Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 711: Pulmonary endothelium is NOT concerned with which of the following?

A. Lipoprotein lipase
B. Plasminogen activator
C. Thrombin
D. Factor X (Correct Answer)

Explanation: **Explanation:** The pulmonary endothelium is not merely a passive barrier for gas exchange; it is a metabolically active organ responsible for the synthesis, activation, and inactivation of various substances. **Why Factor X is the correct answer:** **Factor X** is a clotting factor synthesized primarily in the **liver**. It is not produced, stored, or significantly metabolized by the pulmonary endothelium. While the lungs play a role in the coagulation cascade (e.g., through the production of tissue factor or thrombomodulin), Factor X itself is a systemic plasma protein of hepatic origin. **Analysis of Incorrect Options:** * **A. Lipoprotein Lipase (LPL):** The pulmonary capillaries contain high levels of LPL. This enzyme is responsible for the hydrolysis of circulating triglycerides into free fatty acids and glycerol, playing a key role in lipid metabolism. * **B. Plasminogen Activator:** Pulmonary endothelial cells synthesize and release **Tissue Plasminogen Activator (tPA)**. This is a vital fibrinolytic function that helps dissolve microthrombi, ensuring the pulmonary circulation remains patent. * **C. Thrombin:** The pulmonary endothelium interacts with thrombin through receptors like **thrombomodulin**. This interaction converts thrombin from a procoagulant to an anticoagulant (by activating Protein C), effectively "clearing" or modulating thrombin levels in the lungs. **High-Yield Facts for NEET-PG:** * **ACE Activity:** The pulmonary endothelium is the primary site for **Angiotensin-Converting Enzyme (ACE)**, which converts Angiotensin I to II and inactivates Bradykinin. * **Vasoactive Substances:** It inactivates **Serotonin, Norepinephrine, and Bradykinin**, but does *not* affect Epinephrine, Dopamine, or Histamine. * **Prostaglandins:** It inactivates Prostaglandins E and F, but synthesizes **Prostacyclin (PGI2)**, a potent vasodilator and inhibitor of platelet aggregation.

Question 712: An increase in the concentration of 2,3 DPG may be seen in all of the following, except?

A. Anemia
B. Hypoxia
C. Inosine
D. Hypoxanthine (Correct Answer)

Explanation: **Explanation:** The concentration of **2,3-Bisphosphoglycerate (2,3-DPG)** in red blood cells is a critical regulator of hemoglobin's affinity for oxygen. An increase in 2,3-DPG shifts the Oxygen-Dissociation Curve (ODC) to the **right**, facilitating the unloading of oxygen to tissues. **Why Hypoxanthine is the Correct Answer:** Hypoxanthine is a purine derivative and a breakdown product of adenosine monophosphate (AMP) metabolism. It does not play a role in the glycolytic pathway (Luebering-Rapoport shunt) where 2,3-DPG is synthesized. Therefore, it does not increase 2,3-DPG levels. **Analysis of Incorrect Options:** * **Anemia:** In anemia, the reduced hemoglobin concentration leads to tissue hypoxia. The body compensates by increasing 2,3-DPG production to enhance oxygen delivery to tissues. * **Hypoxia:** Chronic hypoxia (e.g., high altitude or chronic lung disease) stimulates 2,3-DPG production. This is a key adaptive mechanism to maintain oxygenation despite lower arterial oxygen tension. * **Inosine:** In blood banking, inosine is added to stored blood. It can be metabolized into ribose-5-phosphate and eventually into glycolytic intermediates (like glyceraldehyde-3-phosphate), which **increases** the synthesis of 2,3-DPG, thereby restoring the oxygen-carrying efficiency of stored blood. **High-Yield Clinical Pearls for NEET-PG:** * **Luebering-Rapoport Shunt:** The specific pathway in RBCs that produces 2,3-DPG. * **Right Shift Factors:** "CADET, face Right!" (**C**O2, **A**cidosis, **D**PG, **E**xercise, **T**emperature). * **Stored Blood:** 2,3-DPG levels **decrease** in stored blood over time, causing a left shift (increased O2 affinity). This is why inosine is used as a preservative. * **Fetal Hemoglobin (HbF):** Has a lower affinity for 2,3-DPG compared to HbA, resulting in a **left shift**, allowing the fetus to "pull" oxygen from maternal blood.

Question 713: Which of the following changes occur secondary to hypercarbia?

A. Miosis
B. Cool extremities
C. Bradycardia
D. Hypertension (Correct Answer)

Explanation: **Explanation:** Hypercarbia (or hypercapnia) refers to an abnormally high concentration of carbon dioxide ($CO_2$) in the blood. The physiological response to hypercarbia is primarily mediated by the **activation of the sympathetic nervous system**. **Why Hypertension is Correct:** Excess $CO_2$ acts as a potent stimulus for the central and peripheral chemoreceptors. This triggers the vasomotor center in the medulla to increase sympathetic outflow. The resulting release of catecholamines leads to peripheral vasoconstriction and increased cardiac output, manifesting clinically as **hypertension** and tachycardia. While $CO_2$ has a direct local vasodilatory effect on blood vessels, the systemic sympathetic response typically overrides this, leading to an overall rise in blood pressure. **Analysis of Incorrect Options:** * **Miosis:** Hypercarbia typically causes **mydriasis** (pupillary dilation) due to sympathetic overactivity. Miosis (pinpoint pupils) is more characteristic of opioid overdose or pontine hemorrhage. * **Cool extremities:** Hypercarbia causes peripheral vasodilation (direct effect) and increased skin blood flow, leading to **warm, flushed extremities** and a bounding pulse. * **Bradycardia:** The sympathetic surge usually results in **tachycardia**. Bradycardia is generally a late, pre-terminal sign of severe respiratory failure or CO2 narcosis. **High-Yield Clinical Pearls for NEET-PG:** * **CO2 Narcosis:** Extremely high levels of $PaCO_2$ (>70–80 mmHg) can lead to CNS depression, confusion, and coma. * **Cerebral Blood Flow:** $CO_2$ is a potent cerebral vasodilator. Hypercarbia increases intracranial pressure (ICP), which is why therapeutic hyperventilation (to lower $CO_2$) is used to acutely reduce ICP. * **Flapping Tremors (Asterixis):** A classic clinical sign of severe hypercapnia, often seen in COPD patients.

Question 714: When gases flow through an orifice, which factor is least likely to affect turbulence?

A. Density of gas
B. Viscosity of gas
C. Pressure of gas (Correct Answer)
D. Diameter of orifice

Explanation: The tendency of a gas to transition from laminar to turbulent flow is determined by the **Reynolds Number (Re)**. The formula for Reynolds Number is: $$Re = \frac{v \cdot d \cdot \rho}{\eta}$$ *(Where $v$ = velocity, $d$ = diameter, $\rho$ = density, and $\eta$ = viscosity)* ### 1. Why "Pressure of gas" is the correct answer: While pressure gradients drive gas flow, **pressure itself is not a direct variable** in the Reynolds Number equation. Turbulence is fundamentally a function of the physical properties of the fluid (density, viscosity) and the geometry of the airway (diameter). While increasing pressure can increase velocity ($v$), the question asks for the factor *least* likely to affect turbulence inherently. In the context of respiratory physiology, density and viscosity are the primary determinants of flow patterns. ### 2. Analysis of Incorrect Options: * **Density ($\rho$):** High-density gases increase the Reynolds number, promoting turbulence. This is why **Heliox** (low density) is used clinically to reduce turbulence in obstructed airways. * **Viscosity ($\eta$):** Viscosity represents the internal friction of the gas. Higher viscosity promotes laminar flow by "damping" out eddies. It is inversely proportional to the Reynolds number. * **Diameter ($d$):** Turbulence is more likely to occur in larger diameter airways (like the trachea) where the Reynolds number exceeds 2000. ### 3. Clinical Pearls for NEET-PG: * **Heliox Therapy:** A mixture of Helium and Oxygen. Helium has a much lower density than Nitrogen, which lowers the Reynolds number, converting turbulent flow into laminar flow. This reduces the work of breathing in conditions like **status asthmaticus** or **upper airway obstruction**. * **Flow Patterns:** Laminar flow occurs in small peripheral airways (low velocity, small diameter); Turbulent flow occurs in large central airways (high velocity, large diameter). * **Critical Velocity:** The velocity at which laminar flow converts to turbulent flow.

Question 715: Which of the following factors does NOT cause a rightward shift in the oxygen-hemoglobin dissociation curve?

A. Hypoxia
B. Hypocapnia (Correct Answer)
C. Increased temperature
D. Increased 2,3-DPG

Explanation: To understand the oxygen-hemoglobin (O2-Hb) dissociation curve, remember that a **rightward shift** indicates a decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to the tissues. Conversely, a **leftward shift** indicates increased affinity, meaning hemoglobin holds onto oxygen more tightly. ### Why Hypocapnia is the Correct Answer **Hypocapnia** refers to a decrease in the partial pressure of carbon dioxide ($PCO_2$) in the blood. According to the **Bohr Effect**, a decrease in $CO_2$ (and the resulting increase in pH/alkalinity) increases hemoglobin's affinity for oxygen, shifting the curve to the **left**. Therefore, it does not cause a rightward shift. ### Analysis of Incorrect Options (Factors causing a Right Shift) * **Hypoxia:** Chronic hypoxia (e.g., at high altitudes) stimulates the production of **2,3-DPG** in red blood cells, which binds to deoxygenated hemoglobin and stabilizes it, shifting the curve to the **right** to improve tissue oxygenation. * **Increased Temperature:** Higher temperatures (often seen in metabolically active tissues or fever) decrease the stability of the bond between $O_2$ and hemoglobin, shifting the curve to the **right**. * **Increased 2,3-DPG:** This byproduct of glycolysis competes for binding sites on hemoglobin. Higher levels decrease $O_2$ affinity, shifting the curve to the **right**. ### High-Yield Clinical Pearls for NEET-PG * **Mnemonic for Right Shift:** "**CADET**, face Right!" (**C**- $CO_2$ increase, **A**- Acidosis/H+, **D**- 2,3-DPG increase, **E**- Exercise, **T**- Temperature increase). * **$P_{50}$ Value:** A right shift increases the $P_{50}$ (the partial pressure of $O_2$ at which 50% of hemoglobin is saturated). Normal $P_{50}$ is ~26.7 mmHg. * **Fetal Hemoglobin (HbF):** Shifts the curve to the **left** compared to adult hemoglobin (HbA) because HbF does not bind 2,3-DPG effectively, ensuring the fetus can "strip" oxygen from maternal blood.

Question 716: Carbon dioxide (CO2) is primarily transported in the arterial blood as which of the following?

A. Dissolved CO2
B. Carbonic Acid
C. Carbamino-hemoglobin
D. Bicarbonate (Correct Answer)

Explanation: **Explanation:** Carbon dioxide (CO2) is a metabolic waste product that must be transported from the tissues to the lungs. In arterial blood, it exists in three distinct forms, but the distribution is unequal. **1. Why Bicarbonate (D) is Correct:** The majority of CO2 (**approximately 70%**) is transported as **Bicarbonate (HCO3⁻)**. This process occurs primarily within Red Blood Cells (RBCs), where the enzyme **Carbonic Anhydrase** catalyzes the reaction: $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$. The bicarbonate then leaves the RBC in exchange for Chloride ions (the **Chloride Shift** or Hamburger phenomenon), allowing for efficient transport in the plasma. **2. Why the other options are incorrect:** * **A. Dissolved CO2:** Only about **7%** of CO2 is transported physically dissolved in plasma. While small, this portion is crucial because it determines the partial pressure of CO2 ($PaCO_2$). * **B. Carbonic Acid:** This is a transient intermediate molecule. It is highly unstable and rapidly dissociates into $H^+$ and $HCO_3^-$; therefore, it is never a primary transport form. * **C. Carbamino-hemoglobin:** About **23%** of CO2 binds directly to the globin portion (amino groups) of the hemoglobin molecule. This binding is influenced by the **Haldane Effect** (deoxygenated blood has a higher affinity for CO2). **High-Yield NEET-PG Pearls:** * **Chloride Shift (Hamburger Phenomenon):** In systemic tissues, $Cl^-$ enters the RBC as $HCO_3^-$ leaves. In the lungs, this process reverses. * **Haldane Effect:** Oxygenation of Hb in the lungs promotes the dissociation of $CO_2$ from Hb. This is the most important factor for $CO_2$ uptake/release. * **Enzyme Fact:** Carbonic Anhydrase is one of the fastest known enzymes and is absent in plasma (it is found inside RBCs).

Question 717: Which of the following is a cause of shock?

A. Stagnant hypoxia (Correct Answer)
B. Anemic hypoxia
C. Hypoxic hypoxia
D. Histotoxic hypoxia

Explanation: **Explanation:** The core pathophysiology of **Shock** is defined as a state of generalized hypoperfusion where the delivery of oxygen and nutrients is insufficient to meet metabolic demands. **1. Why Stagnant Hypoxia is Correct:** Stagnant hypoxia (also known as ischemic hypoxia) occurs when the arterial oxygen content is normal, but the **blood flow to tissues is reduced**. In shock—whether cardiogenic (pump failure), hypovolemic (low volume), or obstructive—the cardiac output falls significantly. This leads to a "stagnant" flow of blood, preventing adequate oxygen delivery to the peripheral tissues despite normal hemoglobin levels and oxygen saturation. **2. Analysis of Incorrect Options:** * **Anemic Hypoxia:** Occurs when the oxygen-carrying capacity of the blood is reduced (e.g., anemia, CO poisoning). While it reduces oxygen delivery, it is not the primary mechanism defining shock. * **Hypoxic Hypoxia:** Characterized by low arterial $PO_2$ (e.g., high altitude, lung disease). The circulation is usually intact, but the blood itself is poorly oxygenated. * **Histotoxic Hypoxia:** Occurs when tissues cannot utilize oxygen despite adequate delivery (e.g., Cyanide poisoning). The blood flow and oxygen content are normal, but the cellular respiratory chain is inhibited. **Clinical Pearls for NEET-PG:** * **V/Q Mismatch:** The most common cause of Hypoxic Hypoxia. * **Cyanosis:** Stagnant hypoxia often presents with **peripheral cyanosis** (increased oxygen extraction at tissues), whereas hypoxic hypoxia presents with **central cyanosis**. * **Shock Hallmark:** The transition from aerobic to **anaerobic metabolism**, leading to lactic acidosis, is the metabolic hallmark of stagnant hypoxia in shock.

Question 718: What is the pressure gradient during inspiration?

A. Intrapleural pressure (Correct Answer)
B. Transpulmonary pressure
C. Trans-chest wall pressure
D. Alveolar pressure

Explanation: **Explanation:** The process of inspiration is driven by the creation of a pressure gradient between the atmosphere and the alveoli. The primary driver of this gradient is the change in **Intrapleural Pressure (IPP)**. **1. Why Intrapleural Pressure is the correct answer:** During inspiration, the diaphragm and external intercostal muscles contract, increasing the volume of the thoracic cavity. According to Boyle’s Law, as volume increases, pressure decreases. The IPP, which is already sub-atmospheric (approx. -5 cmH₂O), becomes **more negative** (dropping to approx. -7.5 cmH₂O). This "suction" effect pulls the lungs outward, expanding the alveoli and creating the negative alveolar pressure necessary for air to flow into the lungs. **2. Analysis of Incorrect Options:** * **Transpulmonary Pressure (Ptp):** This is the difference between alveolar and intrapleural pressure ($Ptp = Palv - Pip$). While it represents the force keeping the lungs inflated, it is a *resultant* pressure rather than the primary gradient initiator. * **Trans-chest wall pressure:** This is the difference between intrapleural pressure and atmospheric pressure. It relates to the elastic recoil of the chest wall, not the direct gradient for airflow. * **Alveolar pressure:** While alveolar pressure must become negative for air to enter, it is the *fluctuation* in intrapleural pressure that dictates this change. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Normal IPP:** It is always negative during quiet breathing due to the opposing elastic recoils of the lungs (inward) and chest wall (outward). * **Forced Expiration:** IPP can become **positive** during a forceful expiration or a Valsalva maneuver. * **Pneumothorax:** If the pleural cavity is breached, IPP equilibrates with atmospheric pressure, leading to lung collapse (atelectasis). * **Compliance:** The change in lung volume per unit change in transpulmonary pressure is known as lung compliance ($C = \Delta V / \Delta P$).

Question 719: Which of the following oxygen-sensitive channels is present in peripheral chemoreceptors?

A. Ca++
B. Na+
C. K+ (Correct Answer)
D. Cl-

Explanation: ***K+***- **Oxygen-sensitive K+ channels**, specifically members of the **TASK-like potassium channels** family, are central to the response of **glomus cells** in the carotid body to **hypoxia**.- When oxygen levels fall, these channels are **inhibited**, reducing K+ efflux and causing the cell membrane to **depolarize**, initiating the signaling cascade.*Na+*- While **voltage-gated Na+ channels** are essential for action potential generation in the afferent nerve, they are not the primary channels that directly sense changes in **pO2** within the glomus cells.- The initial depolarizing signal stems from the inhibition of K+ channels, not the activation or inhibition of Na+ channels.*Ca++*- **Voltage-gated Ca++ channels** open in response to the **depolarization** caused by K+ channel inhibition upon hypoxia.- The resulting **calcium influx** is mandatory for the final step: triggering the release of **neurotransmitters** (e.g., dopamine, ATP) that signal the brainstem.*Cl-*- **Cl- channels** (Chloride channels) are present in glomus cells and help regulate cell volume and membrane potential, but they do not function as the mechanism's primary oxygen sensor.- The entire chemosensing process is primarily governed by the modulation of **cation** movement (K+ efflux and subsequent Ca++ influx) rather than chloride flux.

Question 720: Which of the following factors causes a rightward shift in the oxygen-hemoglobin dissociation curve?

A. Increase in O₂
B. Increase in CO₂ (Correct Answer)
C. Decrease in CO₂
D. Decrease in temperature

Explanation: ***Increase in CO₂*** - An increase in the partial pressure of **carbon dioxide (PCO₂)** in the blood leads to a decrease in pH (increased H⁺ concentration), a phenomenon known as the **Bohr effect**. - This acidic environment stabilizes the **taut (T) state** of hemoglobin, reducing its affinity for oxygen and facilitating oxygen unloading to metabolically active tissues, thus causing a **rightward shift**. *Increase in O₂* - An increase in the partial pressure of **oxygen (PO₂)** represents a movement *along* the existing curve to the right, leading to a higher hemoglobin saturation percentage. - It does not alter the intrinsic affinity of hemoglobin for oxygen and therefore does not cause a shift of the entire curve. *Decrease in CO₂* - A decrease in **PCO₂** leads to an increase in blood pH (respiratory alkalosis), which increases hemoglobin's affinity for oxygen. - This increased affinity impairs oxygen release to tissues and causes a **leftward shift** of the curve, promoting oxygen uptake in the lungs. *Decrease in temperature* - A decrease in body **temperature** (hypothermia) increases the affinity of hemoglobin for oxygen. - This makes it more difficult for hemoglobin to release oxygen to the tissues, resulting in a **leftward shift** of the curve.

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