Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 651: How is the volume of inspired air that actually ventilates the alveoli calculated?

A. Single breath N2 method (Correct Answer)
B. Dalton's law
C. Bohr equation
D. Boyle's law

Explanation: The volume of inspired air that actually reaches the alveoli is determined by subtracting the **Anatomical Dead Space** from the Tidal Volume. The **Single Breath Nitrogen (N₂) Method** (also known as Fowler’s Method) is the gold standard for measuring this anatomical dead space. ### Why Option A is Correct: In Fowler’s method, a subject takes a single breath of 100% oxygen. As they exhale, a nitrogen analyzer measures the N₂ concentration. * The initial part of the breath contains 0% N₂ (pure oxygen from the dead space). * As exhalation continues, N₂ levels rise as alveolar air (which contains N₂) mixes in. * By plotting N₂ concentration against expired volume, the **Anatomical Dead Space** is calculated. Subtracting this from the Tidal Volume gives the volume that effectively ventilates the alveoli. ### Why Other Options are Incorrect: * **B. Dalton’s Law:** States that the total pressure of a gas mixture is the sum of the partial pressures of individual gases. It is used to calculate $PO_2$ at different altitudes, not lung volumes. * **C. Bohr Equation:** This measures **Physiological Dead Space** using $CO_2$ concentrations. While related, "Anatomical Dead Space" (measured by Fowler's) specifically refers to the volume of the conducting airways. * **D. Boyle’s Law:** States that pressure is inversely proportional to volume ($P \propto 1/V$). This is the principle behind **Body Plethysmography** used to measure Functional Residual Capacity (FRC). ### High-Yield Clinical Pearls for NEET-PG: * **Anatomical Dead Space:** Volume of conducting zones (approx. 150 ml or 2 ml/kg). Measured by **Fowler’s Method**. * **Physiological Dead Space:** Anatomical dead space + Alveolar dead space (wasted ventilation). Measured by **Bohr’s Equation**. * In healthy individuals, Anatomical $\approx$ Physiological dead space. In lung diseases (like PE or COPD), Physiological dead space increases significantly.

Question 652: What is the most important stimulus controlling the level of resting ventilation?

A. PO2 on peripheral chemoreceptors
B. PCO2 on peripheral chemoreceptors
C. pH on peripheral chemoreceptors
D. pH of CSF on central chemoreceptors (Correct Answer)

Explanation: **Explanation:** The primary drive for resting ventilation is the concentration of **hydrogen ions (H+) in the brain extracellular fluid**, which is directly reflected by the **pH of the Cerebrospinal Fluid (CSF)**. **Why Option D is Correct:** Central chemoreceptors, located on the ventral surface of the medulla, are exquisitely sensitive to changes in pH. While CO2 can easily cross the blood-brain barrier (BBB), H+ and HCO3- cannot. Once CO2 enters the CSF, it reacts with water to form carbonic acid, which dissociates into H+ and HCO3-. The resulting increase in H+ (drop in pH) stimulates the central chemoreceptors, accounting for approximately **70-80% of the respiratory drive** under normal resting conditions. **Why Other Options are Incorrect:** * **Option A:** PO2 only becomes a major stimulus for ventilation when it drops significantly (Hypoxic drive, <60 mmHg). In healthy individuals at rest, oxygen levels play a minimal role. * **Option B & C:** Peripheral chemoreceptors (Carotid and Aortic bodies) do respond to PCO2 and pH. However, they are responsible for only about **20-30%** of the ventilatory response to CO2. They act faster than central receptors but are not the *most important* for resting ventilation. **High-Yield NEET-PG Pearls:** * **Main Stimulus:** The central chemoreceptor responds to **H+**, but the stimulus that triggers it from the blood is **CO2** (because only CO2 crosses the BBB). * **Location:** Central chemoreceptors are in the **medulla**; Peripheral chemoreceptors are in the **Carotid bodies** (CN IX) and **Aortic bodies** (CN X). * **CO2 Narcosis:** In chronic CO2 retainers (like COPD patients), the central receptors become desensitized, and respiration becomes driven by PO2 (Hypoxic drive). Giving high-flow oxygen to these patients can paradoxically cause respiratory arrest.

Question 653: In caisson disease, joint pain is caused by which of the following?

A. Nitrogen bubbles (Correct Answer)
B. Oxygen bubbles
C. Carbon monoxide
D. Air in the joint

Explanation: **Explanation:** **Caisson Disease** (also known as Decompression Sickness, "the bends," or diver’s paralysis) occurs due to rapid ascent from high-pressure environments (like deep-sea diving or pressurized tunnels). **1. Why Nitrogen bubbles is correct:** According to **Henry’s Law**, the solubility of a gas in a liquid is proportional to its partial pressure. At high depths, the increased atmospheric pressure causes large amounts of **Nitrogen** (which is physiologically inert) to dissolve into body tissues and lipids. During a rapid ascent, the pressure drops quickly, and the dissolved nitrogen comes out of solution, forming **bubbles** in the blood and tissues. When these bubbles accumulate in the joints and periarticular capillaries, they cause ischemia and mechanical stretching of nerve endings, leading to the characteristic severe joint pain known as "the bends." **2. Why the other options are incorrect:** * **Oxygen bubbles:** Oxygen is rapidly metabolized by tissues and bound to hemoglobin; it does not remain dissolved in quantities sufficient to form bubbles during decompression. * **Carbon monoxide:** This is a toxic gas that binds to hemoglobin (forming carboxyhemoglobin) and interferes with oxygen transport; it is not involved in decompression sickness. * **Air in the joint:** While bubbles are present, it is specifically the expansion of dissolved nitrogen gas, not "room air" or atmospheric air introduced into the joint space, that causes the pathology. **Clinical Pearls for NEET-PG:** * **Type I Decompression Sickness:** Involves "the bends" (joint pain) and "the itches" (skin involvement). * **Type II Decompression Sickness:** More severe; includes "the chokes" (pulmonary edema/shortness of breath) and neurological deficits (staggers). * **Treatment:** Hyperbaric oxygen therapy (recompression). * **Prevention:** Slow ascent with decompression stops to allow nitrogen to be exhaled gradually.

Question 654: Oxygen delivery to tissues is decreased by:

A. Increased hemoglobin level (Correct Answer)
B. Increased HCO3
C. Increased PaCO2
D. All of the above

Explanation: **Explanation:** The delivery of oxygen to tissues depends on the **Oxygen Delivery Index ($DO_2$)**, which is calculated by the formula: **$DO_2 = \text{Cardiac Output (CO)} \times \text{Arterial Oxygen Content } (CaO_2)$** 1. **Why Option A is Correct:** While hemoglobin (Hb) carries oxygen, an excessive increase in hemoglobin (as seen in Polycythemia) significantly increases **blood viscosity**. According to Poiseuille’s Law, increased viscosity leads to increased peripheral resistance and a subsequent **decrease in Cardiac Output**. When the rise in viscosity outweighs the oxygen-carrying capacity, the net oxygen delivery to tissues decreases. 2. **Why Options B and C are Incorrect:** * **Increased $PaCO_2$ (Hypercapnia):** An increase in $CO_2$ causes a **rightward shift** of the Oxygen-Dissociation Curve (Bohr Effect). This decreases the affinity of Hb for oxygen, actually **facilitating** the unloading and delivery of oxygen to the tissues. * **Increased $HCO_3$:** This typically reflects a metabolic alkalosis or a compensatory response to respiratory acidosis. While alkalosis shifts the curve to the left (increasing affinity), it does not inherently decrease the total delivery index in the same systemic manner as increased viscosity. **High-Yield Clinical Pearls for NEET-PG:** * **The Bohr Effect:** Shift to the **Right** (facilitates delivery) is caused by: **↑** $CO_2$, **↑** H+ (Acidosis), **↑** 2,3-DPG, and **↑** Temperature (**CADET**, face Right!). * **Optimal Hematocrit:** For maximal oxygen delivery, a hematocrit of approximately 40-45% is ideal; beyond this, viscosity becomes the limiting factor. * **Formula for $CaO_2$:** $(1.34 \times Hb \times SaO_2) + (0.003 \times PaO_2)$. Note that dissolved oxygen ($PaO_2$) contributes minimally compared to Hb-bound oxygen.

Question 655: Lung compliance is increased in which of the following conditions?

A. The presence of intra-alveolar fluid
B. Acute respiratory distress syndrome
C. Idiopathic pulmonary fibrosis
D. Emphysema (Correct Answer)

Explanation: **Explanation:** **Compliance** is defined as the change in lung volume per unit change in transpulmonary pressure ($C = \Delta V / \Delta P$). It represents the "stretchability" or ease with which the lungs expand. **Why Emphysema is Correct:** In **Emphysema**, there is permanent destruction of the alveolar walls and loss of **elastic recoil** due to the breakdown of elastin fibers (often by elastase). Since elasticity and compliance are inversely related, a loss of elastic "snap-back" makes the lung highly distensible. Therefore, the lung expands very easily at low pressures, resulting in **increased compliance**. **Why Other Options are Incorrect:** * **A. Intra-alveolar fluid:** Fluid (as seen in pulmonary edema) increases surface tension and occupies air space, making the lungs stiffer and **decreasing** compliance. * **B. Acute Respiratory Distress Syndrome (ARDS):** ARDS involves a massive inflammatory response and loss of surfactant. Increased surface tension and "wet lungs" lead to a significant **decrease** in compliance (often called "stiff lungs"). * **C. Idiopathic Pulmonary Fibrosis:** This is a restrictive lung disease where healthy lung tissue is replaced by thick, scarred collagen fibers. This increases the work required to expand the lungs, thereby **decreasing** compliance. **High-Yield Clinical Pearls for NEET-PG:** * **Static Compliance** is primarily affected by the elastic properties of the lung and surface tension. * **Surfactant** increases compliance by reducing alveolar surface tension. * **Aging:** Compliance increases with age due to the natural loss of elastic fibers. * **Pressure-Volume Loop:** In Emphysema, the curve shifts **upward and to the left** (higher volume for lower pressure). In Fibrosis, it shifts **downward and to the right**.

Question 656: What is the typical tidal volume in ml?

A. 300
B. 400
C. 900
D. None of the above (Correct Answer)

Explanation: **Explanation:** The **Tidal Volume (TV)** is the volume of air inspired or expired during a single breath under normal, resting conditions. In a healthy young adult male, the standard tidal volume is approximately **500 ml**. **Why "None of the above" is correct:** The options provided (300 ml, 400 ml, and 900 ml) do not represent the physiological norm for tidal volume. Since the standard value is 500 ml (or roughly 6–8 ml/kg of ideal body weight), none of the specific numerical choices are accurate. **Analysis of Incorrect Options:** * **A (300 ml) & B (400 ml):** These values are lower than the average resting TV. Such volumes might be seen in restrictive lung diseases or shallow breathing but do not represent the "typical" value. * **C (900 ml):** This is significantly higher than the resting TV. A volume of 900 ml would be more characteristic of an increased depth of breathing (hyperpnea) during mild exertion. **High-Yield Clinical Pearls for NEET-PG:** * **Anatomical Dead Space:** Out of the 500 ml of TV, approximately **150 ml** remains in the conducting airways (dead space) and does not participate in gas exchange. Only **350 ml** reaches the alveoli. * **Minute Ventilation:** Calculated as $TV \times \text{Respiratory Rate}$. (e.g., $500 \text{ ml} \times 12 \text{ bpm} = 6000 \text{ ml/min}$). * **Alveolar Ventilation:** A more accurate measure of gas exchange: $(TV - \text{Dead Space}) \times \text{Respiratory Rate}$. * **Measurement:** Tidal volume is measured using a **Spirometer**, though it cannot measure Residual Volume (RV) or Functional Residual Capacity (FRC).

Question 657: Which of the following does NOT cause a rightward shift of the oxyhemoglobin dissociation curve?

A. Increased hydrogen ions
B. Decreased carbon dioxide (Correct Answer)
C. Increased temperature
D. Increased 2,3-bisphosphoglycerate (BPG)

Explanation: The oxyhemoglobin dissociation curve (ODC) represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **rightward shift** indicates a decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to the tissues. ### Why "Decreased Carbon Dioxide" is the Correct Answer A **decrease** in $PCO_2$ (hypocapnia) causes a **leftward shift** of the curve. According to the **Bohr Effect**, lower levels of $CO_2$ increase hemoglobin's affinity for oxygen, making it harder for oxygen to be released into the tissues. This typically occurs in conditions like hyperventilation or at high altitudes (initially). ### Analysis of Incorrect Options (Causes of Rightward Shift) All other options decrease hemoglobin's affinity for oxygen, shifting the curve to the **Right** (Mnemonic: **"CADET, face Right!"**): * **A. Increased Hydrogen ions (Low pH/Acidosis):** Higher acidity stabilizes the T-state (tense) of hemoglobin, promoting oxygen release. * **C. Increased Temperature:** Hyperthermia (e.g., during exercise or fever) weakens the bond between hemoglobin and oxygen. * **D. Increased 2,3-BPG:** This byproduct of glycolysis binds to the beta chains of deoxyhemoglobin, stabilizing the T-state and pushing oxygen off the molecule. ### High-Yield Clinical Pearls for NEET-PG * **Left Shift Causes:** Decreased $H^+$ (Alkalosis), decreased $PCO_2$, decreased Temperature, decreased 2,3-BPG, and **Fetal Hemoglobin (HbF)**, **Carboxyhemoglobin**, and **Methemoglobin**. * **HbF:** Fetal hemoglobin has a higher affinity for $O_2$ than adult hemoglobin (HbA) to facilitate oxygen transfer across the placenta; thus, its curve is always to the **left** of the adult curve. * **P50 Value:** The $PO_2$ at which 50% of hemoglobin is saturated. A right shift **increases** the P50, while a left shift **decreases** it.

Question 658: Curve A signifies which of the following?

A. Pulmonary fibrosis
B. Atelectasis
C. Emphysema (Correct Answer)
D. ARDS

Explanation: ***Emphysema*** - Curve A shows **increased lung compliance** with an upward-left shift, characteristic of emphysema due to **destruction of alveolar walls** and reduced **elastic recoil**. - The **loss of elastic tissue** allows lungs to expand more easily at lower pressures, resulting in higher lung volumes at any given pressure. *Pulmonary fibrosis* - Shows **decreased compliance** with a downward-right shift on pressure-volume curves due to **increased collagen deposition** and lung stiffness. - Requires **higher pressures** to achieve the same lung volumes, opposite to what Curve A demonstrates. *Atelectasis* - Characterized by **collapsed alveoli** leading to **reduced lung volumes** and decreased compliance. - Pressure-volume curve would shift **downward-right**, showing smaller volumes at given pressures, not the pattern seen in Curve A. *ARDS* - Results in **severely decreased compliance** due to **inflammatory fluid accumulation** and **surfactant dysfunction**. - Shows markedly **reduced lung volumes** and rightward shift, requiring much higher pressures for ventilation.

Question 659: What is the normal functional residual capacity?

A. 2.3 L (Correct Answer)
B. 1.3 L
C. 2.9 L
D. 4.5 L

Explanation: **Explanation:** **Functional Residual Capacity (FRC)** is the volume of air remaining in the lungs at the end of a normal, quiet expiration (tidal expiration). It represents the equilibrium point where the inward elastic recoil of the lungs exactly balances the outward chest wall recoil. **Why 2.3 L is correct:** In a healthy adult male of average build, the FRC is approximately **2.2 to 2.4 Liters** (average **2.3 L**). It is calculated as the sum of **Expiratory Reserve Volume (ERV ≈ 1.1 L)** and **Residual Volume (RV ≈ 1.2 L)**. **Analysis of Incorrect Options:** * **1.3 L (Option B):** This value is close to the normal **Residual Volume (RV)** alone, which is the air remaining after a maximal forced expiration. * **2.9 L (Option C):** This is higher than the average FRC. FRC can increase in obstructive diseases like emphysema due to air trapping, but it is not the "normal" physiological value. * **4.5 L (Option D):** This value approximates the **Vital Capacity (VC)**, which is the maximum volume of air a person can expel from the lungs after a maximum inhalation. **High-Yield NEET-PG Pearls:** 1. **Measurement:** FRC cannot be measured by simple spirometry because it contains the Residual Volume. It is measured via **Helium Dilution**, **Nitrogen Washout**, or **Body Plethysmography**. 2. **Clinical Significance:** FRC acts as a buffer to prevent large fluctuations in alveolar gas tensions ($PO_2$ and $PCO_2$) during the respiratory cycle. 3. **Positioning:** FRC **decreases** when moving from a standing to a supine position (due to abdominal contents pushing against the diaphragm). 4. **Anesthesia:** FRC decreases significantly during general anesthesia, which can lead to atelectasis and shunting.

Question 660: What type of blood flow is typically seen in the apex of the lung?

A. No flow, alveolar pressure greater than pulmonary capillary pressure
B. Intermittent flow, alveolar pressure less than pulmonary capillary pressure during diastole
C. Intermittent flow, alveolar pressure less than pulmonary capillary pressure during systole (Correct Answer)
D. Continuous flow, alveolar pressure less than pulmonary capillary pressure

Explanation: ### Explanation This question pertains to **West Zones of the Lung**, which describe the relationship between alveolar pressure ($P_A$), arterial pressure ($P_a$), and venous pressure ($P_v$) influenced by gravity. **1. Why Option C is Correct:** In a healthy, upright individual, the apex of the lung corresponds to **Zone 2**. Here, blood flow is **intermittent**. * **During Systole:** Pulmonary arterial pressure rises enough to exceed alveolar pressure ($P_a > P_A$), allowing blood to flow. * **During Diastole:** Arterial pressure drops below alveolar pressure ($P_A > P_a$), causing the capillaries to be compressed and flow to stop. Thus, flow occurs only when $P_a$ is high enough to overcome the "Starling Resistor" effect of the alveoli. **2. Analysis of Incorrect Options:** * **Option A (Zone 1):** This represents "Dead Space" where $P_A > P_a > P_v$. Under normal physiological conditions, Zone 1 does **not** exist. It only occurs during severe hemorrhage (low $P_a$) or positive pressure ventilation (high $P_A$). * **Option B:** This is physiologically reversed. Flow occurs during systole (high pressure), not diastole. * **Option D (Zone 3):** This describes the **base of the lung**. Here, gravity increases hydrostatic pressure such that $P_a > P_v > P_A$. Since arterial pressure always exceeds alveolar pressure, flow is **continuous**. **3. High-Yield Clinical Pearls for NEET-PG:** * **Zone 1 (Top):** $P_A > P_a > P_v$ (No flow; Alveolar Dead Space). * **Zone 2 (Middle):** $P_a > P_A > P_v$ (Intermittent flow; Waterfall effect). * **Zone 3 (Bottom):** $P_a > P_v > P_A$ (Continuous flow; Maximum flow). * **V/Q Ratio:** Highest at the **apex** (~3.3) and lowest at the **base** (~0.6). * **Tuberculosis:** *M. tuberculosis* prefers the apex because the high V/Q ratio results in higher local $PO_2$, favoring the growth of this aerobe.

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