Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 641: All the following are false regarding physiological dead space except:

A. In healthy adults, it is 20-50ml more than anatomical dead space
B. It is detected by Bohr's method (Correct Answer)
C. It constitutes 10-15% of tidal volume
D. It is increased by endotracheal intubation

Explanation: **Physiological dead space** refers to the total volume of the respiratory system that does not participate in gas exchange. It includes the **Anatomical Dead Space** (conducting airways) and the **Alveolar Dead Space** (non-perfused or poorly perfused alveoli). ### Why Option B is Correct: **Bohr’s Method** is the standard technique used to measure physiological dead space. It is based on the principle that all expired $CO_2$ comes from the alveolar gas, as the air in the dead space contains virtually no $CO_2$. The formula used is: $V_D/V_T = (PaCO_2 - PeCO_2) / PaCO_2$ *(Where $V_D$ = Dead space, $V_T$ = Tidal volume, $PaCO_2$ = Arterial $CO_2$, and $PeCO_2$ = Mixed expired $CO_2$)*. ### Why Other Options are Incorrect: * **Option A:** In healthy adults, alveolar dead space is negligible. Therefore, physiological dead space is **nearly equal** to anatomical dead space (approx. 150 ml). A difference of 20-50 ml would indicate significant ventilation-perfusion mismatch. * **Option C:** Physiological dead space typically constitutes about **30% (1/3rd)** of the tidal volume (e.g., 150 ml out of 500 ml), not 10-15%. * **Option D:** Endotracheal intubation **decreases** anatomical dead space because the tube bypasses the upper respiratory tract (nose, pharynx, larynx), which accounts for a large portion of the dead space. ### High-Yield Pearls for NEET-PG: * **Fowler’s Method:** Used to measure **Anatomical Dead Space** (uses single-breath nitrogen washout). * **Anatomical Dead Space** is roughly **2 ml/kg** of body weight. * **Factors increasing dead space:** Upright position (increased apical dead space), aging, pulmonary embolism, and drugs like atropine (bronchodilation). * **Instrumental Dead Space:** Added by equipment like a snorkel or breathing circuits.

Question 642: Anemic hypoxia is due to which of the following?

A. Decreased partial pressure of oxygen in arterial blood
B. Increased partial pressure of oxygen in arterial blood
C. Increased partial pressure of carbon dioxide in arterial blood
D. Decreased oxygen content in arterial blood (Correct Answer)

Explanation: **Explanation:** **Anemic hypoxia** is a condition where the oxygen-carrying capacity of the blood is reduced, despite the lungs functioning normally. 1. **Why the correct answer is right:** The total **Oxygen Content ($CaO_2$)** of arterial blood is determined by the formula: $CaO_2 = (1.34 \times Hb \times SaO_2) + (0.003 \times PaO_2)$. In anemia, the hemoglobin ($Hb$) concentration is low. Since $Hb$ is the primary vehicle for oxygen transport, a decrease in $Hb$ directly leads to **decreased oxygen content** in the blood. However, the lungs still oxygenate the available $Hb$ normally, and the dissolved oxygen remains constant. 2. **Why the incorrect options are wrong:** * **Option A & B:** Partial pressure of oxygen ($PaO_2$) represents the oxygen dissolved in plasma. In anemic hypoxia, the $PaO_2$ remains **normal** because the alveolar-capillary gas exchange is unaffected. * **Option C:** Increased $PaCO_2$ (Hypercapnia) is typically seen in hypoventilation or type II respiratory failure, not specifically in anemia. **High-Yield Clinical Pearls for NEET-PG:** * **Causes of Anemic Hypoxia:** Anemia, Hemorrhage, Carbon Monoxide (CO) poisoning (CO occupies $Hb$ binding sites), and Methemoglobinemia. * **Key Distinction:** In anemic hypoxia, **$PaO_2$ is normal**, but **$CaO_2$ is decreased**. * **Cyanosis:** Patients with severe anemia often **do not** show cyanosis because cyanosis requires at least 5g/dL of *reduced* (deoxygenated) hemoglobin, which anemic patients may not reach due to overall low $Hb$ levels. * **CO Poisoning:** A classic "trap" in exams; it causes anemic hypoxia because it reduces the amount of $Hb$ available for $O_2$ transport, even though the $PaO_2$ remains normal.

Question 643: Small airways have laminar air flow because?

A. Reynolds' number > 2000
B. Very small diameter
C. Extremely low velocity (Correct Answer)
D. Low cross-sectional area

Explanation: **Explanation:** The nature of airflow in the respiratory tract is determined by the **Reynolds’ number (Re)**, a dimensionless value calculated as: $Re = \frac{\text{Density} \times \text{Velocity} \times \text{Diameter}}{\text{Viscosity}}$ **Why "Extremely low velocity" is correct:** While individual small airways (bronchioles) have tiny diameters, they exist in massive numbers in parallel. This arrangement creates a **massive total cross-sectional area** (the "bell-shaped" expansion of the airway tree). According to the law of continuity, as the total cross-sectional area increases, the **velocity of airflow decreases** significantly. In the terminal bronchioles, the velocity becomes so low that the Reynolds' number drops well below 2000, ensuring purely **laminar flow**. **Analysis of Incorrect Options:** * **A. Reynolds' number > 2000:** This indicates **turbulent flow**, typically found in the trachea and large airways where velocity is high. Laminar flow occurs when $Re < 2000$. * **B. Very small diameter:** While diameter is in the numerator of the Reynolds equation, the drastic reduction in *velocity* (due to the area increase) outweighs the diameter factor in ensuring laminar flow. * **D. Low cross-sectional area:** This is factually incorrect. Small airways collectively have the **highest** total cross-sectional area in the lungs. **High-Yield Clinical Pearls for NEET-PG:** 1. **Silent Zone:** The small airways (beyond the 10th–12th generation) contribute very little to total airway resistance due to their massive parallel arrangement. Disease here is often asymptomatic until advanced. 2. **Maximum Resistance:** Airway resistance is highest in the **medium-sized bronchi** (generations 2–5), not the smallest ones. 3. **Flow Types:** Trachea = Turbulent; Small airways = Laminar; Branching points = Transitional/Tracheated flow.

Question 644: What is the pleural pressure at the end of quiet respiration?

A. Zero
B. More negative (Correct Answer)
C. Positive
D. Less negative

Explanation: **Explanation:** The pleural pressure (intrapleural pressure) is the pressure within the fluid-filled space between the visceral and parietal pleura. To understand why it becomes **more negative** at the end of inspiration, we must look at the balance of elastic forces. 1. **Why "More Negative" is Correct:** At the start of inspiration (functional residual capacity), the pleural pressure is approximately **-5 cm H₂O**. This negativity is due to the opposing elastic recoils: the lungs want to collapse inward, while the chest wall wants to expand outward. During quiet inspiration, the diaphragm contracts, increasing the thoracic volume. According to Boyle’s Law, as volume increases, pressure decreases. By the end of inspiration, the lungs are stretched further, increasing their elastic recoil. To overcome this and keep the airways open, the pleural pressure must drop further, reaching approximately **-7.5 cm H₂O**. Thus, it becomes "more negative." 2. **Why Other Options are Incorrect:** * **Zero:** Pleural pressure is never zero under physiological conditions; if it were, the lungs would collapse. * **Positive:** Positive pleural pressure only occurs during forced expiration (e.g., Valsalva maneuver) or in pathological states like a tension pneumothorax. * **Less Negative:** Pleural pressure becomes "less negative" (returning from -7.5 to -5 cm H₂O) during **expiration**, not at the end of inspiration. **High-Yield Clinical Pearls for NEET-PG:** * **Transpulmonary Pressure:** Defined as Alveolar Pressure minus Pleural Pressure ($P_{tp} = P_{alv} - P_{ip}$). It is always positive, keeping the lungs inflated. * **Gravity Effect:** In a standing position, pleural pressure is **most negative at the apex** (approx. -10 cm H₂O) and **least negative at the base** (approx. -2.5 cm H₂O). * **Pneumothorax:** If the pleural cavity communicates with the atmosphere, pleural pressure equilibrates with atmospheric pressure (becomes zero), leading to lung collapse.

Question 645: Which of the following is not responsible for causing a right shift of the oxygen-hemoglobin dissociation curve?

A. Increased carbon dioxide
B. Increased temperature
C. Increased exercise
D. Alkalosis (Correct Answer)

Explanation: ### Explanation The **Oxygen-Hemoglobin (O₂-Hb) dissociation curve** represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **right shift** indicates a decreased affinity of hemoglobin for oxygen, meaning oxygen is released more easily to the tissues. #### Why Alkalosis is the Correct Answer **Alkalosis** (increased pH/decreased $H^+$ concentration) causes a **left shift** of the curve. According to the **Bohr Effect**, a decrease in hydrogen ion concentration increases hemoglobin’s affinity for oxygen, making it bind more tightly and shifting the curve to the left. Since the question asks for the factor **not** responsible for a right shift, Alkalosis is the correct choice. #### Analysis of Incorrect Options (Factors causing a Right Shift) A right shift occurs in conditions where tissues need more oxygen (mnemonic: **"CADET, face Right!"** — **C**O₂, **A**cid, **D**PG, **E**xercise, **T**emperature). * **A. Increased Carbon Dioxide:** High $PCO_2$ leads to increased $H^+$ production (Carbamino effect), decreasing O₂ affinity. * **B. Increased Temperature:** Denatures the bond between hemoglobin and oxygen, facilitating unloading. * **C. Increased Exercise:** Exercise involves a combination of hypercapnia (high $CO_2$), hyperthermia (high temp), and lactic acidosis, all of which drive a right shift to meet metabolic demands. #### High-Yield Clinical Pearls for NEET-PG * **2,3-BPG:** Increased levels (seen in chronic hypoxia, high altitude, and anemia) cause a **Right Shift**. * **Fetal Hemoglobin (HbF):** Has a higher affinity for oxygen than adult hemoglobin (HbA), causing a **Left Shift**. * **Carbon Monoxide (CO) Poisoning:** Causes a **Left Shift** and changes the curve from sigmoid to hyperbolic, preventing oxygen release to tissues. * **P50 Value:** The $PO_2$ at which Hb is 50% saturated. A **Right Shift** increases the P50 (normal is ~26.7 mmHg).

Question 646: Moderate exercise tachypnea is primarily due to stimulation of which receptor?

A. Proprioceptors (Correct Answer)
B. J receptors
C. Pulmonary stretch receptors
D. Baroreceptors

Explanation: ### Explanation **Why Proprioceptors are the Correct Answer:** During **moderate exercise**, the initial and primary stimulus for increased ventilation (tachypnea) is the activation of **proprioceptors** located in the joints, tendons, and muscles. As soon as exercise begins, these receptors send excitatory impulses to the medullary respiratory centers. This is considered a "feed-forward" or neurogenic mechanism, as it increases breathing even before metabolic changes (like increased $PCO_2$ or decreased $PO_2$) occur in the blood. This rapid response ensures that oxygen delivery and $CO_2$ removal stay ahead of metabolic demand. **Analysis of Incorrect Options:** * **B. J Receptors (Juxtacapillary):** These are located in the alveolar walls near capillaries. They are stimulated by pulmonary congestion, edema, or engorgement (e.g., heart failure), leading to rapid, shallow breathing (dyspnea), not the physiological tachypnea of moderate exercise. * **C. Pulmonary Stretch Receptors:** These are involved in the **Hering-Breuer reflex**. They are stimulated by lung inflation and send inhibitory signals to the dorsal respiratory group to prevent over-inflation. They regulate the depth of breathing rather than initiating exercise-induced tachypnea. * **D. Baroreceptors:** These primarily sense changes in blood pressure. While a significant drop in blood pressure can reflexively increase respiration, they are not the primary mediators of the respiratory response to exercise. **High-Yield Clinical Pearls for NEET-PG:** * **Phases of Exercise Hyperpnea:** Phase I (Immediate) is neurogenic (proprioceptors/cerebral cortex); Phase II (Slow) is due to chemical changes; Phase III is the steady state. * **Arterial Blood Gases:** In moderate exercise, mean arterial $PO_2$, $PCO_2$, and $pH$ remain remarkably **normal**. The stimulus is not a change in blood gases but the neural input from moving limbs. * **Oscillatory Hypothesis:** Some believe that while *mean* $PCO_2$ is constant, the *oscillations* in $PCO_2$ levels during the respiratory cycle stimulate peripheral chemoreceptors during exercise.

Question 647: Which of the following factors does NOT shift the oxygen dissociation curve to the right?

A. Hypoxia
B. Acidosis
C. Increase in 2, 3-DPG
D. Alkalosis (Correct Answer)

Explanation: The **Oxygen-Hemoglobin Dissociation Curve (ODC)** represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **shift to the right** indicates a decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to the tissues. ### Why Alkalosis is the Correct Answer: **Alkalosis** (increased pH/decreased $H^+$ concentration) increases the affinity of hemoglobin for oxygen, making it harder for oxygen to be released. This causes a **shift to the left**, not the right. ### Explanation of Incorrect Options (Factors shifting the curve to the Right): * **Hypoxia:** Chronic hypoxia (e.g., at high altitudes) stimulates the production of **2,3-DPG** within red blood cells. This molecule binds to the beta chains of deoxyhemoglobin, stabilizing the "T" (Tense) state and shifting the curve to the right to enhance tissue oxygenation. * **Acidosis:** An increase in $H^+$ ions (decreased pH) reduces hemoglobin's affinity for oxygen. This is known as the **Bohr Effect**, which ensures that metabolically active tissues receiving acidic byproducts get more oxygen. * **Increase in 2,3-DPG:** As mentioned, 2,3-DPG is a key allosteric effector that decreases oxygen affinity, shifting the curve to the right. ### High-Yield Clinical Pearls for NEET-PG: * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis, **D**PG (2,3-DPG) increase, **E**xercise, **T**emperature increase. * **Left Shift Factors:** Hypothermia, Alkalosis, decreased 2,3-DPG, Fetal Hemoglobin (HbF), and Carbon Monoxide (CO) poisoning. * **P50 Value:** The $PO_2$ at which hemoglobin is 50% saturated. A right shift **increases** the P50 (normal is ~26.6 mmHg).

Question 648: All are functions of surfactant except?

A. Reduces work of breathing
B. Keeps alveoli dry
C. Provides innate immunity
D. Prevents overexpansion of alveoli (Correct Answer)

Explanation: **Explanation:** Surfactant is a surface-active lipoprotein complex (primarily Dipalmitoylphosphatidylcholine - DPPC) secreted by **Type II pneumocytes**. Its primary role is to reduce surface tension at the air-liquid interface of the alveoli. **Why Option D is the Correct Answer:** Surfactant does **not** prevent overexpansion; instead, it prevents **atelectasis (collapse)**. According to the **Law of Laplace ($P = 2T/r$)**, smaller alveoli have a higher collapsing pressure. Surfactant reduces surface tension ($T$) more effectively in smaller alveoli than in larger ones, equalizing pressure between different-sized alveoli and preventing small ones from collapsing into larger ones. Overexpansion is primarily prevented by the **Hering-Breuer inflation reflex** and the chest wall's elastic recoil. **Analysis of Incorrect Options:** * **A. Reduces work of breathing:** By lowering surface tension, surfactant increases **lung compliance**. This makes the lungs easier to inflate, significantly reducing the muscular effort required for inspiration. * **B. Keeps alveoli dry:** Surface tension creates an inward "sucking" force that tends to pull fluid from capillaries into the alveolar space. By reducing this tension, surfactant prevents pulmonary edema and keeps the gas exchange surface dry. * **C. Provides innate immunity:** Surfactant contains proteins **SP-A and SP-D** (collectins), which act as opsonins to neutralize bacteria and viruses, aiding alveolar macrophages. **High-Yield Clinical Pearls for NEET-PG:** * **Synthesis:** Starts at 24–28 weeks; mature levels reached by **35 weeks**. * **L/S Ratio:** A Lecithin/Sphingomyelin ratio **>2** in amniotic fluid indicates fetal lung maturity. * **Glucocorticoids:** Accelerate surfactant production (used in preterm labor). * **Deficiency:** Leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease.

Question 649: The exchange of gases between plasma and tissue fluid is primarily determined by which of the following factors?

A. Partial pressures of gases (Correct Answer)
B. Hydrostatic pressures
C. Osmotic pressure differentials
D. Difference in volume percent of gases

Explanation: ### Explanation **1. Why Option A is Correct:** The exchange of gases (Oxygen and Carbon Dioxide) across biological membranes, including the interface between plasma and tissue fluid, is a passive process governed by **Fick’s Law of Diffusion**. The primary driving force for this movement is the **partial pressure gradient**. Gases move from an area of higher partial pressure to an area of lower partial pressure. For instance, $PO_2$ in systemic capillaries (~95-100 mmHg) is higher than in the interstitial fluid (~40 mmHg), causing oxygen to diffuse into the tissues. **2. Why Other Options are Incorrect:** * **Option B (Hydrostatic pressure):** This pressure, generated by the heart's pumping action, governs the bulk flow of **fluids** (water and solutes) through capillary pores (Starling forces), not the diffusion of individual gas molecules. * **Option C (Osmotic pressure):** Specifically oncotic pressure (exerted by plasma proteins), this force pulls water back into the capillaries. While it regulates fluid balance, it does not determine gas exchange. * **Option D (Volume percent):** While volume percent represents the total content of gas in the blood (including that bound to hemoglobin), the diffusion gradient is strictly determined by the **dissolved fraction** of the gas, which is reflected by its partial pressure. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diffusion Capacity ($D_L$):** $CO_2$ is **20 to 25 times more soluble** than $O_2$, meaning $CO_2$ diffuses much faster across the respiratory membrane even with a smaller pressure gradient. * **Limiting Factor:** Under normal physiological conditions, gas exchange in the lungs is **perfusion-limited**, not diffusion-limited. * **Henry’s Law:** States that the amount of dissolved gas in a liquid is proportional to its partial pressure.

Question 650: True regarding vascularity of the lung is:

A. Hypoxia causes vasodilation
B. Pulmonary resistance is half of the systemic vascular resistance
C. Perfusion is more in the apical lobe than in the base
D. Distended pulmonary veins in the lower lobe (Correct Answer)

Explanation: **Explanation:** The vascularity of the lung is governed by gravity and unique local regulatory mechanisms. **Why Option D is Correct:** In an upright position, **gravity** significantly influences pulmonary blood flow. Hydrostatic pressure is highest at the base (lower lobes) of the lung. This increased pressure leads to the recruitment and distension of pulmonary vessels (West Zone 3), resulting in **distended pulmonary veins in the lower lobes** compared to the apex. **Analysis of Incorrect Options:** * **Option A:** Unlike systemic vessels which vasodilate in response to hypoxia, pulmonary arterioles undergo **Hypoxic Pulmonary Vasoconstriction (HPV)**. This shunts blood away from poorly ventilated alveoli to well-ventilated ones to optimize V/Q matching. * **Option B:** Pulmonary vascular resistance (PVR) is significantly lower than systemic vascular resistance (SVR). PVR is approximately **1/10th to 1/12th** of SVR, not half. This allows the right ventricle to pump the same cardiac output at much lower pressures. * **Option C:** Due to gravity, both ventilation and perfusion increase from the apex to the base. However, **perfusion increases more steeply** than ventilation. Therefore, the base of the lung is better perfused than the apex. **High-Yield Clinical Pearls for NEET-PG:** * **West Zones:** Zone 1 (Apex: $P_A > P_a > P_v$), Zone 2 (Middle: $P_a > P_A > P_v$), Zone 3 (Base: $P_a > P_v > P_A$). * **Cephalization:** On a chest X-ray, if pulmonary veins in the upper lobes become distended (Antler sign), it indicates pulmonary venous hypertension (e.g., Mitral Stenosis or Left Heart Failure). * **V/Q Ratio:** Highest at the apex (~3.3) and lowest at the base (~0.6).

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