Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 461: Which of the following statements regarding zones of blood flow in the lung is true?

A. When changing from a standing to a supine position, the entire lung exhibits zone II blood flow.
B. During intermittent positive pressure ventilation (IPPV), the entire lung is converted to zone III.
C. In an upright posture, zone III is absent throughout the lung.
D. In mitral stenosis, the apex of the lung may be converted to zone III. (Correct Answer)

Explanation: **Explanation:** The distribution of pulmonary blood flow is determined by the relationship between alveolar pressure (**PA**), arterial pressure (**Pa**), and venous pressure (**Pv**), as described by West’s Zones. **Why Option D is Correct:** In **Mitral Stenosis**, there is an increase in left atrial pressure, which leads to a retrograde increase in pulmonary venous pressure (**Pv**). In a normal upright lung, the apex is typically Zone 1 or 2. However, when **Pv** rises significantly (as in mitral stenosis or left heart failure), it can exceed alveolar pressure even at the apex. This converts the apex into **Zone 3 (Pa > Pv > PA)**, characterized by continuous blood flow throughout the cardiac cycle. **Analysis of Incorrect Options:** * **Option A:** In a **supine position**, the effect of gravity is neutralized along the vertical axis of the lung. Most of the lung converts to **Zone 3**, not Zone 2, because the hydrostatic pressure in the vessels exceeds alveolar pressure. * **Option B:** During **IPPV**, alveolar pressure (**PA**) is increased. This tends to compress pulmonary capillaries, potentially converting Zone 2 or 3 areas into **Zone 1 (PA > Pa > Pv)**, especially in hypovolemic patients. * **Option C:** In a healthy **upright posture**, the base of the lung is always **Zone 3** due to gravity increasing hydrostatic pressure in the lower pulmonary vessels. **High-Yield Pearls for NEET-PG:** * **Zone 1:** (PA > Pa > Pv) – Functional dead space; usually absent in healthy individuals but occurs in hemorrhage or positive pressure ventilation. * **Zone 2:** (Pa > PA > Pv) – "Waterfall effect"; flow is determined by the Pa-PA gradient. * **Zone 3:** (Pa > Pv > PA) – Continuous flow; the physiological state of the lung base. * **Zone 4:** Occurs at extremely low lung volumes (base of the lung) where interstitial pressure compresses extra-alveolar vessels, reducing flow.

Question 462: Which of the following best characterizes the pulmonary circulation?

A. Flow - Low, Pressure - High, Resistance - Low, Compliance - High
B. Flow - High, Pressure - Low, Resistance - High, Compliance - Low
C. Flow - Low, Pressure - Low, Resistance - Low, Compliance - High
D. Flow - High, Pressure - Low, Resistance - Low, Compliance - High (Correct Answer)

Explanation: **Explanation:** The pulmonary circulation is a unique vascular system designed to facilitate gas exchange by receiving the entire cardiac output from the right ventricle. 1. **Flow (High):** The pulmonary circulation receives 100% of the cardiac output (approx. 5 L/min). Therefore, the flow is equal to that of the systemic circulation, which is considered "High." 2. **Pressure (Low):** Unlike the systemic system, the pulmonary system is a low-pressure circuit. The mean pulmonary arterial pressure is only about 15 mmHg (compared to 93 mmHg in the aorta). This prevents pulmonary edema and reduces the workload on the right ventricle. 3. **Resistance (Low):** According to Ohm’s Law ($R = \Delta P / Q$), because the pressure gradient ($\Delta P$) is low and the flow ($Q$) is high, the resistance must be significantly lower (about 1/10th of systemic resistance). This is due to shorter, wider vessels and a massive capillary network. 4. **Compliance (High):** Pulmonary vessels are thin-walled and contain less smooth muscle, making them highly distensible. This high compliance allows the lungs to accommodate increases in stroke volume without a significant rise in pressure. **Analysis of Incorrect Options:** * **Option A & C:** Incorrect because flow is high (equal to systemic CO), not low. * **Option B:** Incorrect because resistance is low and compliance is high in the pulmonary circuit. High resistance is a characteristic of the systemic arterial system. **NEET-PG High-Yield Pearls:** * **Recruitment and Distension:** These are the two primary mechanisms by which pulmonary vascular resistance (PVR) decreases further when cardiac output increases (e.g., during exercise). * **Hypoxic Pulmonary Vasoconstriction (HPV):** Unlike systemic vessels which dilate in response to hypoxia, pulmonary vessels **constrict**. This shunts blood away from poorly ventilated alveoli to well-ventilated ones (V/Q matching). * **West Zones:** Blood flow in the lungs is gravity-dependent, being highest at the base (Zone 3) and lowest at the apex (Zone 1).

Question 463: Arterial PO2 is decreased in hypoxia due to which of the following conditions?

A. Cyanide poisoning
B. Carbon monoxide poisoning
C. COPD (Chronic Obstructive Pulmonary Disease) (Correct Answer)
D. Shock

Explanation: **Explanation:** The core concept in this question is differentiating between the types of hypoxia based on **Arterial Oxygen Tension ($PaO_2$)**. **1. Why COPD is Correct:** COPD causes **Hypoxic Hypoxia**. In COPD, structural damage to the alveoli (emphysema) and airway obstruction (bronchitis) lead to ventilation-perfusion ($V/Q$) mismatch and impaired gas exchange. This results in a failure to oxygenate the blood in the lungs, leading to a **decreased $PaO_2$**. **2. Why the other options are incorrect:** * **Cyanide Poisoning (Histotoxic Hypoxia):** The $PaO_2$ and oxygen content are normal. Hypoxia occurs because cyanide inhibits *cytochrome oxidase* in the mitochondria, preventing tissues from utilizing the oxygen delivered to them. * **Carbon Monoxide Poisoning (Anemic Hypoxia):** CO binds to hemoglobin with high affinity, reducing oxygen-carrying capacity. However, since $PaO_2$ represents oxygen dissolved in plasma (not bound to Hb), the **$PaO_2$ remains normal**. * **Shock (Stagnant Hypoxia):** In shock, the $PaO_2$ is typically normal initially. Hypoxia occurs because of reduced cardiac output and slow blood flow, leading to inadequate delivery of oxygen to tissues despite normal arterial oxygenation. **High-Yield NEET-PG Pearls:** * **Hypoxic Hypoxia** (e.g., High altitude, COPD, Hypoventilation) is the **only** type where $PaO_2$ is decreased. * In **Anemic Hypoxia**, $PaO_2$ is normal, but total oxygen content ($CaO_2$) is decreased. * **Pulse Oximetry ($SpO_2$)** is falsely normal or high in CO poisoning because the sensor cannot distinguish between carboxyhemoglobin and oxyhemoglobin. * **A-a Gradient:** Useful for narrowing down causes of decreased $PaO_2$; it is normal in hypoventilation/high altitude but increased in COPD/V-Q mismatch.

Question 464: The functional residual capacity is best defined as the sum of which two lung volumes?

A. Tidal volume and Inspiratory Reserve volume
B. Residual Volume and Expiratory Reserve Volume (Correct Answer)
C. Residual Volume and Inspiratory Reserve Volume
D. Tidal volume, Inspiratory Reserve volume, and Expiratory Reserve Volume

Explanation: **Explanation** **Functional Residual Capacity (FRC)** is the volume of air remaining in the lungs at the end of a normal, quiet expiration (at the end of a tidal breath). It represents the equilibrium point where the inward elastic recoil of the lungs exactly balances the outward chest wall expansion. 1. **Why Option B is Correct:** FRC is the sum of **Expiratory Reserve Volume (ERV)** and **Residual Volume (RV)**. * **ERV:** The additional volume that can be exhaled forcefully after a normal tidal expiration. * **RV:** The volume remaining in the lungs after maximal forced expiration (which cannot be measured by simple spirometry). Mathematically: **FRC = ERV + RV**. 2. **Analysis of Incorrect Options:** * **Option A:** Tidal Volume (TV) + Inspiratory Reserve Volume (IRV) = **Inspiratory Capacity (IC)**. * **Option C:** This combination does not represent a standard physiological capacity. * **Option D:** TV + IRV + ERV = **Vital Capacity (VC)**. This is the maximum volume of air a person can expel from the lungs after a maximum inhalation. **High-Yield Clinical Pearls for NEET-PG:** * **Measurement:** FRC cannot be measured by simple spirometry because it contains Residual Volume. It is measured via **Helium Dilution**, **Nitrogen Washout**, or **Body Plethysmography** (the gold standard). * **Clinical Significance:** FRC acts as a "buffer" for gas exchange, preventing large fluctuations in O₂ and CO₂ levels during the breathing cycle. * **Pathology:** FRC is **increased** in obstructive diseases (e.g., Emphysema due to air trapping) and **decreased** in restrictive diseases (e.g., Pulmonary Fibrosis) and conditions like obesity or pregnancy.

Question 465: Dead space is increased by all, except:

A. Anticholinergic drugs
B. Standing
C. Hyperextension of neck
D. Endotracheal intubation (Correct Answer)

Explanation: **Explanation:** **Anatomical dead space** refers to the volume of the conducting airways (from the nose/mouth down to the terminal bronchioles) where no gas exchange occurs. The correct answer is **Endotracheal intubation** because it **decreases** anatomical dead space. 1. **Why Endotracheal Intubation is correct:** An endotracheal tube bypasses the upper respiratory tract (nose, pharynx, and larynx), which accounts for a significant portion of the anatomical dead space. By providing a direct, shorter route to the trachea, the total volume of non-perfused conducting zone is reduced. (Note: Tracheostomy also decreases dead space for the same reason). 2. **Why the other options are incorrect:** * **Anticholinergic drugs (e.g., Atropine):** These drugs cause bronchodilation. By increasing the diameter of the conducting airways, the internal volume (dead space) increases. * **Standing:** Due to gravity, there is increased ventilation-perfusion (V/Q) mismatch at the apices of the lungs. Alveoli at the apex are ventilated but poorly perfused, increasing **physiological dead space**. * **Hyperextension of neck:** This physical maneuver stretches and widens the caliber of the upper airway, thereby increasing the anatomical dead space volume. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Dead Space:** Approximately **2 ml/kg** or 150 ml in a healthy adult. * **Fowler’s Method:** Used to measure **Anatomical** dead space (using Nitrogen washout). * **Bohr’s Equation:** Used to measure **Physiological** dead space (using $CO_2$ levels). * **Physiological Dead Space = Anatomical + Alveolar Dead Space.** In healthy individuals, physiological and anatomical dead space are nearly equal.

Question 466: Decreased oxygen carrying capacity of blood with normal pO2 in arterial blood is a feature of:

A. Carbon monoxide poisoning
B. COPD
C. Hypoxic hypoxia
D. Anemic hypoxia (Correct Answer)

Explanation: **Explanation:** The correct answer is **Anemic hypoxia**. This condition is characterized by a decrease in the total amount of hemoglobin (Hb) available to carry oxygen, or a reduction in the ability of Hb to bind oxygen, despite the lungs functioning normally. **1. Why Anemic Hypoxia is correct:** In anemic hypoxia, the **Arterial $pO_2$ (partial pressure of dissolved oxygen)** remains **normal** because $pO_2$ depends solely on the diffusion of oxygen from the alveoli into the plasma, which is unaffected. However, the **Oxygen Content** of the blood is significantly reduced because the majority of oxygen is carried bound to hemoglobin. Since Hb levels are low, the total oxygen-carrying capacity is decreased. **2. Analysis of Incorrect Options:** * **Carbon Monoxide (CO) Poisoning:** While this also features normal $pO_2$ and decreased oxygen content, it is technically a form of anemic hypoxia (functional anemia). However, "Anemic hypoxia" is the broader, more definitive category for this physiological state. * **Hypoxic Hypoxia:** This is characterized by a **decreased arterial $pO_2$**, usually due to high altitude or hypoventilation. * **COPD:** This leads to hypoxic hypoxia due to ventilation-perfusion mismatch, resulting in a **low arterial $pO_2$**. **High-Yield Clinical Pearls for NEET-PG:** * **$pO_2$** measures only oxygen dissolved in plasma (approx. 2-3%), not oxygen bound to Hb. * **Anemic Hypoxia Causes:** Anemia, Hemorrhage, and Methemoglobinemia. * **Cyanosis** is usually **absent** in anemic hypoxia because there isn't enough total hemoglobin to reach the threshold of 5g/dL of deoxygenated Hb required to see the blue tint. * **Key Distinction:** In CO poisoning, the $O_2$ dissociation curve shifts to the **left**, making it harder for tissues to offload oxygen.

Question 467: The oxygen buffer function of hemoglobin is related to which of the following?

A. Dissociation curve shape (Correct Answer)
B. Haldane effect
C. Bohr effect
D. Respiratory exchange ratio

Explanation: ### Explanation The **oxygen buffer function of hemoglobin** refers to the ability of hemoglobin to maintain a relatively constant partial pressure of oxygen ($PO_2$) in the tissues, even when the atmospheric oxygen supply or metabolic demands fluctuate. **Why the correct answer is right:** This buffering capacity is primarily due to the **sigmoid (S-shaped) Oxygen-Hemoglobin Dissociation Curve**. 1. **In the Lungs:** Even if alveolar $PO_2$ drops significantly (e.g., from 100 to 60 mmHg), the flat upper portion of the curve ensures that hemoglobin remains nearly 90% saturated. 2. **In the Tissues:** The steep portion of the curve allows for the release of large amounts of oxygen with only a small drop in $PO_2$. If tissue $PO_2$ falls slightly below the normal 40 mmHg, hemoglobin immediately releases a massive amount of $O_2$ to stabilize the tissue $PO_2$. This "automatic" regulation keeps tissue $PO_2$ within a narrow range (approx. 15–40 mmHg). **Why the other options are wrong:** * **Haldane effect:** Describes how the deoxygenation of blood increases its ability to carry $CO_2$. It is related to $CO_2$ transport, not $O_2$ buffering. * **Bohr effect:** Refers to the shift of the curve to the right due to increased $CO_2$ or $H^+$, enhancing $O_2$ delivery. While it aids delivery, it is a *shift* mechanism rather than the inherent *buffering* property of the curve's shape. * **Respiratory exchange ratio (R):** This is the ratio of $CO_2$ produced to $O_2$ consumed ($VCO_2/VO_2$) and is a metabolic parameter, not a hemoglobin function. **High-Yield Clinical Pearls for NEET-PG:** * **$P_{50}$:** The $PO_2$ at which hemoglobin is 50% saturated (Normal: **26-27 mmHg**). An increase in $P_{50}$ indicates a right shift (decreased affinity). * **Sigmoid Shape:** Due to **positive cooperativity** (binding of one $O_2$ molecule increases the affinity for the next). * **Myoglobin:** Has a **hyperbolic** curve, making it a great storage unit but a poor buffer/transporter compared to hemoglobin.

Question 468: Which spirometry feature is characteristic of asthma?

A. FEV shows an upward trend
B. FEV1/FEV ratio shows a downward trend (Correct Answer)
C. PEF shows an upward trend
D. RV shows a downward trend

Explanation: **Explanation:** Asthma is a classic example of an **obstructive lung disease** characterized by reversible airway narrowing. The hallmark of any obstructive pathology is an increased resistance to airflow, which is most pronounced during expiration. **Why Option B is Correct:** In obstructive diseases like asthma, the **FEV1** (Forced Expiratory Volume in 1 second) decreases significantly more than the **FVC** (Forced Vital Capacity). Because the numerator (FEV1) drops more drastically than the denominator (FVC), the **FEV1/FVC ratio decreases** (typically <70%). This downward trend is the most sensitive spirometric indicator of airway obstruction. **Analysis of Incorrect Options:** * **Option A:** FEV1 (or FEV) shows a **downward** trend during an asthma attack due to bronchoconstriction and mucus plugging. * **Option C:** **PEF (Peak Expiratory Flow)** measures the maximum speed of expiration. In asthma, PEF shows a **downward** trend. Monitoring PEF variability is a key tool for assessing asthma severity and control. * **Option D:** **RV (Residual Volume)** actually shows an **upward** trend in asthma. This occurs due to "air trapping," where narrowed airways close prematurely during expiration, leaving more air behind in the lungs. **High-Yield Clinical Pearls for NEET-PG:** * **Reversibility Criteria:** A definitive diagnosis of asthma via spirometry requires an improvement in FEV1 of **>12% and >200 mL** after inhalation of a short-acting beta-agonist (e.g., Salbutamol). * **Flow-Volume Loop:** In asthma, the expiratory limb of the loop typically shows a **"scooped-out"** appearance. * **DLCO:** Unlike Emphysema (where DLCO is decreased), DLCO in Asthma is usually **normal or slightly increased**.

Question 469: Conversion of angiotensin-I to angiotensin-II occurs in which organ?

A. Kidney
B. Lung (Correct Answer)
C. Liver
D. RBCs

Explanation: **Explanation:** The conversion of Angiotensin-I to Angiotensin-II is a critical step in the **Renin-Angiotensin-Aldosterone System (RAAS)**. This reaction is catalyzed by the **Angiotensin-Converting Enzyme (ACE)**. 1. **Why Lungs are correct:** While ACE is present in various vascular beds, its highest concentration is found on the **luminal surface of the vascular endothelial cells of the lungs**. As the entire cardiac output passes through the pulmonary circulation, the lungs serve as the primary site for this conversion. 2. **Why other options are incorrect:** * **Kidney:** The kidneys produce **Renin** (from juxtaglomerular cells), which converts Angiotensinogen to Angiotensin-I. They do not primarily convert Ang-I to Ang-II. * **Liver:** The liver synthesizes **Angiotensinogen**, the precursor protein. * **RBCs:** Red blood cells are not involved in the enzymatic pathways of the RAAS. **High-Yield NEET-PG Pearls:** * **Dual Function of ACE:** ACE is also known as **Kininase II**. Besides producing Angiotensin-II (a potent vasoconstrictor), it is responsible for the **degradation of Bradykinin** (a vasodilator). * **Clinical Correlation:** ACE Inhibitors (e.g., Enalapril) lead to an accumulation of Bradykinin in the lungs, which is the primary cause of the characteristic **dry cough** seen as a side effect. * **ACE2 vs. ACE:** ACE2 (a different enzyme) acts as the functional receptor for the **SARS-CoV-2** virus to enter host cells.

Question 470: Which reflex is associated with hyperinflation of the lung?

A. J-reflex (Correct Answer)
B. Cushing reflex
C. Bezold-Jaisch reflex
D. Bainbridge reflex

Explanation: ### Explanation **Correct Option: A. J-reflex** The **J-reflex (Juxtacapillary reflex)** is mediated by **J-receptors** located in the alveolar walls, in close proximity to the pulmonary capillaries. These receptors are sensory nerve endings of the unmyelinated **vagus nerve (C-fibers)**. They are stimulated by **hyperinflation of the lungs**, pulmonary edema, pulmonary congestion, or pneumonia. * **Mechanism:** Stimulation leads to a reflex response characterized by **apnea followed by rapid shallow breathing (tachypnea)**, bradycardia, and hypotension. It is a protective mechanism to prevent lung overdistension. **Why other options are incorrect:** * **B. Cushing reflex:** This is a physiological nervous system response to **increased intracranial pressure (ICP)**. It is characterized by the "Cushing triad": hypertension, bradycardia, and irregular respirations. * **C. Bezold-Jarisch reflex:** This involves receptors in the **ventricles of the heart**. It is triggered by noxious chemical stimuli or stretch, leading to a triad of bradycardia, hypotension, and apnea. * **D. Bainbridge reflex:** Also known as the atrial distension reflex, it occurs when an increase in venous return stretches the **right atrium**, leading to an **increase in heart rate** to pump the excess blood. **High-Yield Clinical Pearls for NEET-PG:** * **Hering-Breuer Inflation Reflex:** Often confused with the J-reflex, this is triggered by stretch receptors in the smooth muscles of the airways to **terminate inspiration** and prevent over-inflation. * **J-receptors** are specifically sensitive to **interstitial volume changes** (e.g., pulmonary edema), which is why patients with left heart failure experience rapid shallow breathing. * **Key Stimulant:** Chemicals like **Capsaicin** can also pharmacologically trigger J-receptors.

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