Respiratory System Practice Questions

Q: What will be hemoglobin saturation if P02 is 60 mm Hg at pH 7.4 and temperature 37°C?

90%. The correct answer is **90%**. This question tests your understanding of the **Oxyhemoglobin Dissociation Curve (ODC)**, which describes the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin ($SaO_2$). ### 1. Why 90% is Correct The ODC is sigmoid-shaped due to the "cooperative binding" of hemoglobin. There are three high-yield "anchor points" on the curve that every NEET-PG aspirant must memorize: * **$P_{50}$:** At a $PO_2$ of **27 mmHg**, hemoglobin is **50%** saturated. * **Venous Point:** At a $PO_2$ of **40 mmHg**, hemoglobin is **75%** saturated. * **The "Shoulder" Point:** At a $PO_2$ of **60 mmHg**, hemoglobin is **90%** saturated. At $PO_2$ 60 mmHg, the curve begins to flatten. This is clinically significant because it means that even if $PO_2$ drops from 100 to 60 mmHg, oxygen saturation remains relatively high (90%), providing a safety buffer for oxygen delivery. ### 2. Why Other Options are Incorrect * **50%:** This occurs at a $PO_2$ of 27 mmHg (the $P_{50}$ value). * **60%:** This is a distractor; students often confuse $PO_2$ values with saturation percentages. * **75%:** This corresponds to a $PO_2$ of 40 mmHg, which is typical for mixed venous blood. ### 3. Clinical Pearls for NEET-PG * **Right Shift (Decreased Affinity):** Caused by "CADET, face Right!" (**C**O2 increase, **A**cidosis/Low pH, **D**PG (2,3-BPG) increase, **E**xercise, **T**emperature increase). * **Left Shift (Increased Affinity):** Caused by Alkalosis, Hypothermia, decreased 2,3-BPG, and Fetal Hemoglobin (HbF). * **Critical Threshold:** A $PO_2$ below 60 mmHg marks the beginning of the "steep" portion of the curve, where small further drops in $PO_2$ lead to drastic desaturation and potential tissue hypoxia.

Q: Which of the following statements regarding the pneumotaxic center is true?

Both of the above.. The respiratory center in the brainstem is divided into the medullary and pontine centers. The **Pneumotaxic Center**, located in the upper pons (specifically the nucleus parabrachialis and Kolliker-Fuse nucleus), plays a critical role in regulating the respiratory pattern. ### **Explanation of the Correct Answer** **Option C (Both A and B) is correct because:** 1. **Neuronal Composition:** Unlike the Dorsal Respiratory Group (DRG), which is primarily inspiratory, the pneumotaxic center contains a heterogeneous population of **both inspiratory (I) and expiratory (E) neurons**. These neurons fire at different phases of the respiratory cycle to coordinate the transition between inspiration and expiration. 2. **Fine-tuning Function:** Its primary physiological role is to act as an **"off-switch" for inspiration**. By limiting the duration of inspiration, it indirectly controls the tidal volume and respiratory rate. This "fine-tuning" ensures a smooth, rhythmic transition, preventing gasping or irregular breathing patterns. ### **Analysis of Options** * **Option A:** True. Electrophysiological studies confirm the presence of both I and E neurons in the pontine nuclei. * **Option B:** True. It modulates the medullary rhythm generators to adapt to various physiological needs, effectively fine-tuning the breath. ### **High-Yield Facts for NEET-PG** * **Location:** Upper Pons. * **Effect of Stimulation:** Strong stimulation of the pneumotaxic center **increases respiratory rate** (by shortening inspiration) and **decreases tidal volume**. * **Lesion Effect:** A lesion in the pneumotaxic center (especially if the Vagus nerve is also cut) leads to **Apneusis**—prolonged inspiratory gasps with brief expiratory movements. * **Antagonist:** It functions antagonistically to the **Apneustic Center** (located in the lower pons), which promotes inspiration.

Q: In body plethysmography, a person is asked to expire against a closed glottis. What will be the change in the pressure in the lung and the box?

Increase in lung, decrease in box. ### Explanation **1. Underlying Concept: Boyle’s Law** Body plethysmography is based on **Boyle’s Law**, which states that at a constant temperature, the pressure ($P$) and volume ($V$) of a gas are inversely proportional ($P \times V = K$). When a person attempts to **expire against a closed glottis** (a maneuver similar to the Valsalva maneuver), they are compressing the air within their lungs. * **In the Lungs:** The expiratory effort decreases the volume of the thoracic cavity. According to Boyle’s Law, as volume decreases, the **pressure in the lungs increases**. * **In the Box:** As the chest wall and lungs compress (occupying less space), the volume of the air *outside* the body but *inside* the airtight box increases. Consequently, the **pressure in the box decreases**. **2. Analysis of Incorrect Options** * **Option A & B:** These are incorrect because the system is closed. A change in the thoracic volume must result in an equal and opposite change in the box volume. Therefore, pressure changes cannot occur in the same direction for both. * **Option D:** This describes the opposite physiological event—**inspiration** against a closed glottis (Müller’s maneuver). During inspiration, lung volume increases (decreasing lung pressure) and the chest expands, compressing the air in the box (increasing box pressure). **3. Clinical Pearls & High-Yield Facts** * **Gold Standard:** Body plethysmography is the gold standard for measuring **Functional Residual Capacity (FRC)**, especially in patients with obstructive lung diseases (e.g., COPD) where helium dilution methods underestimate volume due to "trapped air." * **Total Lung Capacity (TLC):** Once FRC is determined via plethysmography, TLC and Residual Volume (RV) can be calculated. * **Maneuver:** The specific maneuver used during the test is often referred to as "panting" against a shutter.

Q: The intrapleural pressure is negative both during inspiration and expiration because:

The thoracic cage and lungs are elastic structures. ### Explanation **1. Why the Correct Answer is Right:** The intrapleural pressure (IPP) is negative (sub-atmospheric) due to the **opposing elastic recoil forces** of the lungs and the thoracic cage. * The **lungs** have a natural tendency to recoil inward (collapse) due to elastic fibers and surface tension. * The **thoracic cage** has a natural tendency to recoil outward (expand). These two structures are held together by the thin film of pleural fluid. As they pull in opposite directions, they create a "vacuum" effect in the potential space between them, resulting in a negative IPP. This negativity persists even during expiration because the equilibrium point (Functional Residual Capacity) is only reached when these opposing forces are balanced, not eliminated. **2. Why Incorrect Options are Wrong:** * **Option A:** Intrapulmonary pressure is *not* always negative. It becomes negative during inspiration (to pull air in) and positive during expiration (to push air out). * **Option C:** Transpulmonary pressure is the *result* of the difference between alveolar and intrapleural pressure ($P_{tp} = P_{alv} - P_{ip}$); it is a measure of the distending force, not the primary cause of IPP negativity. * **Option D:** While surfactant reduces surface tension and prevents alveolar collapse, it does not generate the negative pressure in the pleural cavity; that is a function of the chest wall-lung interaction. **3. NEET-PG High-Yield Pearls:** * **Normal Values:** IPP is approximately **-5 cm $H_2O$** at the start of inspiration and drops to **-7.5 cm $H_2O$** at the end of inspiration. * **Pneumothorax:** If the pleural cavity is breached, air enters the space, IPP becomes equal to atmospheric pressure, and the lung collapses due to its unopposed inward recoil. * **Gravity Effect:** IPP is **most negative at the apex** of the lung and least negative at the base in an upright position.

Q: Diffusion hypoxia is a type of hypoxia?

Hypoxic. **Explanation:** **Diffusion Hypoxia** (also known as the Fink effect) occurs primarily during recovery from general anesthesia when using **Nitrous Oxide (N₂O)**. 1. **Why Hypoxic Hypoxia is correct:** Hypoxic hypoxia is defined by a decrease in the arterial partial pressure of oxygen ($PaO_2$). When N₂O administration is stopped, it rushes out of the blood into the alveoli due to its low blood solubility. This rapid influx of N₂O **dilutes the concentration of Oxygen** and Carbon Dioxide within the alveoli. The resulting drop in alveolar $PO_2$ leads to a decrease in arterial $PaO_2$, fitting the definition of hypoxic hypoxia. 2. **Why other options are incorrect:** * **Anemic Hypoxia:** Occurs when the oxygen-carrying capacity of the blood is reduced (e.g., low hemoglobin, CO poisoning). In diffusion hypoxia, hemoglobin levels are normal. * **Stagnant (Ischemic) Hypoxia:** Results from reduced blood flow to tissues (e.g., heart failure, shock). Here, cardiac output and systemic perfusion remain unchanged. * **Histotoxic Hypoxia:** Occurs when tissues cannot utilize oxygen despite adequate delivery (e.g., Cyanide poisoning). In diffusion hypoxia, the cellular machinery is functional; the supply is simply diluted. **Clinical Pearls for NEET-PG:** * **Prevention:** To prevent diffusion hypoxia, clinicians administer **100% Oxygen** for 3–5 minutes after discontinuing N₂O. * **Mechanism:** It is the reverse of the "Second Gas Effect." * **CO₂ Dilution:** N₂O also dilutes alveolar $CO_2$, which can decrease the respiratory drive, further exacerbating the hypoxia.

Q: Shift of the oxygen dissociation curve to the right is caused by the following factors EXCEPT?

Increased concentration of oxygen. The Oxygen Dissociation Curve (ODC) represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **shift to the right** indicates a decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to the tissues. ### Why "Increased concentration of oxygen" is the Correct Answer: An increase in the concentration of oxygen ($PO_2$) does not shift the curve to the right; rather, it moves the point **along the curve** toward the right (higher saturation). A rightward shift is caused by factors that stabilize the **Tense (T) state** of hemoglobin. Increased oxygen stabilizes the **Relaxed (R) state**, which actually increases hemoglobin's affinity for more oxygen (cooperativity). ### Explanation of Incorrect Options (Factors that DO shift the curve to the right): * **Increased 2,3-BPG:** This byproduct of glycolysis binds to the beta chains of hemoglobin, stabilizing the deoxygenated T-state and promoting $O_2$ release. * **Increased Temperature:** Higher metabolic activity (e.g., exercise or fever) increases temperature, which weakens the bond between hemoglobin and oxygen, shifting the curve to the right. * **Increased $CO_2$ (Bohr Effect):** High $CO_2$ levels lead to increased $H^+$ concentration (decreased pH). Both $CO_2$ and $H^+$ bind to hemoglobin, reducing its affinity for oxygen. ### High-Yield NEET-PG Pearls: * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2, **A**cid ($H^+$), **D**PG (2,3-BPG), **E**xercise, and **T**emperature. * **Left Shift:** Occurs in conditions like fetal hemoglobin (HbF), carbon monoxide poisoning, and alkalosis (decreased $H^+$). * **P50 Value:** The $PO_2$ at which hemoglobin is 50% saturated. A right shift **increases** the P50 (normal is ~26.7 mmHg).

Q: What is the partial pressure of oxygen in the alveoli?

103 mmHg. **Explanation:** The partial pressure of oxygen in the alveoli ($PAO_2$) is determined by the balance between the delivery of oxygen via ventilation and its removal by pulmonary capillary blood [1]. At sea level, atmospheric air has a $PO_2$ of approximately 160 mmHg. As this air is inhaled, it is humidified in the upper airways, adding water vapor pressure (47 mmHg), which reduces the $PO_2$ to ~149 mmHg. Once it reaches the alveoli, it mixes with residual carbon dioxide ($CO_2$) diffusing out of the blood. According to the **Alveolar Gas Equation**, the final $PAO_2$ is approximately **103–104 mmHg**. **Analysis of Options:** * **A (60 mmHg):** This is the threshold below which significant hypoxemia occurs and is often the target $PaO_2$ in clinical respiratory failure management. * **B (103 mmHg):** **Correct.** This represents the standard alveolar oxygen tension under normal physiological conditions at sea level [1]. * **C (136 mmHg):** This value does not correspond to standard physiological oxygen pressures in the respiratory cycle. * **D (160 mmHg):** This is the $PO_2$ of **dry atmospheric air** at sea level (21% of 760 mmHg) before it enters the respiratory tract. **NEET-PG High-Yield Pearls:** 1. **Alveolar Gas Equation:** $PAO_2 = FiO_2(P_{atm} - PH_2O) - (PaCO_2 / R)$. 2. **A-a Gradient:** The difference between alveolar ($PAO_2$) and arterial ($PaO_2$) oxygen. A normal gradient is <15 mmHg (increases with age). An increased gradient suggests shunting, V/Q mismatch, or diffusion defects. 3. **Venous $PO_2$:** The partial pressure of oxygen in mixed venous blood is typically **40 mmHg**.

Question 1

What will be hemoglobin saturation if P02 is 60 mm Hg at pH 7.4 and temperature 37°C?

Accepted Answer

90%

Answer

50%

Answer

60%

Answer

75%

Question 2

Which of the following statements regarding the pneumotaxic center is true?

Accepted Answer

Both of the above.

Answer

It contains both inspiratory (I) and expiratory (E) neurons.

Answer

It may play a role in the fine-tuning of breathing.

Answer

None of the above.

Question 3

In body plethysmography, a person is asked to expire against a closed glottis. What will be the change in the pressure in the lung and the box?

Accepted Answer

Increase in lung, decrease in box

Answer

Increase in both

Answer

Decrease in both

Answer

Decrease in lung, increase in box

Question 4

The intrapleural pressure is negative both during inspiration and expiration because:

Accepted Answer

The thoracic cage and lungs are elastic structures

Answer

Intrapulmonary pressure is always negative

Answer

Transpulmonary pressure determines the negativity

Answer

Surfactant prevents the lungs from collapsing

Question 5

Extrapulmonary restrictive defects lead to the development of which acid-base disturbance?

Accepted Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Answer

Increased DLco

Answer

Reduced DLco

Question 6

Diffusion hypoxia is a type of hypoxia?

Accepted Answer

Hypoxic

Answer

Anemic

Answer

Histotoxic

Answer

Stagnant

Question 7

Pleural pressure is positive during which phase of respiration?

Accepted Answer

End of forced expiration

Answer

End of inspiration

Answer

End of expiration

Answer

Start of inspiration

Question 8

Shift of the oxygen dissociation curve to the right is caused by the following factors EXCEPT?

Accepted Answer

Increased concentration of oxygen

Answer

Increased 2,3-BPG

Answer

Increased temperature

Answer

Increased concentration of carbon dioxide

Question 9

What is the partial pressure of oxygen in the alveoli?

Accepted Answer

103 mmHg

Answer

60 mmHg

Answer

136 mmHg

Answer

160 mmHg

Question 10

Laminar flow is directly proportional to:

Accepted Answer

Viscosity

Answer

Density

Answer

Radius

Answer

Velocity

Respiratory System — MCQs

Respiratory System — MCQs

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