Respiratory System Practice Questions

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).

Q: The pneumotaxic center is located in which part of the brain?

Pons. **Explanation:** The regulation of respiration is controlled by the respiratory centers located in the brainstem. The **Pons** houses two critical centers: the **Pneumotaxic center** (located in the upper pons/nucleus parabrachialis) and the **Apneustic center** (lower pons). **1. Why Pons is Correct:** The Pneumotaxic center acts as a "switch-off" point for inspiration. It sends inhibitory signals to the dorsal respiratory group (DRG) in the medulla, limiting the duration of inspiration and increasing the respiratory rate. By controlling the "filling" time of the lungs, it prevents over-inflation and regulates the breathing pattern. **2. Why other options are incorrect:** * **Medulla:** While the medulla is the primary site for rhythm generation (containing the Dorsal Respiratory Group for inspiration and Ventral Respiratory Group for expiration), it does not house the pneumotaxic center. The Pre-Bötzinger complex in the medulla is the actual pacemaker. * **Midbrain:** The midbrain contains centers for visual and auditory reflexes but has no direct role in the primary neural control of respiratory rhythm. * **Cerebellum:** This region is responsible for motor coordination and balance; it does not regulate the involuntary respiratory cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Hering-Breuer Reflex:** This is a protective reflex triggered by stretch receptors in the lungs to prevent over-inflation, similar in function to the pneumotaxic center but mediated via the Vagus nerve. * **Apneustic Center:** If the pneumotaxic center or vagus nerves are damaged, the apneustic center causes "apneustic breathing" (prolonged inspiratory gasps). * **Location Summary:** * Upper Pons = Pneumotaxic Center. * Lower Pons = Apneustic Center. * Medulla = DRG, VRG, and Pre-Bötzinger complex.

Q: Which physiological parameter is almost the same at the apex and base of the lung?

Oxygen concentration in blood. ### Explanation The correct answer is **Oxygen concentration in blood** (Option B). In a standing position, gravity creates a vertical gradient in the lung. Both ventilation (V) and perfusion (Q) increase from the apex to the base. However, perfusion increases much more steeply than ventilation. This results in a high **Ventilation-Perfusion (V/Q) ratio** at the apex (~3.3) and a low V/Q ratio at the base (~0.6). **Why Oxygen Concentration is the same:** While the partial pressure of oxygen ($pO_2$) is significantly higher at the apex (approx. 130 mmHg) than at the base (approx. 89 mmHg), the **oxygen concentration (content)** remains nearly identical. This is due to the **Sigmoid shape of the Oxyhemoglobin Dissociation Curve**. At a $pO_2$ above 70–80 mmHg, hemoglobin is already almost fully saturated (96–98%). Therefore, the extra $pO_2$ at the apex does not significantly increase the actual amount of oxygen carried by the blood. **Analysis of Incorrect Options:** * **Ventilation (C) and Perfusion (D):** Both are significantly higher at the base than the apex due to gravity and greater alveolar compliance at the base. * **pCO2 (A):** Due to the lower V/Q ratio at the base, $CO_2$ is not "washed out" as effectively, making $pCO_2$ higher at the base (approx. 42 mmHg) compared to the apex (approx. 28 mmHg). **High-Yield NEET-PG Pearls:** * **V/Q Ratio:** Highest at the apex, lowest at the base. * **Gas Exchange:** Most efficient where V/Q is closest to 1.0 (middle zones). * **Tuberculosis:** *M. tuberculosis* prefers the **apex** because the high V/Q ratio provides a high $pO_2$ environment favorable for its growth. * **Zone of West:** In Zone 1 (Apex), Alveolar pressure > Arterial pressure, which can lead to "dead space" ventilation.

Q: The physiological dead space is decreased by:

Neck flexion. **Explanation:** **Physiological Dead Space** refers to the total volume of the respiratory system that does not participate in gas exchange. It is the sum of **Anatomical Dead Space** (conducting airways) and **Alveolar Dead Space** (ventilated but non-perfused alveoli). **Why Neck Flexion is Correct:** Anatomical dead space is directly proportional to the volume of the conducting airways. **Neck flexion** shortens the upper airway and reduces its internal volume, thereby decreasing the anatomical dead space. Conversely, neck extension or protruding the jaw increases it. **Analysis of Incorrect Options:** * **Upright Position:** In the standing position, gravity causes blood to shift to the lung bases, leaving the apices poorly perfused but well-ventilated. This increases alveolar dead space, thus increasing physiological dead space. * **Positive Pressure Ventilation (PPV):** PPV increases the volume of the conducting airways (due to high pressure) and can over-distend alveoli, potentially compressing adjacent capillaries (increasing alveolar dead space). * **Emphysema:** This condition involves the destruction of alveolar walls and capillary beds. While ventilation continues, the loss of gas-exchange surface area significantly increases alveolar dead space. **High-Yield Pearls for NEET-PG:** * **Formula:** Physiological Dead Space is calculated using the **Bohr Equation**: $V_D/V_T = (PaCO_2 - PeCO_2) / PaCO_2$. * **Normal Value:** In a healthy individual, physiological dead space roughly equals anatomical dead space (approx. 2 ml/kg or 150 ml). * **Factors Increasing Dead Space:** Age, upright posture, pulmonary embolism, anticholinergic drugs (bronchodilation), and instrument dead space (e.g., long ventilator circuits). * **Factors Decreasing Dead Space:** Supine position, tracheostomy (bypasses upper airway), and neck flexion.

Q: The slope of the pressure-volume curve of the lung represents which of the following?

Compliance of the lung. **Explanation:** **1. Why Option A is Correct:** In respiratory physiology, **Compliance (C)** is defined as the change in lung volume ($\Delta V$) per unit change in pressure ($\Delta P$). Mathematically, $C = \Delta V / \Delta P$. On a Pressure-Volume (P-V) curve, the volume is plotted on the Y-axis and pressure on the X-axis. Therefore, the **slope** of this curve ($\text{rise}/\text{run}$) represents the compliance of the lung. A steeper slope indicates high compliance (the lung expands easily), while a flatter slope indicates low compliance (a "stiff" lung). **2. Why Other Options are Incorrect:** * **B. Conductance:** This is the reciprocal of airway resistance ($G = 1/R$). It relates to the ease with which air flows through the airways, not the static elastic properties of the lung tissue represented by the P-V curve. * **C. Surface Tension:** While surface tension (governed by surfactant) is a major *determinant* of compliance, it is a force acting at the air-liquid interface, not the slope of the P-V curve itself. * **D. Transpulmonary Pressure:** This is the pressure gradient ($P_{alveolar} - P_{intrapleural}$) required to keep the lungs inflated. It is represented by the values on the X-axis of the P-V curve, not the slope. **NEET-PG High-Yield Pearls:** * **Increased Compliance:** Seen in **Emphysema** (due to loss of elastic fibers) and with aging. * **Decreased Compliance:** Seen in **Pulmonary Fibrosis**, Pulmonary Edema, and Neonatal Respiratory Distress Syndrome (NRDS) due to lack of surfactant. * **Hysteresis:** The P-V curve for inspiration is different from expiration; the lung volume at any given pressure is higher during expiration than inspiration due to the effects of surface tension.

Q: A 5-year-old child at sea level has the following arterial blood gas results: pH 7.41, PaO2 100 mmHg, and PaCO2 40 mmHg. The child is being ventilated with 80% oxygen. What is the alveolar-arterial oxygen gradient (A-a PO2)?

420 mmHg. ### Explanation To calculate the **Alveolar-arterial (A-a) oxygen gradient**, we must first determine the Alveolar Oxygen Tension ($PAO_2$) using the **Alveolar Gas Equation**: $$PAO_2 = [FiO_2 \times (P_{atm} - PH_2O)] - (PaCO_2 / R)$$ **1. Calculate $PAO_2$:** * **$FiO_2$ (Fraction of inspired oxygen):** 80% = 0.8 * **$P_{atm}$ (Atmospheric pressure at sea level):** 760 mmHg * **$PH_2O$ (Water vapor pressure at body temp):** 47 mmHg * **$PaCO_2$ (Arterial $CO_2$):** 40 mmHg * **$R$ (Respiratory quotient):** Assume 0.8 (standard) $$PAO_2 = [0.8 \times (760 - 47)] - (40 / 0.8)$$ $$PAO_2 = [0.8 \times 713] - 50$$ $$PAO_2 = 570.4 - 50 = 520.4 \text{ mmHg}$$ **2. Calculate A-a Gradient:** $$\text{A-a Gradient} = PAO_2 - PaO_2$$ $$\text{A-a Gradient} = 520.4 - 100 = 420.4 \text{ mmHg}$$ Rounding to the nearest option gives **420 mmHg**. --- #### Why the other options are incorrect: * **Option A (570.4 mmHg):** This represents only the inspired oxygen component ($PiO_2$) before accounting for $CO_2$ displacement in the alveoli. * **Option B (520.4 mmHg):** This is the calculated $PAO_2$ (Alveolar oxygen). It fails to subtract the $PaO_2$ (Arterial oxygen) to find the gradient. * **Option C (470.4 mmHg):** This value results from a calculation error, likely subtracting the $PaCO_2$ (40) directly instead of the $PaCO_2/R$ (50). --- #### High-Yield Clinical Pearls for NEET-PG: * **Normal A-a Gradient:** Increases with age. A quick formula is **(Age/4) + 4**. For a 5-year-old, the normal gradient is ~5 mmHg. * **Significance:** A high A-a gradient (like 420 mmHg) indicates a **gas exchange defect** (e.g., V/Q mismatch, Shunt, or Diffusion limitation). * **Hypoxemia with Normal A-a Gradient:** Occurs in **Alveolar Hypoventilation** (e.g., opioid overdose) or **High Altitude**. * **FiO2 Rule:** For every 10% increase in $FiO_2$, the $PAO_2$ increases by approximately 60–70 mmHg.

Q: The condition in which the arterial PO2 is normal but the amount of hemoglobin available to carry O2 is reduced?

Anaemic hypoxia. ### Explanation The correct answer is **Anaemic hypoxia**. #### 1. Why Anaemic Hypoxia is Correct Hypoxia is defined as a deficiency of oxygen at the tissue level. In **Anaemic hypoxia**, the lungs are functioning normally, so the **arterial $PO_2$ (partial pressure of dissolved oxygen) remains normal**. However, the total **oxygen-carrying capacity** of the blood is reduced because there is either a decrease in the total amount of hemoglobin (as in anemia) or the hemoglobin is unable to bind oxygen (as in Carbon Monoxide poisoning or Methemoglobinemia). Since $O_2$ content = $(1.34 \times Hb \times Saturation) + (0.003 \times PO_2)$, a drop in functional Hb leads to tissue hypoxia despite normal $PO_2$. #### 2. Why Other Options are Incorrect * **Stagnant (Ischemic) Hypoxia:** The $PO_2$ and Hb levels are normal, but blood flow to the tissues is inadequate (e.g., heart failure, shock, or local embolism). * **Histotoxic Hypoxia:** The $PO_2$ and $O_2$ delivery are normal, but the tissues cannot utilize the oxygen because cellular enzymes (like cytochrome oxidase) are inhibited (e.g., Cyanide poisoning). * **Hypoxic Hypoxia:** This is characterized by a **low arterial $PO_2$**. It occurs due to low atmospheric $O_2$ (high altitude), hypoventilation, or V/Q mismatch. #### 3. NEET-PG High-Yield Pearls * **CO Poisoning:** A classic cause of anaemic hypoxia. It shifts the Oxygen-Hemoglobin Dissociation Curve (OHDC) to the **left**, making it harder for the remaining $O_2$ to be released to tissues. * **Arterial $PO_2$ vs. $O_2$ Content:** Remember that $PO_2$ only measures dissolved oxygen, not oxygen bound to hemoglobin. * **Cyanosis:** Usually **absent** in anaemic hypoxia because there isn't enough total hemoglobin to produce the 5g/dL of deoxygenated Hb required to see a blue tint.

Q: The provided diagram shows the depth of respiration of a 45-year-old man who suffered a head injury in an automobile accident. This crescendo-decrescendo pattern of breathing is known as:

Cheyne-Stokes breathing. ***Cheyne-Stokes breathing*** • This **crescendo-decrescendo waxing-and-waning** pattern is characteristic of Cheyne-Stokes breathing, often seen after **head injury** or **brainstem damage**. • Results from altered **CO2 sensitivity** in the respiratory control centers, causing cycles of **hyperpnea** followed by **apnea**. *Apnea* • Refers to **complete cessation of breathing** for periods typically lasting more than 10 seconds. • Does not describe a **crescendo-decrescendo pattern** but rather absence of respiratory effort. *Biot's breathing* • Characterized by **irregular** breathing with periods of **apnea** followed by clusters of rapid, shallow breaths. • Lacks the **gradual waxing and waning** pattern seen in the crescendo-decrescendo description. *Hyperpnea* • Refers to **increased depth and rate** of breathing that is sustained and regular. • Does not involve the **cyclic pattern** of crescendo-decrescendo with intervening periods of reduced breathing.

Question 1

What is the anatomical dead space of a normal lung?

Accepted Answer

150 ml

Answer

250 ml

Answer

300 ml

Answer

350 ml

Question 2

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

Question 3

The pneumotaxic center is located in which part of the brain?

Accepted Answer

Pons

Answer

Medulla

Answer

Midbrain

Answer

Cerebellum

Question 4

Which physiological parameter is almost the same at the apex and base of the lung?

Accepted Answer

Oxygen concentration in blood

Answer

Partial pressure of carbon dioxide (pCO2)

Answer

Ventilation

Answer

Perfusion

Question 5

The physiological dead space is decreased by:

Accepted Answer

Neck flexion

Answer

Upright position

Answer

Positive pressure ventilation

Answer

Emphysema

Question 6

The slope of the pressure-volume curve of the lung represents which of the following?

Accepted Answer

Compliance of the lung

Answer

Conductance

Answer

Surface tension

Answer

Transpulmonary pressure

Question 7

A 5-year-old child at sea level has the following arterial blood gas results: pH 7.41, PaO2 100 mmHg, and PaCO2 40 mmHg. The child is being ventilated with 80% oxygen. What is the alveolar-arterial oxygen gradient (A-a PO2)?

Accepted Answer

420 mmHg

Answer

570.4 mmHg

Answer

520.4 mmHg

Answer

470.4 mmHg

Question 8

The condition in which the arterial PO2 is normal but the amount of hemoglobin available to carry O2 is reduced?

Accepted Answer

Anaemic hypoxia

Answer

Stagnant hypoxia

Answer

Histotoxic hypoxia

Answer

Hypoxic hypoxia

Question 9

Which of the following statements is true regarding the Hb/O2 dissociation curve?

Accepted Answer

Hypothermia shifts the curve to the left.

Answer

Fetal hemoglobin shifts the curve to the left.

Answer

Hypercarbia shifts the curve to the left.

Answer

Acidosis shifts the curve to the left.

Question 10

The provided diagram shows the depth of respiration of a 45-year-old man who suffered a head injury in an automobile accident. This crescendo-decrescendo pattern of breathing is known as:

Accepted Answer

Cheyne-Stokes breathing

Answer

Apnea

Answer

Biot's breathing

Answer

Hyperpnea

Respiratory System — MCQs

Respiratory System — MCQs

On this page

Practice by Chapter

Want unlimited practice?