Respiratory System Practice Questions

Q: Anatomical dead space is:

1/3rd of tidal volume. **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 because there are no alveoli. 1. **Why Option A is correct:** In a healthy adult, the average **Tidal Volume (TV)** is approximately **500 ml**. The anatomical dead space is roughly **150 ml**. Mathematically, $150/500 = 0.3$, which is approximately **1/3rd of the tidal volume**. This ratio ($V_D/V_T$) is a standard physiological constant used to estimate the efficiency of ventilation. 2. **Why the other options are incorrect:** * **Option B:** 2/5th (40%) of TV would be 200 ml, which is higher than the physiological norm for anatomical dead space in a healthy individual. * **Options C & D:** These values are significantly overestimated. The standard rule of thumb for anatomical dead space is **2 ml/kg** of ideal body weight (e.g., a 70 kg man has ~140-150 ml of dead space). 10-15 ml/kg would exceed the total tidal volume. **High-Yield NEET-PG Pearls:** * **Fowler’s Method:** Used to measure **Anatomical Dead Space** (using Single Breath Nitrogen Washout). * **Bohr’s Equation:** Used to measure **Physiological Dead Space** (using $PCO_2$ levels). * **Physiological Dead Space** = Anatomical Dead Space + Alveolar Dead Space. In healthy individuals, they are nearly equal. * **Dead Space Increase:** It increases with upright posture, age, and drugs like atropine (bronchodilation). It decreases in the supine position and after a tracheostomy.

Q: Which of the following statements regarding carbon monoxide poisoning is incorrect?

Oxygen dissociation curve shifts to the right. ### Explanation **1. Why Option A is the Correct Answer (The Incorrect Statement)** In carbon monoxide (CO) poisoning, the Oxygen Dissociation Curve (ODC) shifts to the **left**, not the right. CO has an affinity for hemoglobin (Hb) that is approximately **200–250 times greater** than that of oxygen. When CO binds to one of the four heme sites, it causes a conformational change in the Hb molecule that increases the affinity of the remaining heme sites for oxygen. This prevents the unloading of oxygen into the tissues, leading to cellular hypoxia. A shift to the left signifies "increased affinity/decreased unloading." **2. Analysis of Other Options** * **Option B (Shift to the left):** This is a correct statement. As explained above, CO increases the affinity of remaining heme sites for oxygen, shifting the curve to the left and making the curve **hyperbolic** rather than sigmoidal. * **Option C (Carboxyhemoglobin is formed):** This is correct. The complex formed by the binding of CO to hemoglobin is called carboxyhemoglobin (COHb). * **Option D (Hyperbaric oxygen therapy):** This is a standard treatment. High-pressure oxygen reduces the half-life of COHb significantly (from ~320 minutes in room air to ~20 minutes in a hyperbaric chamber) by physically displacing CO from the hemoglobin. **3. NEET-PG High-Yield Pearls** * **The "Double Whammy":** CO poisoning causes hypoxia by two mechanisms: (1) reducing the oxygen-carrying capacity of blood and (2) shifting the ODC to the left (inhibiting O2 release). * **Pulse Oximetry Pitfall:** Standard pulse oximeters cannot distinguish between oxyhemoglobin and carboxyhemoglobin, often giving **falsely normal SpO2 readings**. * **Clinical Sign:** Classic "cherry-red" skin discoloration (rarely seen in living patients; more common post-mortem). * **P50 Value:** In CO poisoning, the P50 (partial pressure of O2 at which 50% Hb is saturated) **decreases**.

Q: Which of the following conditions is characterized by obstructive lung function?

Asthma. ### Explanation **Correct Answer: D. Asthma** **1. Why Asthma is Correct:** Asthma is a classic example of an **obstructive lung disease**. The underlying pathophysiology involves chronic airway inflammation, bronchial hyperresponsiveness, and reversible bronchoconstriction. This leads to increased resistance to airflow, particularly during expiration. On Pulmonary Function Tests (PFTs), obstructive diseases are characterized by a **decreased FEV1/FVC ratio (<0.7)** and an increased Total Lung Capacity (TLC) due to air trapping. **2. Why Other Options are Incorrect:** Options A, B, and C are all causes of **Restrictive Lung Disease**, where the primary issue is reduced lung expansion and decreased lung volumes (decreased TLC), but the FEV1/FVC ratio remains normal or increased. * **Obesity (A):** Causes "Extraparenchymal Restriction" due to the mechanical weight of excess adipose tissue on the chest wall and diaphragm. * **Kyphoscoliosis (B):** A chest wall deformity that limits the thoracic cage's ability to expand, leading to restrictive impairment. * **Pleural Effusion (C):** Fluid in the pleural space compresses the underlying lung parenchyma, preventing full expansion. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Obstructive Diseases (CBABE):** **C**ystic Fibrosis, **B**ronchitis (Chronic), **A**sthma, **B**ronchiectasis, **E**mphysema. * **Flow-Volume Loops:** In obstruction, the loop shows a "scooped-out" appearance during expiration. In restriction, the loop is narrow (witch’s hat appearance) but maintains its shape. * **Reversibility:** A hallmark of Asthma is a >12% and >200ml improvement in FEV1 after bronchodilator administration, distinguishing it from COPD.

Q: CO2 is primarily transported in the arterial blood as:

Bicarbonate. **Explanation:** Carbon dioxide ($CO_2$) is a metabolic waste product transported from the tissues to the lungs via three primary mechanisms. Understanding the quantitative distribution of these forms is crucial for NEET-PG. **1. Why Bicarbonate is Correct:** The majority of $CO_2$ (**approx. 70%**) is transported as **Bicarbonate ($HCO_3^-$)**. When $CO_2$ enters the RBCs, it reacts with water to form carbonic acid ($H_2CO_3$), a reaction catalyzed by the enzyme **Carbonic Anhydrase**. This acid quickly dissociates into $H^+$ and $HCO_3^-$. The bicarbonate then exits the RBC into the plasma in exchange for Chloride ions (the **Hamburger phenomenon** or Chloride Shift). **2. Analysis of Incorrect Options:** * **Dissolved $CO_2$ (Option A):** Only about **7%** of $CO_2$ is transported physically dissolved in the plasma. While small, this portion is responsible for the partial pressure ($PCO_2$) that exerts chemical control over ventilation. * **Carbonic Acid (Option B):** This is a transient intermediate state. Due to its rapid dissociation, the concentration of $H_2CO_3$ in the blood is negligible. * **Carbamino-hemoglobin (Option C):** About **23%** of $CO_2$ binds directly to the amino groups of globin chains (not the heme iron). This binding is influenced by the **Haldane Effect** (deoxygenation of blood increases its ability to carry $CO_2$). **High-Yield Clinical Pearls for NEET-PG:** * **Haldane Effect:** Occurs in the lungs; oxygenation of Hb promotes $CO_2$ dissociation. * **Chloride Shift:** In systemic tissues, $Cl^-$ moves **into** the RBC; in the lungs, $Cl^-$ moves **out** of the RBC (Reverse Chloride Shift). * **Enzyme:** Carbonic Anhydrase is one of the fastest known enzymes; Type II is the predominant isoform in RBCs. * **Solubility:** $CO_2$ is roughly **20-25 times** more soluble in plasma than Oxygen.

Q: As blood passes through the tissues, all of the following physiological changes occur EXCEPT:

The bicarbonate concentration of the blood decreases.. **Explanation:** This question tests your understanding of the **Chloride Shift (Hamburger Phenomenon)** and the **Bohr/Haldane effects** occurring at the tissue level. **1. Why Option A is the Correct Answer (The Exception):** As blood passes through tissues, $CO_2$ diffuses from cells into RBCs. Inside the RBC, $CO_2$ reacts with water (catalyzed by carbonic anhydrase) to form $H_2CO_3$, which dissociates into $H^+$ and $HCO_3^-$. To maintain osmotic and electrical balance, the **bicarbonate concentration in the blood actually increases** as it is pumped out of the RBC into the plasma in exchange for Chloride ($Cl^-$). Therefore, saying the concentration decreases is physiologically incorrect. **2. Analysis of Incorrect Options:** * **Option B (Affinity for $O_2$ decreases):** This is the **Bohr Effect**. Increased $PCO_2$, $H^+$ (lower pH), and temperature at the tissue level shift the oxygen-dissociation curve to the right, decreasing hemoglobin's affinity for $O_2$ to facilitate unloading. * **Option C & D (Binding of $CO_2$ and $H^+$ increases):** This is the **Haldane Effect**. When hemoglobin releases $O_2$ (becoming deoxyhemoglobin), it becomes a better buffer for $H^+$ and has a higher affinity for $CO_2$ (forming carbaminohemoglobin). This facilitates $CO_2$ transport away from tissues. **High-Yield Facts for NEET-PG:** * **Chloride Shift:** Occurs at tissues ($Cl^-$ enters RBC, $HCO_3^-$ leaves). * **Reverse Chloride Shift:** Occurs in lungs ($Cl^-$ leaves RBC, $HCO_3^-$ enters). * **Enzyme:** Carbonic anhydrase is one of the fastest enzymes; it is absent in plasma but abundant in RBCs. * **Bohr Effect:** $CO_2/H^+$ affecting $O_2$ binding (Tissues). * **Haldane Effect:** $O_2$ affecting $CO_2$ binding (Lungs).

Q: What is the anatomical dead space of a normal lung?

150 ml. ### Explanation **Correct Answer: A. 150 ml** **Underlying Medical Concept:** Anatomical dead space refers to the volume of the conducting airways (nose, pharynx, larynx, trachea, bronchi, and bronchioles) where no gas exchange occurs because these structures lack alveoli. In a healthy adult, this volume is approximately **150 ml**, or roughly **2 ml/kg** of ideal body weight. During a normal tidal volume ($V_T$) of 500 ml, only 350 ml reaches the alveoli for gas exchange, while the remaining 150 ml stays in the anatomical dead space. **Analysis of Incorrect Options:** * **B (250 ml):** This value is significantly higher than normal. Such an increase might be seen in pathological conditions or if "Physiological Dead Space" increases due to ventilation-perfusion ($V/Q$) mismatch. * **C & D (300 ml & 350 ml):** These values are incorrect for anatomical dead space. 350 ml is actually the volume of fresh air that reaches the alveoli during a normal breath ($V_T$ - Dead Space). **High-Yield Clinical Pearls for NEET-PG:** 1. **Physiological Dead Space:** This is the sum of Anatomical Dead Space + Alveolar Dead Space. In a healthy lung, it is nearly equal to anatomical dead space because alveolar dead space is negligible. 2. **Bohr’s Equation:** Used to measure **Physiological** dead space ($V_D/V_T = [PaCO_2 - PeCO_2] / PaCO_2$). 3. **Fowler’s Method:** Used to measure **Anatomical** dead space using single-breath nitrogen washout. 4. **Factors Increasing Dead Space:** Upright position (due to apical $V/Q$ changes), neck extension, and drugs like Atropine (bronchodilation). 5. **Factors Decreasing Dead Space:** Tracheostomy and supine position.

Q: In a normal resting person breathing air at sea level, what is the alveolar partial pressure of oxygen (in mm Hg)?

100 mm Hg. ### Explanation The alveolar partial pressure of oxygen ($P_AO_2$) is determined by the balance between the rate of oxygen delivery to the alveoli (via ventilation) and the rate of oxygen removal by the pulmonary capillaries. **1. Why 100 mm Hg is Correct:** At sea level, the atmospheric pressure is 760 mm Hg. As air is inhaled, it is humidified in the conducting airways, adding water vapor pressure (47 mm Hg). The partial pressure of inspired oxygen ($PiO_2$) becomes: $0.21 \times (760 - 47) \approx 150 \text{ mm Hg}$. Once in the alveoli, oxygen is constantly being absorbed into the blood while $CO_2$ enters the alveoli. Using the **Alveolar Gas Equation**: $P_AO_2 = PiO_2 - (PaCO_2 / R)$ Assuming a normal $PaCO_2$ of 40 mm Hg and a Respiratory Quotient (R) of 0.8: $P_AO_2 = 150 - (40 / 0.8) = 150 - 50 = \mathbf{100 \text{ mm Hg}}$. **2. Why the other options are incorrect:** * **90 mm Hg:** This is closer to the normal **arterial** $PO_2$ ($PaO_2$). There is a physiological **A-a gradient** (5–10 mm Hg) due to anatomical shunts and V/Q mismatch, meaning arterial blood always has slightly less oxygen than the alveoli. * **120 mm Hg:** This value is too high for a person breathing room air; it would require hyperventilation or supplemental oxygen. * **80 mm Hg:** This represents mild hypoxemia and is significantly lower than the standard physiological value for a healthy resting individual at sea level. **3. High-Yield Clinical Pearls for NEET-PG:** * **A-a Gradient:** Normal is <15 mm Hg. An increased gradient with hypoxia suggests intrinsic lung disease (e.g., fibrosis, pneumonia) or V/Q mismatch. * **Humidification:** Always remember to subtract 47 mm Hg (water vapor pressure) from total barometric pressure when calculating $PiO_2$. * **Alveolar $PCO_2$ ($P_ACO_2$):** In a healthy person, this is essentially equal to arterial $PCO_2$ (40 mm Hg).

Question 1

Anatomical dead space is:

Accepted Answer

1/3rd of tidal volume

Answer

2/5th of tidal volume

Answer

10 ml/kg of body weight

Answer

15 ml/kg of body weight

Question 2

Which of the following statements regarding carbon monoxide poisoning is incorrect?

Accepted Answer

Oxygen dissociation curve shifts to the right

Answer

Oxygen dissociation curve shifts to the left

Answer

Carboxyhemoglobin (COHb) is formed

Answer

Hyperbaric oxygen therapy can be used

Question 3

Which of the following conditions is characterized by obstructive lung function?

Accepted Answer

Asthma

Answer

Obesity

Answer

Kyphoscoliosis

Answer

Pleural effusion

Question 4

CO2 is primarily transported in the arterial blood as:

Accepted Answer

Bicarbonate

Answer

Dissolved CO2

Answer

Carbonic Acid

Answer

Carbamino-hemoglobin

Question 5

Lung compliance is greatest at/during which phase of respiration?

Accepted Answer

End-expiration

Answer

Start of inspiration

Answer

Mid-inspiration

Answer

End-inspiration

Question 6

Thickening of the respiratory membrane is seen in which condition?

Accepted Answer

Asthma

Answer

Emphysema

Answer

Empyema

Answer

Bronchiectasis

Question 7

What is the normal oxygen tension of mixed venous blood?

Accepted Answer

40 mm Hg

Answer

25 mm Hg

Answer

55 mm Hg

Answer

70 mm Hg

Question 8

As blood passes through the tissues, all of the following physiological changes occur EXCEPT:

Accepted Answer

The bicarbonate concentration of the blood decreases.

Answer

The affinity of hemoglobin for O2 decreases.

Answer

The ability of hemoglobin to bind CO2 increases.

Answer

The ability of hemoglobin to bind H+ increases.

Question 9

What is the anatomical dead space of a normal lung?

Accepted Answer

150 ml

Answer

250 ml

Answer

300 ml

Answer

350 ml

Question 10

In a normal resting person breathing air at sea level, what is the alveolar partial pressure of oxygen (in mm Hg)?

Accepted Answer

100 mm Hg

Answer

90 mm Hg

Answer

120 mm Hg

Answer

80 mm Hg

Respiratory System — MCQs

Respiratory System — MCQs

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