolic pressure matches the pulmonary artery, the diastolic pressure in the ventricle drops to near zero as it relaxes to fill with blood. * **Option C (120/0 mm Hg):** This represents **Left Ventricular (LV) Pressure**. The LV reaches high systolic pressures but, like the RV, its diastolic pressure drops to near zero to allow for filling. **3. High-Yield Clinical Pearls for NEET-PG** * **PCWP (Pulmonary Capillary Wedge Pressure):** Normal is **6–12 mm Hg**. It is a clinical proxy for Left Atrial Pressure. * **West Zones of the Lung:** Blood flow distribution is determined by the relationship between Alveolar pressure (PA), Arterial pressure (Pa), and Venous pressure (Pv). * **Hypoxic Pulmonary Vasoconstriction:** Unlike systemic vessels (which dilate), pulmonary arterioles **constrict** in response to low alveolar oxygen to shunt blood to better-ventilated areas.

Question 116: Transport of carbon monoxide (CO) is diffusion limited because?

A. High affinity of CO for hemoglobin (Correct Answer)
B. Alveolar membrane is less permeable to CO
C. CO crosses the epithelial barrier slowly
D. On exposure to air, there is a sudden increase in partial pressure

Explanation: **Explanation:** The transport of a gas across the alveolar-capillary membrane is determined by whether it is **perfusion-limited** or **diffusion-limited**. **Why the correct answer is right:** Carbon monoxide (CO) is the classic example of a **diffusion-limited** gas. This is because CO has an extremely high affinity for hemoglobin (approximately 240 times that of oxygen). As soon as CO molecules cross the alveolar-capillary membrane, they are rapidly bound to hemoglobin. This "sequestering" effect ensures that the partial pressure of CO in the plasma ($P_{c}CO$) remains near zero throughout the length of the capillary. Since the pressure gradient between the alveoli and the blood ($P_A - P_c$) remains high and never reaches equilibrium, the only factor limiting its uptake is the physical properties of the diffusion barrier itself. **Why the incorrect options are wrong:** * **B & C:** These are physiologically incorrect. CO is highly lipid-soluble and crosses the alveolar epithelium and capillary endothelium very rapidly. The barrier is not the limiting factor; the lack of partial pressure buildup in the blood is. * **D:** While a sudden increase in partial pressure affects the gradient, it does not define the "limiting" mechanism of transport. Diffusion limitation is a property of the gas's interaction with blood components (hemoglobin). **High-Yield Clinical Pearls for NEET-PG:** * **DLCO (Diffusion Capacity of the Lung for CO):** Because CO is diffusion-limited, it is the gas of choice used in pulmonary function tests to measure the integrity of the alveolar-capillary membrane. * **Perfusion-limited gases:** Nitrous oxide ($N_2O$) is the classic example. It does not bind to hemoglobin, so partial pressure in the blood rises rapidly, reaching equilibrium with the alveoli early in the capillary. * **Oxygen ($O_2$):** Under normal resting conditions, $O_2$ is perfusion-limited. However, in states of disease (fibrosis) or extreme exercise (high cardiac output), it can become diffusion-limited.

Question 117: Surfactant is secreted by which of the following cells?

A. Type I pneumocyte
B. Type II pneumocyte (Correct Answer)
C. Seoli cell
D. Leydig cell

Explanation: **Explanation:** The correct answer is **Type II pneumocyte**. These cells are cuboidal epithelial cells found in the alveolar walls, comprising only about 5% of the alveolar surface area but representing approximately 60% of the alveolar cell population. **1. Why Type II Pneumocytes?** Type II pneumocytes act as the "caretakers" of the alveoli. Their primary function is the synthesis and secretion of **surfactant** (mainly dipalmitoylphosphatidylcholine - DPPC). Surfactant is stored in intracellular organelles called **lamellar bodies**. By reducing surface tension at the air-liquid interface, surfactant prevents alveolar collapse (atelectasis) during expiration and increases lung compliance. Additionally, Type II cells serve as stem cells, proliferating to replace damaged Type I pneumocytes. **2. Analysis of Incorrect Options:** * **Type I pneumocyte:** These are thin, squamous cells covering 95% of the alveolar surface. Their primary role is facilitating gas exchange; they do not secrete surfactant. * **Sertoli cell:** Located in the seminiferous tubules of the testes, these cells support sperm development (spermatogenesis) and form the blood-testis barrier. * **Leydig cell:** Also found in the testes (interstitial space), these cells are responsible for the production of testosterone. **Clinical Pearls for NEET-PG:** * **Surfactant synthesis** begins around 24–26 weeks of gestation, but adequate levels are usually reached only after **35 weeks**. * **Infant Respiratory Distress Syndrome (IRDS):** Caused by surfactant deficiency in premature neonates. * **Lecithin/Sphingomyelin (L/S) ratio:** A ratio > 2.0 in amniotic fluid indicates fetal lung maturity. * **Glucocorticoids** (e.g., Betamethasone) are administered to mothers in preterm labor to accelerate surfactant production by stimulating Type II pneumocytes.

Question 118: What is the immediate response of J receptor stimulation?

A. Tachycardia
B. Hypertension
C. Tachypnea
D. Apnea (Correct Answer)

Explanation: **Explanation:** **J receptors (Juxtacapillary receptors)** are sensory nerve endings located in the alveolar walls, in close proximity to the pulmonary capillaries. They are innervated by **unmyelinated C-fibers** of the vagus nerve. **Why Apnea is the Correct Answer:** The classic response to J receptor stimulation is known as the **"Pulmonary Chemoreflex."** When these receptors are stimulated—typically by pulmonary edema, congestion, pneumonia, or chemical irritants (like capsaicin)—they trigger a rapid reflex triad. The **immediate** respiratory response is a brief period of **Apnea** (cessation of breathing), which is then followed by rapid, shallow breathing (tachypnea). **Analysis of Incorrect Options:** * **Tachycardia & Hypertension (A & B):** J receptor stimulation actually produces the opposite effect. It triggers a parasympathetic response leading to **Bradycardia** (decreased heart rate) and **Hypotension** (decreased blood pressure). * **Tachypnea (C):** While tachypnea does occur, it is the *secondary* or delayed response. The question asks for the *immediate* response, which is the transient apnea. **High-Yield Clinical Pearls for NEET-PG:** * **Stimulus:** The most common physiological stimulus is **interstitial fluid volume expansion** (e.g., Left Heart Failure leading to pulmonary edema). * **The Triad:** Remember the J-reflex triad: **Apnea, Bradycardia, and Hypotension.** * **Sensation of Dyspnea:** J receptor activation is a major contributor to the feeling of "shortness of breath" in patients with pulmonary congestion. * **Location:** They are located in the alveolar interstitium, unlike Irritant Receptors (found in the epithelium of the upper airways).

Question 119: What percentage of oxygen is inhaled?

A. 5%
B. 20% (Correct Answer)
C. 50%
D. 78%

Explanation: **Explanation:** The composition of atmospheric air is a fundamental concept in respiratory physiology. Dry atmospheric air consists of approximately **78% Nitrogen, 21% Oxygen, and 0.04% Carbon Dioxide**, with the remainder being trace gases like Argon. In the context of medical exams, the fraction of inspired oxygen ($FiO_2$) in room air is rounded to **21% or roughly 20%**. * **Option B (20%) is Correct:** This represents the standard concentration of oxygen in the air we breathe at sea level. Regardless of altitude, the *percentage* of oxygen remains constant at ~21%, though the *partial pressure* ($PO_2$) decreases as total atmospheric pressure drops. * **Option A (5%) is Incorrect:** This value is closer to the concentration of **Carbon Dioxide in exhaled air** (~4-5%). * **Option C (50%) is Incorrect:** This concentration is only achieved through supplemental oxygen therapy (e.g., via a simple face mask or Venturi mask). * **Option D (78%) is Incorrect:** This is the approximate concentration of **Nitrogen** in the atmosphere. **High-Yield Clinical Pearls for NEET-PG:** 1. **Exhaled Air Composition:** Exhaled air still contains about **16% Oxygen** and **4% $CO_2$**. This is why mouth-to-mouth resuscitation is effective. 2. **Water Vapor:** As air enters the respiratory tract, it is humidified. The partial pressure of water vapor ($PH_2O$) at body temperature ($37^\circ C$) is **47 mmHg**. This dilutes the partial pressure of other gases. 3. **Alveolar Gas Equation:** To calculate the partial pressure of alveolar oxygen ($PAO_2$), clinicians use: $PAO_2 = FiO_2(P_{atm} - PH_2O) - (PaCO_2 / R)$.

Question 120: Conducting zone of the respiratory system is defined as:

A. 1st generation of airways
B. 0th generation to 16th generation of airways (Correct Answer)
C. 0th generation to 23rd generation of airways
D. 16th to 23rd generation of airways

Explanation: ### Explanation The respiratory system is divided into two functional zones based on the **Weibel Model** of airway branching, which describes 23 generations of airways. **1. Why Option B is Correct:** The **Conducting Zone** extends from the **trachea (0th generation)** to the **terminal bronchioles (16th generation)**. Its primary functions are to warm, humidify, and filter inspired air and distribute it to the gas-exchange surfaces. Crucially, this zone contains no alveoli; therefore, no gas exchange occurs here. This volume of air is known as the **Anatomical Dead Space** (approximately 150 mL). **2. Why the Other Options are Incorrect:** * **Option A:** The 1st generation refers specifically to the mainstem bronchi. The conducting zone begins at the trachea (0th). * **Option C:** This describes the **entire respiratory tree** (0 to 23). It conflates the conducting zone with the respiratory zone. * **Option D:** This describes the **Respiratory Zone**, which starts from the **respiratory bronchioles (17th generation)** and ends at the **alveolar sacs (23rd generation)**. This is where gas exchange actually takes place. **High-Yield Facts for NEET-PG:** * **Cartilage:** Present up to the 10th generation (bronchi); absent in bronchioles (11th generation onwards). * **Cilia:** Present up to the respiratory bronchioles, but disappear before the alveolar ducts. * **Smooth Muscle:** Highest relative amount is found in the **terminal bronchioles** (the last part of the conducting zone). * **Velocity of Airflow:** It is highest in the trachea and **lowest in the respiratory zone** due to the massive increase in total cross-sectional area, allowing time for diffusion.

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