Respiratory System Practice Questions

Q: What is the normal partial pressure of carbon dioxide (pCO2) in exhaled air?

27 mm Hg. The partial pressure of carbon dioxide in exhaled air ($P_E CO_2$) is approximately **27 mm Hg**. This value is a result of the mixing of alveolar air with air from the anatomical dead space. ### **Detailed Explanation** 1. **The Concept of Mixing:** During expiration, the first portion of air expelled comes from the **anatomical dead space** (trachea and bronchi), where $PCO_2$ is nearly **0 mm Hg** (similar to atmospheric air). The latter portion is **alveolar air**, which has a $PCO_2$ of **40 mm Hg**. 2. **The Resultant Value:** The total exhaled air (mixed expired air) is a combination of these two. Since dead space accounts for roughly one-third of a normal tidal volume, the $PCO_2$ is diluted from 40 mm Hg down to approximately **27–28 mm Hg**. ### **Analysis of Options** * **A. 36 mm Hg:** This is too high for mixed expired air; it is closer to the $PCO_2$ of end-tidal air (which represents pure alveolar air). * **B. 27 mm Hg (Correct):** This represents the average $PCO_2$ in a full breath of expired air after dilution by dead space. * **C. 40 mm Hg:** This is the $PCO_2$ of **alveolar air** and **systemic arterial blood ($PaCO_2$)**. * **D. 17 mm Hg:** This value is too low and would only be seen in states of extreme hyperventilation or increased dead space ventilation. ### **NEET-PG High-Yield Pearls** * **Bohr Equation:** Uses the difference between $PaCO_2$ (40) and $P_E CO_2$ (27) to calculate the **Physiological Dead Space**. * **End-Tidal $CO_2$ ($EtCO_2$):** In clinical anesthesia, $EtCO_2$ monitors the very last portion of expired air. It is usually **35–40 mm Hg**, reflecting alveolar $CO_2$ levels. * **Inspired Air $PCO_2$:** Is negligible (~0.3 mm Hg).

Q: Decreased O2 affinity of Hb in blood with decreased pH is known as which effect?

Bohr effect. ### Explanation **1. Why the Bohr Effect is Correct:** The **Bohr effect** describes the phenomenon where an increase in $CO_2$ concentration or a **decrease in pH** (increased $H^+$) leads to a decreased affinity of hemoglobin (Hb) for oxygen. * **Mechanism:** When $H^+$ ions bind to specific amino acid residues in the Hb molecule, they stabilize the **T-state (Tense state)** or deoxygenated form of hemoglobin. This causes the Oxygen-Dissociation Curve (ODC) to **shift to the right**, facilitating the unloading of oxygen to metabolically active tissues. **2. Why the Other Options are Incorrect:** * **Haldane Effect (Option A):** This is the mirror image of the Bohr effect but concerns $CO_2$ transport. It states that deoxygenated Hb has an increased affinity for $CO_2$. Essentially, oxygen displacement from Hb in the lungs promotes $CO_2$ unloading. * **Double Haldane Effect (Option B):** This is a physiological occurrence in the **placenta**. It involves the simultaneous operation of the Haldane effect in both maternal and fetal blood to facilitate $CO_2$ transfer from fetus to mother. * **Double Bohr Effect (Option D):** This also occurs in the **placenta**. As maternal blood takes up $CO_2$ (shifting its ODC to the right), the fetal blood loses $CO_2$ (shifting its ODC to the left). This "double shift" ensures maximal oxygen transfer from mother to fetus. **3. High-Yield Clinical Pearls for NEET-PG:** * **Right Shift of ODC (Mnemonic: CADET, face Right!):** **C**O2 increase, **A**cidosis (low pH), **D**PG (2,3-BPG) increase, **E**xercise, **T**emperature increase. * **P50 Value:** The partial pressure of $O_2$ at which Hb is 50% saturated. A **Bohr effect increases the P50**, indicating decreased affinity. * **Site of Action:** Bohr effect occurs at the **tissue level** (unloading $O_2$), while the Haldane effect occurs at the **lung level** (unloading $CO_2$).

Q: Oxygen release to tissues is affected by all of the following EXCEPT:

Bicarbonate. **Explanation:** The release of oxygen to tissues is primarily governed by the **Oxygen-Dissociation Curve (ODC)** and the affinity of hemoglobin for oxygen. Factors that shift the curve to the right decrease hemoglobin's affinity for oxygen, thereby increasing oxygen unloading to the tissues. **Why Bicarbonate (C) is the correct answer:** While bicarbonate ($HCO_3^-$) is the primary form in which carbon dioxide is transported in the blood (70%), it does not directly influence the release of oxygen from hemoglobin. It is the **partial pressure of $CO_2$ ($PCO_2$)** and the resulting **pH (hydrogen ion concentration)** that affect oxygen affinity via the Bohr effect, not the bicarbonate ion itself. **Analysis of Incorrect Options:** * **2,3-DPG (A):** An increase in 2,3-Diphosphoglycerate (produced during glycolysis) binds to the beta chains of deoxyhemoglobin, stabilizing the "T" (Tense) state and shifting the ODC to the right, promoting oxygen release. * **pH (B):** According to the **Bohr Effect**, a decrease in pH (acidosis) reduces hemoglobin's affinity for oxygen, facilitating its release to metabolically active tissues. * **Globin chain (D):** The structure of the globin chain determines oxygen affinity. For example, **Fetal Hemoglobin (HbF)** has gamma chains instead of beta chains, which prevents 2,3-DPG binding, leading to a higher affinity for $O_2$ (left shift) compared to adult hemoglobin (HbA). **High-Yield Clinical Pearls for NEET-PG:** * **Right Shift (Increased $O_2$ release):** "CADET, face Right!" (**C**-CO2, **A**-Acidosis, **D**-DPG, **E**-Exercise, **T**-Temperature). * **Left Shift (Decreased $O_2$ release):** Fetal Hb, CO poisoning, Methemoglobinemia, and Hypothermia. * **Haldane Effect:** Describes how oxygenation of blood in the lungs displaces $CO_2$ from hemoglobin (opposite of the Bohr effect).

Q: Regarding lung volumes, which of the following is true?

Residual volume keeps alveoli inflated between breaths. **Explanation:** **Correct Option (B):** Residual Volume (RV) is the volume of air remaining in the lungs after a maximal forced expiration. Its primary physiological function is to provide a continuous gas exchange surface and prevent the collapse of alveoli (atelectasis) at the end of expiration. By keeping the alveoli partially inflated, it reduces the "work of breathing" required to re-expand them during the next inspiration. **Analysis of Incorrect Options:** * **Option A:** Functional Residual Capacity (FRC) is the sum of RV and Expiratory Reserve Volume (ERV). It typically accounts for approximately **40%** of Total Lung Capacity (TLC), not 75%. * **Option C:** Vital Capacity (VC) **decreases** with age. This is due to a decrease in lung compliance (stiffening of chest wall), weakening of respiratory muscles, and an increase in Residual Volume. * **Option D:** The average value for Residual Volume is approximately **1100–1200 ml**. The value of 500 ml refers to the **Tidal Volume (TV)**, which is the volume of air inspired or expired during a normal breath. **High-Yield Clinical Pearls for NEET-PG:** * **Measurement:** RV, FRC, and TLC **cannot** be measured by simple spirometry because they contain air that cannot be exhaled. They are measured using Helium Dilution, Nitrogen Washout, or Body Plethysmography. * **Obstructive vs. Restrictive:** RV and FRC are characteristically **increased** in obstructive lung diseases (e.g., Emphysema) due to air trapping, but **decreased** in restrictive lung diseases (e.g., Pulmonary Fibrosis). * **Closing Capacity:** In the elderly, the "Closing Volume" increases and may exceed the FRC, leading to small airway collapse during normal breathing.

Q: Which of the following statements regarding dyspnoea are True or False?

a,b,d,e - True; c - False. Dyspnoea is a subjective experience of breathing discomfort. Understanding its physiological mechanisms is crucial for NEET-PG. * **Statement (a) True:** Dyspnoea results from an imbalance between the **"drive" (afferent signaling)** to breathe and the **"actual" (efferent feedback)** ventilation achieved [1]. This is known as the **neuromechanical dissociation** theory. * **Statement (b) True:** Chemoreceptors in the carotid bodies and medulla are sensitive to **Hypercapnia (↑CO2)** and **Acidosis (↓pH)** [2]. Stimulation of these receptors increases the respiratory drive, leading to the sensation of air hunger. * **Statement (c) False:** While motor cortex activation is involved, the primary sensation of dyspnoea is mediated through the **Limbic System (specifically the Anterior Insula and Anterior Cingulate Cortex)**, which processes the emotional distress associated with breathlessness. * **Statement (d) True:** Pulmonary stretch receptors and J-receptors (juxtacapillary) in the lungs respond to mechanical changes like lung inflation or interstitial edema, sending signals via the **Vagus nerve** to the brainstem [3]. * **Statement (e) True:** In conditions like asthma or COPD, the increased **work of breathing** (due to airway resistance or low compliance) requires increased recruitment of respiratory muscles, which is perceived as "chest tightness" or "effort." **Clinical Pearls for NEET-PG:** 1. **Orthopnoea:** Dyspnoea while lying flat; highly specific for Left Ventricular Failure (LVF) [1]. 2. **Platypnoea:** Dyspnoea while upright; seen in Hepatopulmonary Syndrome and Atrial Septal Defects. 3. **Trepopnoea:** Dyspnoea when lying on one side; seen in unilateral lung disease or pleural effusion. 4. **Paroxysmal Nocturnal Dyspnoea (PND):** Occurs 1-2 hours after falling asleep; pathognomonic for Congestive Heart Failure.

Q: With each quiet inspired breath, what fraction of alveolar air is replaced?

1/7. **Explanation:** The correct answer is **1/7**. This concept relates to the **Functional Residual Capacity (FRC)** and the efficiency of alveolar ventilation during quiet breathing. **1. Why 1/7 is correct:** During quiet inspiration (Tidal Volume), approximately **500 mL** of air is inhaled. However, about 150 mL remains in the anatomical dead space. Thus, only **350 mL** of fresh air actually reaches the alveoli. At the end of a normal expiration, the volume of air remaining in the lungs is the **FRC**, which is approximately **2300 mL**. The fraction of alveolar air replaced is calculated as: * *Fresh air reaching alveoli / FRC* = 350 mL / 2300 mL ≈ **1/6.5 to 1/7**. This slow replacement is physiologically vital as it prevents sudden changes in blood gas concentrations (O₂ and CO₂), ensuring stable gas exchange even if breathing is temporarily interrupted. **2. Why other options are incorrect:** * **1/2 and 1/3:** These fractions are too high. If 30-50% of alveolar air were replaced with each breath, the partial pressures of O₂ and CO₂ would fluctuate wildly, leading to respiratory instability. * **1/10:** This value underestimates the efficiency of normal ventilation. While it might be seen in certain restrictive lung pathologies with very low tidal volumes, it is not the physiological norm. **Clinical Pearls & High-Yield Facts:** * **FRC (Functional Residual Capacity):** Acts as a "buffer" for gas exchange. It is the sum of Expiratory Reserve Volume (ERV) and Residual Volume (RV). * **Nitrogen Washout Method:** This slow replacement rate is the principle behind using 100% oxygen to "wash out" nitrogen from the FRC to measure lung volumes. * **Time Constant:** It takes about 17 seconds (multiple breaths) to replace half of the alveolar gas in a normal person.

Question 1

Medullary chemoreceptors are primarily sensitive to which of the following?

Accepted Answer

H+ concentration in the cerebrospinal fluid (CSF)

Answer

CO2 concentration in the cerebrospinal fluid (CSF)

Answer

H+ concentration in the blood

Answer

CO2 concentration in the blood

Question 2

What is the normal partial pressure of carbon dioxide (pCO2) in exhaled air?

Accepted Answer

27 mm Hg

Answer

36 mm Hg

Answer

40 mm Hg

Answer

17 mm Hg

Question 3

Decreased O2 affinity of Hb in blood with decreased pH is known as which effect?

Accepted Answer

Bohr effect

Answer

Haldane effect

Answer

Double Haldane effect

Answer

Double Bohr effect

Question 4

Oxygen release to tissues is affected by all of the following EXCEPT:

Accepted Answer

Bicarbonate

Answer

2,3-DPG

Answer

pH

Answer

Globin chain

Question 5

Regarding lung volumes, which of the following is true?

Accepted Answer

Residual volume keeps alveoli inflated between breaths

Answer

Functional residual capacity accounts for 75% of total lung capacity

Answer

Vital capacity increases in the elderly

Answer

Residual volume is about 500 ml

Question 6

Oxygen affinity of hemoglobin increases with which of the following?

Accepted Answer

Carbon monoxide

Answer

Acidosis

Answer

Hypoxia

Answer

Anemia

Question 7

Which of the following statements regarding dyspnoea are True or False?

Accepted Answer

a,b,d,e - True; c - False

Answer

c,d,e - True; a,b - False

Answer

a,d,e - True; b,c - False

Answer

a,d - True; b,c,e - False

Question 8

PO2 is maximum in which part of the lung?

Accepted Answer

Apex of lung

Answer

Base of lung

Answer

Posterior lobe

Answer

Middle lobe

Question 9

With each quiet inspired breath, what fraction of alveolar air is replaced?

Accepted Answer

1/7

Answer

1/2

Answer

1/3

Answer

1/10

Question 10

A 49-year-old man has a pulmonary embolism that completely blocks blood flow to his left lung. As a result, which of the following will occur?

Accepted Answer

Alveolar PO2 in the left lung will be approximately equal to the PO2 in inspired air

Answer

Ventilation/perfusion (V/Q) ratio in the left lung will be zero

Answer

Systemic arterial PO2 will be elevated

Answer

V/Q ratio in the left lung will be lower than in the right lung

Respiratory System — MCQs

Respiratory System — MCQs

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