Respiratory System Practice Questions

Q: A 54-year-old man sustains third degree burns in a house fire. His respiratory rate is 30/min, Hb = 17 g/dL, arterial PO2 is 95 mm Hg, and arterial O2 saturation is 50%. What is the most likely cause of his low oxygen saturation?

Carbon monoxide poisoning. ### Explanation **Correct Option: B. Carbon Monoxide (CO) Poisoning** The hallmark of carbon monoxide poisoning is a **normal arterial $PO_2$** (dissolved oxygen) but a **critically low arterial $O_2$ saturation ($SaO_2$)**. * **Mechanism:** CO has an affinity for hemoglobin that is 200–250 times greater than oxygen. It binds to hemoglobin to form **carboxyhemoglobin**, physically displacing oxygen and reducing the $SaO_2$. * **The "Normal $PO_2$" Trap:** $PO_2$ measures oxygen dissolved in plasma, which is unaffected by CO. Therefore, the patient is not technically "hypoxemic" in terms of $PO_2$, but suffers from severe **anemic hypoxia** because the oxygen-carrying capacity is decimated. * **Left Shift:** CO also causes a leftward shift of the Oxyhemoglobin Dissociation Curve (OHDC), making it harder for the remaining oxygen to be released to tissues. **Why Incorrect Options are Wrong:** * **A & C (Airway Obstruction/Pulmonary Edema):** Both conditions interfere with gas exchange at the alveolar-capillary membrane. This would lead to a **low arterial $PO_2$** (hypoxemia), which is not present in this patient ($PO_2$ = 95 mmHg is normal). * **D (Fever):** Fever causes a **Right shift** of the OHDC (decreasing affinity), but it does not cause a massive drop in $SaO_2$ to 50% in the presence of normal $PO_2$. **NEET-PG High-Yield Pearls:** 1. **Pulse Oximetry Gap:** Standard pulse oximeters cannot distinguish between oxyhemoglobin and carboxyhemoglobin; they may show a falsely normal $SaO_2$. Co-oximetry is required for diagnosis. 2. **Cherry Red Skin:** A classic but rare clinical sign of CO poisoning. 3. **Treatment:** 100% Hyperbaric Oxygen (HBO) to reduce the half-life of carboxyhemoglobin. 4. **OHDC Shift:** CO poisoning causes a **Left shift**; Anemia (low Hb) does not shift the curve, but CO poisoning (functional anemia) does.

Q: Which of the following findings demonstrates a difference between pulmonary circulation and systemic circulation?

Pulmonary vasoconstriction in response to hypoxia. **Explanation:** The fundamental difference between pulmonary and systemic circulation lies in their response to low oxygen levels (hypoxia). **1. Why Option B is Correct:** In the **systemic circulation**, hypoxia causes **vasodilation** to increase blood flow and oxygen delivery to tissues. However, in the **pulmonary circulation**, hypoxia triggers **Hypoxic Pulmonary Vasoconstriction (HPV)**. This is a protective mechanism where pulmonary arterioles constrict in poorly ventilated areas of the lung. This shunts blood away from hypoxic alveoli toward well-ventilated alveoli, optimizing **ventilation-perfusion (V/Q) matching** and preventing arterial hypoxemia. **2. Why the other options are incorrect:** * **Option A:** This is the opposite of the physiological reality in the lungs; vasodilation in response to hypoxia occurs in systemic vessels (e.g., skeletal muscle), not pulmonary ones. * **Option C:** Blood volume changes during the cardiac cycle do not define the functional difference between these two circuits in the context of vascular regulation. * **Option D:** The pulmonary circulation is a **low-pressure, low-resistance** system with very little basal vasoconstrictor tone compared to the systemic circulation, which maintains high tone to regulate blood pressure. **High-Yield NEET-PG Pearls:** * **Mechanism of HPV:** Hypoxia inhibits voltage-gated potassium channels in pulmonary artery smooth muscle cells, leading to depolarization and calcium influx, causing contraction. * **Clinical Correlation:** Global alveolar hypoxia (e.g., at high altitudes) causes generalized pulmonary vasoconstriction, leading to **High Altitude Pulmonary Edema (HAPE)** and pulmonary hypertension. * **V/Q Ratio:** The main goal of HPV is to minimize "shunting" by ensuring blood only goes where oxygen is available.

Q: Diffusion of oxygen at the tissue level is affected in all the following poisoning except?

Curare. This question tests the understanding of the **Oxygen Cascade** and the specific mechanisms by which toxins interfere with oxygen delivery and utilization. ### **Explanation** The correct answer is **Curare (B)**. Curare is a non-depolarizing neuromuscular blocking agent that acts at the nicotinic receptors of the motor endplate [2]. It causes **respiratory failure** by paralyzing the diaphragm and intercostal muscles. However, it does **not** interfere with the biochemical or physical diffusion of oxygen at the tissue level; if the patient is mechanically ventilated, oxygen diffusion remains perfectly normal. ### **Why the other options are incorrect:** * **Carbon Monoxide (A):** CO binds to hemoglobin with 200x higher affinity than $O_2$, forming Carboxyhemoglobin. It causes a **leftward shift** of the oxygen-dissociation curve, preventing the release (diffusion) of $O_2$ from hemoglobin to the tissues [1]. * **Phosgene (C):** This is a pulmonary irritant that causes severe **alveolar-capillary membrane damage** and non-cardiogenic pulmonary edema. This physical barrier and the destruction of the membrane directly impair the diffusion of gases. * **Cyanides (D):** Cyanide inhibits **Cytochrome Oxidase $a_3$** in the electron transport chain. This prevents the tissues from utilizing oxygen (histotoxic hypoxia). While $O_2$ is present in the blood, the concentration gradient for diffusion is abolished because the cellular "sink" for oxygen is blocked. ### **NEET-PG High-Yield Pearls** * **Cyanide Poisoning:** Characterized by a narrow arterial-venous $O_2$ difference (venous blood remains bright red). Antidote: Nitrites + Sodium Thiosulfate or Hydroxocobalamin. * **CO Poisoning:** "Cherry red" skin discoloration (post-mortem). Treatment: 100% Hyperbaric Oxygen. * **Curare Antidote:** Neostigmine (acetylcholinesterase inhibitor) combined with Glycopyrrolate.

Q: The provided pressure-volume curves represent a normal subject and a patient with a pulmonary disease. Based on these curves, what is the likely pulmonary condition of the patient?

Emphysema. ***Emphysema*** - **Increased lung compliance** causes the pressure-volume curve to shift **left and upward**, allowing greater volume at lower pressures due to **alveolar wall destruction**. - Loss of **elastic recoil** from damaged alveolar septa results in easier lung expansion but impaired expiration. *Pulmonary fibrosis* - **Decreased lung compliance** shifts the pressure-volume curve **right and downward**, requiring higher pressures to achieve the same volume. - **Collagen deposition** and **fibrotic tissue** make the lungs stiffer and harder to expand, opposite to the pattern seen in emphysema. *Asbestosis* - A **restrictive lung disease** that decreases compliance, shifting the curve **right and downward** like other fibrotic conditions. - **Asbestos fiber deposition** causes **pulmonary fibrosis** and scarring, making lungs less distensible, not more compliant. *Pulmonary congestion* - **Fluid accumulation** in pulmonary interstitium and alveoli decreases compliance, shifting the curve **right and downward**. - **Increased pulmonary capillary pressure** from heart failure causes **interstitial edema**, making lungs stiffer and harder to ventilate.

Q: CO2 retention is seen in which of the following conditions?

Respiratory failure. **Explanation:** **1. Why Respiratory Failure is correct:** Carbon dioxide (CO2) retention, or **hypercapnia**, occurs when alveolar ventilation is inadequate to remove the CO2 produced by metabolism. In **Type II Respiratory Failure** (Ventilatory Failure), there is a primary failure of the "pump" (respiratory muscles, chest wall, or central drive), leading to reduced minute ventilation. This results in the hallmark clinical finding of **hypoxemia with hypercapnia (PaCO2 >45 mmHg).** **2. Why other options are incorrect:** * **Carbon Monoxide (CO) Poisoning:** CO binds to hemoglobin with an affinity 200–250 times greater than oxygen, causing a leftward shift of the oxygen-dissociation curve. While it causes severe tissue hypoxia, the ventilation drive usually remains intact or increases, meaning CO2 levels are typically **normal or decreased** (due to compensatory hyperventilation). * **High Altitude:** At high altitudes, the low barometric pressure leads to hypoxia. This stimulates peripheral chemoreceptors, causing **hyperventilation**. Increased breathing "washes out" CO2, leading to **hypocapnia** and respiratory alkalosis, not retention. **3. High-Yield Clinical Pearls for NEET-PG:** * **Type I Respiratory Failure:** Hypoxemia with normal/low PaCO2 (e.g., Pneumonia, Pulmonary Edema). * **Type II Respiratory Failure:** Hypoxemia with high PaCO2 (e.g., COPD, Myasthenia Gravis, Opioid overdose). * **CO2 Narcosis:** High levels of PaCO2 (>70–80 mmHg) can cause confusion, tremors, and eventually coma. * **Haldane Effect:** Deoxygenated hemoglobin has a higher affinity for CO2; this helps in CO2 loading at tissues and unloading at lungs.

Q: Severe pulmonary congestion and edema is seen when PCWP rises above which value?

25 mm Hg. **Explanation:** The correct answer is **25 mm Hg**. **1. Underlying Medical Concept:** Pulmonary Capillary Wedge Pressure (PCWP) is an indirect estimate of left atrial pressure. Under normal physiological conditions, the **Colloid Osmotic Pressure (Oncotic Pressure)** of the plasma is approximately **25–28 mm Hg**. This pressure acts to keep fluid inside the pulmonary capillaries. As long as the PCWP (hydrostatic pressure) remains below the plasma oncotic pressure, fluid filtration into the interstitium is minimal and easily cleared by lymphatics. When PCWP exceeds **25 mm Hg**, the hydrostatic pressure overcomes the oncotic pressure, causing a massive shift of fluid into the alveoli, resulting in **severe pulmonary edema**. **2. Analysis of Incorrect Options:** * **A (5 mm Hg):** This is within the normal range of PCWP (typically 5–12 mm Hg). At this level, the lungs remain "dry." * **B (10 mm Hg):** This is still within the normal physiological range and does not cause congestion. * **C (15 mm Hg):** While this represents mild elevation (seen in early heart failure), the lymphatic system can usually compensate for the slight increase in fluid transudation. Clinical congestion may begin, but "severe edema" does not occur until the oncotic threshold is breached. **3. NEET-PG High-Yield Pearls:** * **Normal PCWP:** 5–12 mm Hg. * **Cephalization (Antler Sign):** Seen on X-ray when PCWP is 12–18 mm Hg. * **Kerley B Lines:** Seen when PCWP is 18–25 mm Hg (interstitial edema). * **Bat-wing Opacity:** Seen when PCWP >25 mm Hg (alveolar edema). * **Gold Standard:** PCWP is measured using a **Swan-Ganz catheter** (Pulmonary Artery Catheterization).

Q: The initiation of the first breath in a newborn is due to changes in which of the following parameters?

pH, PaO2, and PaCO2. ### Explanation The initiation of the first breath at birth is a complex physiological event triggered by a combination of chemical, thermal, and mechanical stimuli. **Why the correct answer is right:** The primary chemical drive for the first breath is a result of the transient **asphyxia** that occurs during the final stages of labor and the clamping of the umbilical cord. This leads to: 1. **Decrease in $PaO_2$ (Hypoxia):** Stimulates peripheral chemoreceptors (carotid and aortic bodies). 2. **Increase in $PaCO_2$ (Hypercapnia):** Stimulates central chemoreceptors. 3. **Decrease in pH (Acidosis):** Further sensitizes the respiratory centers. These chemical changes act on the respiratory center in the medulla to trigger the first inspiratory effort. While other factors like cold stress (thermal) and tactile stimulation (mechanical) play a role, the alteration in blood gases is the fundamental physiological trigger. **Why other options are wrong:** In this specific question format, all options provided the same parameters ($pH$, $PaO_2$, and $PaCO_2$). In a standard NEET-PG scenario, incorrect options might include "Decreased $PaCO_2$" or "Increased $PaO_2$," which are physiologically incorrect as they would suppress the respiratory drive rather than initiate it. **High-Yield Facts for NEET-PG:** * **Surfactant:** While $pH$ and $PaO_2$ initiate the breath, **Surfactant** (produced by Type II pneumocytes) is essential to *maintain* the breath by reducing alveolar surface tension and preventing collapse. * **Stimuli Hierarchy:** The strongest chemical stimulus for *ongoing* breathing is $PaCO_2$, but for the *first* breath, the synergistic effect of hypoxia and hypercapnia is vital. * **Fetal Lung Fluid:** During birth, mechanical compression of the chest (vaginal squeeze) helps clear about 1/3 of the fetal lung fluid; the rest is absorbed by pulmonary capillaries and lymphatics.

Question 1

A 54-year-old man sustains third degree burns in a house fire. His respiratory rate is 30/min, Hb = 17 g/dL, arterial PO2 is 95 mm Hg, and arterial O2 saturation is 50%. What is the most likely cause of his low oxygen saturation?

Accepted Answer

Carbon monoxide poisoning

Answer

Airway obstruction from smoke inhalation

Answer

Pulmonary edema

Answer

Fever

Question 2

What is the respiratory quotient of protein in the body?

Accepted Answer

0.8

Answer

0.5

Answer

0.75

Answer

1

Question 3

Which of the following findings demonstrates a difference between pulmonary circulation and systemic circulation?

Accepted Answer

Pulmonary vasoconstriction in response to hypoxia

Answer

Pulmonary vasodilation in response to hypoxia

Answer

Decreased blood volume during systole

Answer

Increased basal vasoconstrictor tone

Question 4

Diffusion of oxygen at the tissue level is affected in all the following poisoning except?

Accepted Answer

Curare

Answer

Carbon monoxide

Answer

Phosgene

Answer

Cyanides

Question 5

The provided pressure-volume curves represent a normal subject and a patient with a pulmonary disease. Based on these curves, what is the likely pulmonary condition of the patient?

Accepted Answer

Emphysema

Answer

Pulmonary fibrosis

Answer

Asbestosis

Answer

Pulmonary congestion

Question 6

CO2 retention is seen in which of the following conditions?

Accepted Answer

Respiratory failure

Answer

Carbon monoxide poisoning

Answer

High altitude

Answer

All of the above

Question 7

Closing volume is the volume of lung

Accepted Answer

Above residual volume in dependent part of lung

Answer

Above residual volume in the non-dependent part of lung

Answer

Above tidal volume in non-dependent part of lung

Answer

Above tidal volume in dependent part of lung

Question 8

Severe pulmonary congestion and edema is seen when PCWP rises above which value?

Accepted Answer

25 mm Hg

Answer

5 mm Hg

Answer

10 mm Hg

Answer

15 mm Hg

Question 9

The initiation of the first breath in a newborn is due to changes in which of the following parameters?

Accepted Answer

pH, PaO2, and PaCO2

Question 10

Arrange the following physiological events in the sequence that occurs on exposure to hypoxia:

Accepted Answer

Reduced conductance of hypoxia-sensitive K+ channels - Decreased K+ efflux - Ca2+ influx - Exocytosis

Answer

Reduced conductance of hypoxia-sensitive K+ channels - Ca2+ influx - Decreased K+ efflux - Exocytosis

Answer

Decreased K+ efflux - Exocytosis - Reduced conductance of hypoxia-sensitive K+ channels - Ca2+ influx

Answer

Ca2+ influx - Exocytosis - Reduced conductance of hypoxia-sensitive K+ channels - Decreased K+ efflux

Respiratory System — MCQs

Respiratory System — MCQs

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