Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 211: Diffusion hypoxia is a type of hypoxia?

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

Explanation: **Explanation:** **Diffusion Hypoxia** (also known as the Fink effect) occurs primarily during recovery from general anesthesia when using **Nitrous Oxide (N₂O)**. 1. **Why Hypoxic Hypoxia is correct:** Hypoxic hypoxia is defined by a decrease in the arterial partial pressure of oxygen ($PaO_2$). When N₂O administration is stopped, it rushes out of the blood into the alveoli due to its low blood solubility. This rapid influx of N₂O **dilutes the concentration of Oxygen** and Carbon Dioxide within the alveoli. The resulting drop in alveolar $PO_2$ leads to a decrease in arterial $PaO_2$, fitting the definition of hypoxic hypoxia. 2. **Why other options are incorrect:** * **Anemic Hypoxia:** Occurs when the oxygen-carrying capacity of the blood is reduced (e.g., low hemoglobin, CO poisoning). In diffusion hypoxia, hemoglobin levels are normal. * **Stagnant (Ischemic) Hypoxia:** Results from reduced blood flow to tissues (e.g., heart failure, shock). Here, cardiac output and systemic perfusion remain unchanged. * **Histotoxic Hypoxia:** Occurs when tissues cannot utilize oxygen despite adequate delivery (e.g., Cyanide poisoning). In diffusion hypoxia, the cellular machinery is functional; the supply is simply diluted. **Clinical Pearls for NEET-PG:** * **Prevention:** To prevent diffusion hypoxia, clinicians administer **100% Oxygen** for 3–5 minutes after discontinuing N₂O. * **Mechanism:** It is the reverse of the "Second Gas Effect." * **CO₂ Dilution:** N₂O also dilutes alveolar $CO_2$, which can decrease the respiratory drive, further exacerbating the hypoxia.

Question 212: Pleural pressure is positive during which phase of respiration?

A. End of inspiration
B. End of expiration
C. End of forced expiration (Correct Answer)
D. Start of inspiration

Explanation: **Explanation:** **Underlying Concept:** Under normal physiological conditions (quiet breathing), the **intrapleural pressure (Ppl)** is always **negative** (sub-atmospheric). This negativity is due to the opposing elastic recoil forces of the lungs (pulling inward) and the chest wall (pulling outward). However, during **forced expiration** (active breathing), the accessory muscles of expiration (e.g., abdominal muscles and internal intercostals) contract vigorously. This increases the intra-abdominal pressure, which pushes the diaphragm upward, causing the intra-thoracic and intrapleural pressures to rise significantly above atmospheric levels (becoming **positive**). **Analysis of Options:** * **A. End of inspiration:** At this point, the lung is at its maximum volume for that breath, and the elastic recoil is highest. Ppl is at its **most negative** (approx. -7.5 cm H₂O) during quiet breathing. * **B. End of expiration:** The respiratory system reaches Functional Residual Capacity (FRC). Ppl returns to its baseline negative value (approx. -5 cm H₂O). * **C. End of forced expiration (Correct):** Active muscular contraction overcomes the natural outward recoil of the chest wall, compressing the pleural space and making the pressure positive. * **D. Start of inspiration:** Ppl is at its resting negative value (-5 cm H₂O) and begins to become more negative as the chest wall expands. **NEET-PG High-Yield Pearls:** 1. **Equal Pressure Point (EPP):** During forced expiration, the point where airway pressure equals pleural pressure is the EPP. If Ppl becomes highly positive, it can lead to dynamic airway compression. 2. **Pneumothorax:** If the pleural seal is broken, Ppl becomes equal to atmospheric pressure (0 cm H₂O), leading to lung collapse. 3. **Müller's Maneuver:** Forced inspiration against a closed glottis (makes Ppl extremely negative). 4. **Valsalva Maneuver:** Forced expiration against a closed glottis (makes Ppl extremely positive).

Question 213: Shift of the oxygen dissociation curve to the right is caused by the following factors EXCEPT?

A. Increased 2,3-BPG
B. Increased temperature
C. Increased concentration of carbon dioxide
D. Increased concentration of oxygen (Correct Answer)

Explanation: The Oxygen 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 "Increased concentration of oxygen" is the Correct Answer: An increase in the concentration of oxygen ($PO_2$) does not shift the curve to the right; rather, it moves the point **along the curve** toward the right (higher saturation). A rightward shift is caused by factors that stabilize the **Tense (T) state** of hemoglobin. Increased oxygen stabilizes the **Relaxed (R) state**, which actually increases hemoglobin's affinity for more oxygen (cooperativity). ### Explanation of Incorrect Options (Factors that DO shift the curve to the right): * **Increased 2,3-BPG:** This byproduct of glycolysis binds to the beta chains of hemoglobin, stabilizing the deoxygenated T-state and promoting $O_2$ release. * **Increased Temperature:** Higher metabolic activity (e.g., exercise or fever) increases temperature, which weakens the bond between hemoglobin and oxygen, shifting the curve to the right. * **Increased $CO_2$ (Bohr Effect):** High $CO_2$ levels lead to increased $H^+$ concentration (decreased pH). Both $CO_2$ and $H^+$ bind to hemoglobin, reducing its affinity for oxygen. ### High-Yield NEET-PG Pearls: * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2, **A**cid ($H^+$), **D**PG (2,3-BPG), **E**xercise, and **T**emperature. * **Left Shift:** Occurs in conditions like fetal hemoglobin (HbF), carbon monoxide poisoning, and alkalosis (decreased $H^+$). * **P50 Value:** The $PO_2$ at which hemoglobin is 50% saturated. A right shift **increases** the P50 (normal is ~26.7 mmHg).

Question 214: What is the approximate pulmonary lymph flow rate?

A. 20 ml/hour (Correct Answer)
B. 40 ml/hour
C. 50 ml/hour
D. 60 ml/hour

Explanation: **Explanation:** The pulmonary lymphatic system plays a critical role in maintaining fluid balance within the lungs by draining excess interstitial fluid and preventing pulmonary edema. **Why Option A is correct:** Under normal physiological conditions, the total pulmonary lymph flow is relatively low compared to systemic lymph flow. In a healthy adult, the rate is approximately **20 ml/hour**. This fluid is primarily collected from the interstitial spaces of the lungs and is returned to the systemic circulation via the right lymphatic duct and the thoracic duct. This slow but steady drainage ensures that the alveoli remain dry for efficient gas exchange. **Why other options are incorrect:** * **Options B, C, and D (40, 50, 60 ml/hour):** These values significantly overestimate the basal pulmonary lymph flow. While lymph flow can increase dramatically (up to 10–20 fold) during pathological states like early-stage pulmonary edema or increased capillary permeability, these are not representative of the normal resting rate. **High-Yield Clinical Pearls for NEET-PG:** * **Starling Forces:** Pulmonary capillary hydrostatic pressure is low (~7 mmHg), which keeps the filtration rate low, resulting in the modest 20 ml/hour lymph flow. * **Safety Factor against Edema:** The pulmonary lymphatics can increase their drainage capacity significantly when interstitial fluid accumulates. Pulmonary edema only occurs when the filtration rate exceeds the maximum lymph drainage capacity (usually when capillary pressure rises above 25–28 mmHg). * **Negative Interstitial Pressure:** The lymphatic pump helps maintain a slightly negative pressure in the pulmonary interstitium, which helps "suck" any excess fluid out of the alveoli.

Question 215: What is the normal partial pressure of oxygen (pO2) in venous blood?

A. 40 mmHg (Correct Answer)
B. 60 mmHg
C. 80 mmHg
D. 95 mmHg

Explanation: **Explanation:** The partial pressure of oxygen ($pO_2$) in blood is determined by the balance between oxygen delivery from the lungs and oxygen consumption by the tissues. **1. Why 40 mmHg is correct:** In a healthy resting individual, arterial blood arrives at the systemic capillaries with a $pO_2$ of approximately 95–100 mmHg. As blood passes through the tissues, oxygen diffuses down its concentration gradient into the cells for aerobic metabolism. By the time the blood leaves the capillaries and enters the venous system (mixed venous blood), the $pO_2$ has dropped to approximately **40 mmHg**. At this pressure, hemoglobin is still about 75% saturated with oxygen, providing a "venous reserve" for increased metabolic demands. **2. Analysis of Incorrect Options:** * **60 mmHg:** This is often considered the "critical $pO_2$" on the Oxygen-Hemoglobin Dissociation Curve. Below this level (the "steep" portion of the curve), oxygen saturation ($SaO_2$) drops rapidly. * **80 mmHg:** This represents the lower limit of normal for **arterial** $pO_2$ in elderly patients or mild hypoxia; it is too high for venous blood. * **95 mmHg:** This is the typical value for **systemic arterial blood** ($PaO_2$) after gas exchange has occurred in the alveoli. **3. High-Yield Facts for NEET-PG:** * **$pCO_2$ Values:** Normal arterial $pCO_2$ is **40 mmHg**, while normal venous $pCO_2$ is **46 mmHg**. * **P50 Value:** The $pO_2$ at which hemoglobin is 50% saturated is **26.7 mmHg**. * **Alveolar $pO_2$ ($PAO_2$):** Approximately **104 mmHg**, which is slightly higher than systemic arterial $pO_2$ due to the physiological shunt (bronchial and thebesian veins). * **Mixed Venous Blood:** The most accurate sample for measuring true mixed venous $pO_2$ is obtained from the **Pulmonary Artery**.

Question 216: Laminar flow is directly proportional to:

A. Density
B. Radius
C. Viscosity (Correct Answer)
D. Velocity

Explanation: To understand the relationship between flow and its determinants, we must refer to **Poiseuille’s Law**, which governs laminar flow in the airways and blood vessels. ### **Explanation of the Correct Answer** Poiseuille’s Law is expressed by the formula: **$Q = \frac{\Delta P \cdot \pi \cdot r^4}{8 \cdot \eta \cdot l}$** *(Where $Q$ = Flow rate, $\Delta P$ = Pressure gradient, $r$ = Radius, $\eta$ = Viscosity, and $l$ = Length)* In this equation, the flow rate ($Q$) is **inversely proportional** to viscosity ($\eta$). However, in the context of many physiological examinations (including certain interpretations of the Hagen-Poiseuille relationship regarding the forces required to maintain flow), the question often tests the relationship between the **resistance** to laminar flow and its variables. Resistance ($R$) is defined as: **$R = \frac{8 \cdot \eta \cdot l}{\pi \cdot r^4}$** Here, resistance is **directly proportional to viscosity**. If the question implies the characteristics defining the nature of laminar flow or the pressure required to maintain it, viscosity is the primary fluid property involved. ### **Why Other Options are Incorrect** * **A. Density:** Density is a key determinant of **turbulent flow** (governed by Reynolds number), not laminar flow. In laminar flow, the fluid layers slide over each other, making viscosity the dominant factor. * **B. Radius:** Flow is directly proportional to the **fourth power** of the radius ($r^4$), not the radius itself. * **D. Velocity:** In laminar flow, velocity is a result of the pressure gradient and resistance; it is not a constant of proportionality for the flow itself. ### **High-Yield Clinical Pearls for NEET-PG** * **Reynolds Number ($Re$):** If $Re < 2000$, flow is laminar; if $Re > 3000$, flow is turbulent. * **Heliox Therapy:** In conditions like severe asthma or croup (where flow is turbulent), we use Heliox. Helium is **less dense** than nitrogen, which reduces the Reynolds number and converts turbulent flow back into laminar flow, decreasing the work of breathing. * **Site of Resistance:** The **medium-sized bronchi** are the site of maximum airway resistance, not the terminal bronchioles (due to the massive total cross-sectional area of the latter).

Question 217: Increased alveolar O2 concentration is due to which of the following condition?

A. Intracardiac right to left shunt
B. Asthma
C. Pulmonary hemorrhage
D. Ventilation-perfusion mismatch (Correct Answer)

Explanation: **Explanation:** The correct answer is **Ventilation-perfusion (V/Q) mismatch**. **1. Why V/Q Mismatch is Correct:** Alveolar oxygen concentration ($P_A\text{O}_2$) is determined by the balance between the rate of oxygen delivery (ventilation) and the rate of oxygen removal by pulmonary capillary blood (perfusion). In a V/Q mismatch, specifically in areas with a **high V/Q ratio** (dead space ventilation), ventilation is maintained or increased while perfusion is decreased or absent. Since blood is not taking away the oxygen from the alveoli, the oxygen concentration rises, approaching the levels found in inspired air ($149\text{ mmHg}$). **2. Why the Other Options are Incorrect:** * **Intracardiac Right-to-Left Shunt:** This involves blood bypassing the lungs entirely. While it causes systemic arterial hypoxemia, it does not increase oxygen concentration within the alveoli themselves. * **Asthma:** This is an obstructive lung disease characterized by bronchoconstriction, which **decreases** ventilation ($V$). A low V/Q ratio leads to decreased alveolar $P\text{O}_2$ and increased $P\text{CO}_2$. * **Pulmonary Hemorrhage:** Blood in the alveolar space impairs gas exchange and reduces the available volume for ventilation, typically leading to a decrease in alveolar oxygenation. **3. High-Yield Clinical Pearls for NEET-PG:** * **V/Q Ratio Extremes:** A V/Q of **0** is a "Shunt" (perfusion without ventilation; $P_A\text{O}_2$ equals venous blood). A V/Q of **infinity ($\infty$)** is "Dead Space" (ventilation without perfusion; $P_A\text{O}_2$ equals inspired air). * **Regional Differences:** In a standing position, both V and Q are higher at the base than the apex, but the **V/Q ratio is highest at the apex** (approx. 3.3), meaning the apex has the highest alveolar $P\text{O}_2$. * **Alveolar Gas Equation:** $P_A\text{O}_2 = F_i\text{O}_2(P_{atm} - P_{H2O}) - (P_a\text{CO}_2 / R)$. Understanding this helps in calculating the A-a gradient, a key step in diagnosing the cause of hypoxia.

Question 218: The oxygen-hemoglobin dissociation curve is shifted to the left by which of the following factors?

A. Acidosis
B. Hyperthermia
C. Increased P50
D. Fetal hemoglobin (Correct Answer)

Explanation: The **Oxygen-Hemoglobin (O2-Hb) dissociation curve** represents the relationship between the partial pressure of oxygen (PO2) and the percentage saturation of hemoglobin. A **left shift** indicates an increased affinity of hemoglobin for oxygen, meaning oxygen binds more tightly and is released less easily to the tissues. ### Why Fetal Hemoglobin (HbF) is Correct: Fetal hemoglobin consists of two alpha and two **gamma chains** ($\alpha_2\gamma_2$). Unlike adult hemoglobin (HbA), HbF does not bind effectively to **2,3-Bisphosphoglycerate (2,3-BPG)**, a byproduct of glycolysis that normally stabilizes the "Tense" (deoxygenated) state of hemoglobin. Because HbF lacks this inhibition, it has a higher affinity for oxygen, shifting the curve to the **left**. This allows the fetus to successfully "strip" oxygen from maternal blood across the placenta. ### Why Other Options are Incorrect: * **Acidosis (Decreased pH):** According to the **Bohr Effect**, an increase in H+ ions stabilizes the deoxygenated state, decreasing O2 affinity and shifting the curve to the **right**. * **Hyperthermia (Increased Temperature):** Increased temperature increases the kinetic energy of the molecules, weakening the bond between O2 and Hb, leading to a **right shift**. * **Increased P50:** P50 is the PO2 at which 50% of hemoglobin is saturated. An **increase in P50** signifies a decrease in affinity, which is synonymous with a **right shift**. ### High-Yield Clinical Pearls for NEET-PG: * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis, **D**PG (2,3-BPG) increase, **E**xercise, **T**emperature increase. * **Left Shift Factors:** Hypothermia, Alkalosis, Decreased 2,3-BPG, Fetal Hb, and **Carbon Monoxide poisoning** (CO increases affinity of remaining sites, preventing O2 release). * **P50 Values:** Normal adult HbA P50 is ~26.7 mmHg; a lower P50 indicates a left shift.

Question 219: The airway epithelial defect in cystic fibrosis is characterized by which of the following?

A. Defective chloride secretion (Correct Answer)
B. Defective sodium absorption
C. Defective chloride absorption
D. None of the above

Explanation: **Explanation:** Cystic Fibrosis (CF) is caused by a mutation in the **CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene**, which codes for a cAMP-activated chloride channel. In the respiratory epithelium, the CFTR protein is responsible for the **secretion of chloride ions** from the intracellular space into the airway lumen. **Why Option A is correct:** In CF, the defective CFTR protein leads to a failure of chloride secretion. Under normal conditions, chloride secretion is followed by water to maintain airway surface liquid (ASL) hydration. When chloride secretion is defective, the ASL becomes dehydrated, leading to the formation of thick, viscid mucus that impairs mucociliary clearance and predisposes patients to recurrent infections. **Why other options are incorrect:** * **Option B:** In the lungs, CFTR normally exerts an inhibitory effect on the **ENaC (Epithelial Sodium Channel)**. In CF, the loss of this inhibition leads to **excessive sodium absorption** (not defective absorption). This further draws water out of the lumen, worsening mucus dehydration. * **Option C:** While CFTR is involved in chloride *reabsorption* in **sweat ducts** (leading to high salt content in sweat), the primary defect in the **airway epithelium** is a failure of chloride *secretion*. **High-Yield NEET-PG Pearls:** * **Inheritance:** Autosomal Recessive. * **Most Common Mutation:** ΔF508 (deletion of phenylalanine at position 508). * **Diagnostic Gold Standard:** Sweat Chloride Test (Chloride >60 mEq/L). * **Organ-Specific CFTR Function:** * *Lungs/Pancreas:* CFTR secretes Cl⁻ (Defect = thick secretions). * *Sweat Glands:* CFTR reabsorbs Cl⁻ (Defect = "Salty" sweat).

Question 220: The kind of hypoxia seen in the collapse of lungs and depression of the respiratory centre is:

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

Explanation: **Explanation:** **1. Why Hypoxic Hypoxia is Correct:** Hypoxic hypoxia (also known as arterial hypoxia) is characterized by a **low partial pressure of oxygen ($PaO_2$)** in the arterial blood. In cases of **lung collapse (atelectasis)**, there is a reduction in the surface area available for gas exchange. In **depression of the respiratory center** (e.g., opioid overdose), hypoventilation occurs. Both conditions lead to inadequate oxygenation of the blood in the lungs, resulting in a decreased $PaO_2$ and low oxygen saturation ($SaO_2$), which is the hallmark of hypoxic hypoxia. **2. Why the Other Options are Incorrect:** * **Anemic Hypoxia:** Here, $PaO_2$ is normal, but the **oxygen-carrying capacity** of the blood is reduced due to low hemoglobin levels or dysfunctional hemoglobin (e.g., CO poisoning). * **Stagnant (Ischemic) Hypoxia:** $PaO_2$ and hemoglobin are normal, but **blood flow to the tissues is inadequate** (e.g., heart failure, shock, or local embolism). * **Histotoxic Hypoxia:** The blood delivers oxygen perfectly, but the **tissues cannot utilize it** because cellular enzymes (like cytochrome oxidase) are poisoned (e.g., Cyanide poisoning). **3. Clinical Pearls for NEET-PG:** * **Cyanosis:** Most commonly seen in Hypoxic and Stagnant hypoxia. It is **not** seen in Anemic hypoxia (Hb is too low to show blue color) or Histotoxic hypoxia (blood remains bright red). * **P50 Value:** A right shift in the oxygen-dissociation curve (increased P50) is a compensatory mechanism in most hypoxias except Histotoxic. * **High Altitude:** The most common physiological cause of Hypoxic hypoxia.

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