Respiratory System Practice Questions

Q: The oxygen-hemoglobin dissociation curve is shifted to the left by:

Alkalosis. The oxygen-hemoglobin (O2-Hb) dissociation curve represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. ### **Why Alkalosis is Correct** A **left shift** indicates an **increased affinity** of hemoglobin for oxygen. This means hemoglobin binds oxygen more tightly in the lungs but releases it less readily to the tissues. **Alkalosis** (increased pH/decreased $H^+$ concentration) stabilizes the "R" (Relaxed) state of hemoglobin, which has a higher affinity for oxygen, thus shifting the curve to the left. ### **Why Other Options are Incorrect** * **Acidosis (A):** Increased $H^+$ ions (decreased pH) shift the curve to the **right** (Bohr Effect). This decreases Hb-O2 affinity, facilitating oxygen unloading to tissues. * **Hyperthermia (C):** Increased temperature increases the kinetic energy of the molecules, weakening the bond between Hb and $O_2$, shifting the curve to the **right**. * **Anemia (D):** While anemia reduces the total oxygen-carrying capacity, the chronic compensatory increase in **2,3-BPG** levels in anemic patients typically shifts the curve to the **right** to improve tissue oxygenation. ### **High-Yield Clinical Pearls for NEET-PG** To remember the factors shifting the curve, use the mnemonic **"CADET, face Right!"** Factors that shift the curve to the **Right** (Decreased affinity): * **C** – $CO_2$ (Hypercapnia) * **A** – Acidosis * **D** – 2,3-DPG (increased) * **E** – Exercise * **T** – Temperature (Hyperthermia) **Note:** Fetal Hemoglobin (HbF) and Carbon Monoxide (CO) poisoning both shift the curve to the **Left**, though CO poisoning also decreases the maximum oxygen-carrying capacity (plateau).

Q: Anemic hypoxia is seen in which of the following conditions?

Carbon monoxide poisoning. **Explanation:** **Anemic hypoxia** occurs when the arterial $PO_2$ is normal, but the total oxygen-carrying capacity of the blood is reduced. This can be due to a decrease in hemoglobin concentration or an alteration in the hemoglobin molecule that prevents it from binding or releasing oxygen effectively. **Why Carbon Monoxide (CO) Poisoning is correct:** In CO poisoning, carbon monoxide binds to hemoglobin with an affinity **200–250 times greater** than oxygen, forming **carboxyhemoglobin**. This results in: 1. A reduction in the number of binding sites available for oxygen. 2. A **leftward shift** of the Oxygen-Hemoglobin Dissociation Curve (OHDC), which prevents the unloading of oxygen to tissues. Despite normal dissolved oxygen ($PO_2$), the total oxygen content is severely reduced, fitting the definition of anemic hypoxia. **Analysis of Incorrect Options:** * **Carbon dioxide poisoning:** High levels of $CO_2$ (hypercapnia) lead to respiratory acidosis but do not primarily cause anemic hypoxia. * **Hydrogen cyanide poisoning:** This causes **histotoxic hypoxia**. Cyanide inhibits the enzyme **cytochrome oxidase** in the mitochondria, preventing tissues from utilizing oxygen despite adequate delivery. * **Nerve gas exposure:** These agents (e.g., Sarin) inhibit acetylcholinesterase, leading to a cholinergic crisis. Death occurs due to respiratory failure (paralysis of respiratory muscles), which leads to **hypoxic hypoxia**. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Sign:** CO poisoning presents with "cherry-red" skin/mucosa (though rare in clinical practice). * **Methemoglobinemia:** Another classic cause of anemic hypoxia (presents with "chocolate-colored" blood and cyanosis). * **Key Distinction:** In anemic hypoxia, **Arterial $PO_2$ is normal**, but **Arterial $O_2$ content is decreased**.

Q: Least arteriovenous oxygen difference is seen in which of the following conditions?

Cyanide poisoning. The arteriovenous (A-V) oxygen difference represents the amount of oxygen extracted by tissues from the blood. **Correct Answer: D. Cyanide Poisoning (Histotoxic Hypoxia)** In cyanide poisoning, the cyanide ion binds to the ferric ($Fe^{3+}$) iron in **cytochrome oxidase a3** within the mitochondrial electron transport chain. This inhibits cellular respiration, rendering tissues unable to utilize the oxygen delivered to them. Since oxygen is not extracted, the venous blood remains highly oxygenated, nearly matching the arterial oxygen content. This results in the **least (minimal) A-V oxygen difference** and classically causes the venous blood to appear "cherry red." **Explanation of Incorrect Options:** * **A. Hypoxic Hypoxia:** Caused by low atmospheric $PO_2$ or hypoventilation. While arterial oxygen is low, tissues still extract oxygen to survive, maintaining a significant A-V difference. * **B. Anemic Hypoxia:** Arterial $PO_2$ is normal, but hemoglobin concentration is low. Tissues extract a larger percentage of the available oxygen, often leading to a normal or slightly increased A-V difference relative to the total oxygen carrying capacity. * **C. Stagnant Hypoxia:** Occurs due to reduced blood flow (e.g., heart failure or shock). Because blood moves slowly through capillaries, tissues have more time to extract oxygen, leading to a **maximal (increased) A-V oxygen difference.** **High-Yield Pearls for NEET-PG:** 1. **Stagnant Hypoxia:** Characterized by the *highest* A-V oxygen difference. 2. **Histotoxic Hypoxia:** The only type where venous $PO_2$ is elevated. 3. **Cyanosis:** Not seen in Anemic hypoxia (low Hb) or Histotoxic hypoxia (high venous $O_2$). 4. **Treatment for Cyanide:** Amyl nitrite/Sodium nitrite (creates methemoglobin to sequester cyanide) and Sodium thiosulfate.

Q: Death due to cyanide poisoning results from which of the following types of anoxia?

Histotoxic anoxia. **Explanation:** **Histotoxic anoxia** is the correct answer because cyanide poisoning interferes with the utilization of oxygen at the cellular level, rather than its delivery. Cyanide binds to the **ferric (Fe³⁺) iron** in the **cytochrome c oxidase** enzyme (Complex IV) of the mitochondrial electron transport chain. This inhibits aerobic respiration, preventing cells from using the oxygen delivered to them. Consequently, the venous blood remains highly oxygenated, often giving the patient a characteristic "cherry-red" skin appearance. **Analysis of Incorrect Options:** * **Anoxic Anoxia:** Occurs when there is a decrease in the arterial partial pressure of oxygen ($PaO_2$). Common causes include high altitude, airway obstruction, or alveolar hypoventilation. * **Anaemic Anoxia:** Occurs when the $PaO_2$ is normal, but the oxygen-carrying capacity of the blood is reduced. Causes include anemia, hemorrhage, or carbon monoxide (CO) poisoning (where Hb is unavailable). * **Stagnant (Ischemic) Anoxia:** Occurs when blood flow to the tissues is slowed or stopped despite normal oxygen content. Causes include heart failure, shock, or local thrombosis. **Clinical Pearls for NEET-PG:** * **Key Finding:** In histotoxic hypoxia, the **Arterio-Venous (A-V) oxygen difference is decreased** because tissues cannot extract oxygen from the blood. * **Antidote for Cyanide:** Amyl nitrite/Sodium nitrite (to create methemoglobin, which sequesters cyanide) followed by Sodium thiosulfate (to convert cyanide to non-toxic thiocyanate). Hydroxocobalamin is also a first-line treatment. * **Classic Sign:** "Cherry-red" discoloration of skin and mucous membranes (due to high venous $O_2$ saturation).

Q: The Hb-O2 dissociation curve is shifted to the left by which of the following?

Decreased 2,3-DPG. **Explanation:** The Hemoglobin-Oxygen (Hb-O2) dissociation curve represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **leftward shift** indicates an **increased affinity** of hemoglobin for oxygen, meaning oxygen binds more tightly and is less easily released to the tissues. **Why Option D is Correct:** **2,3-Diphosphoglycerate (2,3-DPG)** is a byproduct of glycolysis in RBCs that binds to the beta chains of deoxyhemoglobin, stabilizing the "T" (Tense) state and promoting oxygen release. Therefore, a **decrease in 2,3-DPG** stabilizes the "R" (Relaxed) state, increasing oxygen affinity and shifting the curve to the **left**. **Why Other Options are Incorrect:** * **A. Metabolic Acidosis:** A decrease in pH (increased $H^+$ ions) decreases Hb affinity for $O_2$ (the **Bohr Effect**), shifting the curve to the **right**. * **B. Increased Temperature:** Higher temperatures denature the bond between hemoglobin and oxygen, facilitating $O_2$ unloading and shifting the curve to the **right**. * **C. Increased $PCO_2$:** High $CO_2$ levels lead to increased $H^+$ production and direct carbamino-hemoglobin formation, both of which shift 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, **D**- 2,3-DPG increase, **E**- Exercise, **T**- Temperature increase). * **Fetal Hemoglobin (HbF):** Shifts the curve to the **left** because it has a poor binding affinity for 2,3-DPG, allowing the fetus to extract $O_2$ from maternal blood. * **Stored Blood:** Levels of 2,3-DPG decrease in stored blood, causing a left shift; this is why massive transfusions can temporarily impair tissue oxygen delivery.

Q: Which of the following changes does NOT occur in the blood as it passes through the systemic capillaries?

Shift of the oxygen dissociation curve to the left. **Explanation:** The correct answer is **C (Shift of the oxygen dissociation curve to the left)**. In the systemic capillaries, the blood undergoes changes that facilitate oxygen unloading to the tissues. This requires a **rightward shift** of the oxygen-hemoglobin dissociation curve (the **Bohr Effect**), driven by increased $PCO_2$, increased $[H^+]$ (decreased pH), and increased temperature. A leftward shift would increase hemoglobin's affinity for oxygen, hindering its release to the tissues. **Analysis of Incorrect Options:** * **A. Increase in hematocrit:** As blood passes through capillaries, the "Chloride Shift" (Hamburger phenomenon) occurs. $HCO_3^-$ leaves the RBCs while $Cl^-$ enters. This increase in intracellular osmotic pressure causes water to enter the RBCs, making them swell and slightly increasing the hematocrit in venous blood compared to arterial blood. * **B. Decrease in pH:** Tissues produce $CO_2$ as a metabolic byproduct. When $CO_2$ enters the blood, it reacts with water to form carbonic acid, which dissociates into $H^+$ and $HCO_3^-$, thereby lowering the pH (making it more acidic). * **D. Increase in protein content:** Due to hydrostatic pressure, a small amount of protein-free fluid filters out of the capillaries into the interstitial space (ultrafiltration). This leads to a relative increase in the concentration of plasma proteins in the capillary blood. **High-Yield NEET-PG Pearls:** * **Bohr Effect:** Shift to the **Right** (occurs at **Tissues**). Mnemonic: **CADET**, face Right! (**C**O2, **A**cid, 2,3-**D**PG, **E**xercise, **T**emperature). * **Haldane Effect:** Occurs at the **Lungs**; oxygenation of hemoglobin promotes the dissociation of $CO_2$ from hemoglobin. * **Chloride Shift:** $Cl^-$ moves **into** RBCs in systemic capillaries and **out** of RBCs in pulmonary capillaries.

Q: Identify the process indicated by '1' in the illustration depicting lung compliance.

Inspiration. ***Inspiration*** - Curve '1' in the **pressure-volume diagram** represents the **inflation phase** of the lung during inspiration, where volume increases with increasing transpulmonary pressure. - During inspiration, **surfactant** reduces surface tension progressively, allowing easier lung expansion as alveoli recruit and inflate. *Expiration* - This would be represented by curve '2' in the **hysteresis loop**, showing the **deflation phase** where volume decreases with decreasing pressure. - The **expiratory curve** follows a different path due to **surface tension hysteresis** and surfactant properties during deflation. *Inspiratory air* - This refers to the **actual air volume** being inhaled, not the process or curve shown in the compliance diagram. - The curve '1' represents the **mechanical relationship** between pressure and volume, not the air itself. *Expiratory air* - This refers to the **actual air volume** being exhaled, not the process or curve depicted. - The **P-V loop** demonstrates lung mechanics and compliance, not the composition or volume of expired air.

Q: The alveoli are normally kept dry by which of the following mechanisms?

Surfactant. **Explanation:** The dryness of the alveoli is primarily maintained by **Surfactant**, which reduces surface tension at the air-liquid interface. According to the **Law of Laplace ($P = 2T/r$)**, surface tension ($T$) creates an inward collapsing pressure ($P$) that tends to pull fluid from the pulmonary capillaries into the alveolar space (pulmonary edema). By significantly lowering surface tension, surfactant minimizes this inward hydrostatic pressure, preventing fluid transudation and keeping the alveoli dry for efficient gas exchange. **Analysis of Incorrect Options:** * **Macrophages (A):** These are immune cells responsible for phagocytosing debris and pathogens; they do not regulate fluid dynamics or surface tension. * **Negative intrapleural pressure (C):** This pressure keeps the lungs expanded against the chest wall. If anything, excessive negative pressure (as seen in upper airway obstruction) can actually promote pulmonary edema by increasing the pressure gradient that pulls fluid into the alveoli. * **High pCO2 (D):** High alveolar $pCO_2$ typically causes localized vasodilation but does not play a role in maintaining alveolar dryness. **Clinical Pearls for NEET-PG:** * **Composition:** Surfactant is 90% lipids (mainly **Dipalmitoylphosphatidylcholine - DPPC**) and 10% proteins (SP-A, B, C, D). * **Source:** Secreted by **Type II Pneumocytes** (lamellar bodies). * **Clinical Correlation:** Deficiency of surfactant in premature infants leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease, characterized by alveolar collapse and pulmonary edema. * **Other factors keeping alveoli dry:** Low pulmonary capillary hydrostatic pressure (~10 mmHg) and efficient lymphatic drainage.

Q: Which of the following is not true in obstructive lung disease?

TLC is increased. In obstructive lung diseases (e.g., Asthma, COPD, Bronchiectasis), the primary pathology is **increased airway resistance**, making it difficult to exhale air rapidly. ### Why Option B is the Correct Answer The question asks for the statement that is **NOT true**. In obstructive lung disease, **Total Lung Capacity (TLC) is typically increased or normal**, never decreased. This occurs due to **air trapping** and **hyperinflation**; because patients cannot exhale fully, residual air stays in the lungs, increasing the Residual Volume (RV) and consequently the TLC. Therefore, saying an increased TLC is "not true" is technically a flaw in the question's framing—however, in the context of standard PG exams, this question often highlights that while FEV1 and FVC change significantly, an *increase* in TLC is a compensatory finding of hyperinflation, not a diagnostic deficit. ### Analysis of Other Options * **A. FEV1 is decreased:** **True.** Forced Expiratory Volume in 1 second is the hallmark of obstruction. Increased airway resistance significantly slows down the flow of air during the first second of expiration. * **C. FVC is decreased:** **True.** While FEV1 drops more drastically, the Forced Vital Capacity (FVC) also decreases in chronic or severe obstruction due to premature airway closure (dynamic compression). * **D. Reduced timed vital capacity:** **True.** "Timed vital capacity" is another term for FEV1. As established, the rate of air expiration is reduced in obstructive pathologies. ### High-Yield Clinical Pearls for NEET-PG * **The Gold Standard:** The most important parameter for diagnosing obstruction is a **decreased FEV1/FVC ratio (< 0.7)**. * **Restrictive vs. Obstructive:** In Restrictive disease (e.g., Fibrosis), TLC is **decreased**, and the FEV1/FVC ratio is **normal or increased**. * **Flow-Volume Loop:** Obstructive disease shows a characteristic **"scooped-out"** appearance on the expiratory limb.

Q: Alveoli are kept dry because of what?

Surfactants. **Explanation:** The correct answer is **Surfactant**. The primary mechanism keeping the alveoli dry is the reduction of surface tension. According to the **Law of Laplace ($P = 2T/r$)**, surface tension ($T$) creates an inward collapsing pressure ($P$) that tends to pull fluid from the pulmonary capillaries into the alveolar space (pulmonary edema). Pulmonary surfactant, produced by **Type II Pneumocytes**, contains **Dipalmitoylphosphatidylcholine (DPPC)**, which significantly lowers surface tension. By reducing this inward pressure, surfactant prevents the "suction effect" that would otherwise draw interstitial fluid into the alveoli, thereby maintaining a dry environment for efficient gas exchange. **Analysis of Incorrect Options:** * **Glycoproteins:** While surfactants contain proteins (SP-A, B, C, D), glycoproteins in the respiratory tract are primarily components of mucus (mucin) and do not regulate alveolar fluid balance. * **Buffers:** These (like the bicarbonate system) regulate the pH of blood and fluids but have no physical role in preventing fluid accumulation in the alveoli. * **Bohr’s Effect:** This describes the shift in the hemoglobin-oxygen dissociation curve due to changes in $CO_2$ or $pH$. It relates to oxygen unloading at tissues, not alveolar dryness. **Clinical Pearls for NEET-PG:** * **Infant Respiratory Distress Syndrome (IRDS):** Caused by surfactant deficiency in preterm infants, leading to alveolar collapse (atelectasis) and pulmonary edema. * **Lecithin/Sphingomyelin (L/S) Ratio:** A ratio > 2.0 in amniotic fluid indicates fetal lung maturity. * **Negative Interstitial Pressure:** Along with surfactant, the lymphatic system and the negative pressure in the pulmonary interstitium also help keep alveoli dry.

Question 1

The oxygen-hemoglobin dissociation curve is shifted to the left by:

Accepted Answer

Alkalosis

Answer

Acidosis

Answer

Hyperthermia

Answer

Anemia

Question 2

Anemic hypoxia is seen in which of the following conditions?

Accepted Answer

Carbon monoxide poisoning

Answer

Carbon dioxide poisoning

Answer

Hydrogen cyanide poisoning

Answer

Nerve gas exposure

Question 3

Least arteriovenous oxygen difference is seen in which of the following conditions?

Accepted Answer

Cyanide poisoning

Answer

Hypoxic hypoxia

Answer

Anemic hypoxia

Answer

Stagnant hypoxia

Question 4

Death due to cyanide poisoning results from which of the following types of anoxia?

Accepted Answer

Histotoxic anoxia

Answer

Anoxic anoxia

Answer

Anaemic anoxia

Answer

Stagnant anoxia

Question 5

The Hb-O2 dissociation curve is shifted to the left by which of the following?

Accepted Answer

Decreased 2,3-DPG

Answer

Metabolic acidosis

Answer

Increased temperature

Answer

Increased PCO2

Question 6

Which of the following changes does NOT occur in the blood as it passes through the systemic capillaries?

Accepted Answer

Shift of the oxygen dissociation curve to the left

Answer

Increase in hematocrit

Answer

Decrease in pH

Answer

Increase in protein content

Question 7

Identify the process indicated by '1' in the illustration depicting lung compliance.

Accepted Answer

Inspiration

Answer

Expiration

Answer

Inspiratory air

Answer

Expiratory air

Question 8

The alveoli are normally kept dry by which of the following mechanisms?

Accepted Answer

Surfactant

Answer

Macrophages

Answer

Negative intrapleural pressure

Answer

High pCO2 in the alveoli

Question 9

Which of the following is not true in obstructive lung disease?

Accepted Answer

TLC is increased

Answer

FEV1 is decreased

Answer

FVC is decreased

Answer

Reduced timed vital capacity

Question 10

Alveoli are kept dry because of what?

Accepted Answer

Surfactants

Answer

Glycoproteins

Answer

Buffers

Answer

Bohr's effect

Respiratory System — MCQs

Respiratory System — MCQs

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