Respiratory System Practice Questions

Q: The normal intrapleural pressure at the start of inspiration is approximately cm of H2O?

-5.0. **Explanation:** The **intrapleural pressure (IPP)** is the pressure within the pleural cavity (the space between the visceral and parietal pleura). Under normal physiological conditions, this pressure is **subatmospheric (negative)** due to the opposing elastic recoil forces of the lungs (tending to collapse inward) and the chest wall (tending to expand outward). 1. **Why -5.0 cm H₂O is correct:** At the **start of inspiration** (which corresponds to the end of a normal expiration or Functional Residual Capacity), the inward recoil of the lungs and the outward recoil of the chest wall are in equilibrium. This creates a resting negative pressure of approximately **-5 cm H₂O**. This "suction" effect keeps the lungs inflated against the chest wall. 2. **Analysis of Incorrect Options:** * **-7.5 cm H₂O (Option A):** This is the approximate intrapleural pressure at the **end of inspiration**. As the chest wall expands during inspiration, the pleural space volume increases, making the pressure more negative (Boyle’s Law) to pull the lungs open. * **-2.0 cm H₂O (Option C):** This value is too high (less negative) for a healthy adult at rest. * **-0.5 cm H₂O (Option D):** This value typically represents the **intrapulmonary (alveolar) pressure** at the start of inspiration, not the intrapleural pressure. **High-Yield Pearls for NEET-PG:** * **Transpulmonary Pressure:** Defined as Alveolar Pressure minus Intrapleural Pressure ($P_{tp} = P_{alv} - P_{ip}$). It is always positive under normal conditions. * **Pneumothorax:** If the pleural cavity is breached, IPP becomes equal to atmospheric pressure (0 cm H₂O), leading to lung collapse. * **Basal vs. Apical IPP:** Due to gravity, IPP is **more negative at the apex** (~ -10 cm H₂O) and **less negative at the base** (~ -2.5 cm H₂O) in a standing position.

Q: Early in an attack of asthma, which of the following sets of arterial blood values is most likely?

Low PO2 and low PCO2. **Explanation:** The correct answer is **Low PO2 and low PCO2**. **1. Why it is correct:** During an acute asthma attack, bronchospasm and airway inflammation lead to **Ventilation-Perfusion (V/Q) mismatch**. Some areas of the lung are poorly ventilated but well-perfused, leading to a drop in arterial oxygen levels (**Hypoxemia/Low PO2**). In the **early stage**, the body compensates for this hypoxia through a reflex increase in the respiratory rate (tachypnea). Because Carbon Dioxide ($CO_2$) is highly diffusible (20 times more than $O_2$), this hyperventilation effectively "washes out" $CO_2$ from the blood, resulting in **Hypocapnia (Low PCO2)** and respiratory alkalosis. **2. Why the other options are incorrect:** * **Option A & B:** These are incorrect because even a mild asthma attack typically causes enough V/Q mismatch to lower the $PO_2$ below normal levels. * **Option C:** An elevated $PCO_2$ (Hypercapnia) is a **late and ominous sign** in asthma. It indicates that the patient’s respiratory muscles are fatiguing, and they can no longer maintain the high minute ventilation required to blow off $CO_2$. **3. NEET-PG High-Yield Pearls:** * **The "Normal" PCO2 Trap:** In a severe asthma attack, a "normal" $PCO_2$ value is a **danger sign**. It suggests the patient is transitioning from hyperventilation to respiratory failure. * **Acid-Base Status:** Early asthma presents with **Respiratory Alkalosis**; late/status asthmaticus presents with **Respiratory Acidosis**. * **A-a Gradient:** The Alveolar-arterial (A-a) oxygen gradient is typically **increased** in asthma due to V/Q mismatch.

Q: What happens in decreased lung compliance?

Increased respiratory rate and decreased tidal volume. **Explanation:** The core concept behind this question is the **Work of Breathing (WOB)**. In conditions with **decreased lung compliance** (e.g., Pulmonary Fibrosis, ARDS, or Pulmonary Edema), the lungs become "stiff." To expand these stiff lungs, the body must overcome significant **elastic resistance**. 1. **Why Option A is correct:** To minimize the energy expenditure (WOB), the body adopts a **Rapid Shallow Breathing Pattern**. * **Decreased Tidal Volume ($V_T$):** Taking a deep breath requires significant pressure to stretch stiff elastic fibers. By decreasing $V_T$, the elastic work is minimized. * **Increased Respiratory Rate (RR):** To maintain adequate **Minute Ventilation** ($\text{RR} \times V_T$) and prevent hypercapnia, the body compensates for the low $V_T$ by increasing the frequency of breaths. 2. **Why other options are incorrect:** * **Options B & C:** Increasing Tidal Volume in a non-compliant lung would exponentially increase the elastic work of breathing, leading to rapid respiratory muscle fatigue. * **Options C & D:** Decreasing the respiratory rate alongside a low or high $V_T$ would result in inadequate alveolar ventilation, leading to respiratory failure. **NEET-PG High-Yield Pearls:** * **Elastic Work:** Increased in restrictive diseases (decreased compliance). Minimized by rapid, shallow breathing. * **Non-Elastic (Airway) Work:** Increased in obstructive diseases (e.g., Asthma, COPD). Minimized by **slow, deep breathing** (Decreased RR, Increased $V_T$). * **Compliance Formula:** $C = \Delta V / \Delta P$. In fibrosis, the slope of the Pressure-Volume curve shifts to the **right and downwards**. * **Total Work of Breathing:** Normally represents only ~5% of total oxygen consumption but can increase significantly in disease states.

Q: Anatomic dead space in a non-smoker under normal conditions is:

2.2 cc/kg. ### Explanation **1. Understanding the Correct Answer (Option A: 2.2 cc/kg)** Anatomic 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. In a healthy, non-smoking adult, the anatomic dead space is roughly proportional to body size. The standard physiological estimate is **2.2 mL per kg (or 1 mL per pound)** of ideal body weight. For an average 70 kg adult, this equates to approximately **150 mL**. This volume is essential for warming and humidifying inspired air but does not contribute to the respiratory zone. **2. Analysis of Incorrect Options** * **Option B (5.1 cc/kg):** This value is significantly higher than physiological norms. Such a high dead space would imply severe pathological conditions or massive over-inflation of conducting zones, leading to inefficient ventilation. * **Option C (3.1 cc/kg):** While closer, this exceeds the standard 2.2 cc/kg ratio. It might be seen in specific clinical scenarios involving high PEEP (Positive End-Expiratory Pressure) or large instrumental dead space (e.g., long ventilator tubings), but it is not the "normal" value. * **Option D (1.5 cc/kg):** This value is too low for an adult. While children have a slightly different ratio, 2.2 cc/kg remains the standard benchmark for medical examinations. **3. NEET-PG High-Yield Pearls** * **Fowler’s Method:** Used to measure **Anatomic Dead Space** using single-breath nitrogen washout. * **Bohr’s Equation:** Used to measure **Physiological Dead Space** using arterial and expired $CO_2$ levels. * **Physiological vs. Anatomic:** In healthy individuals, Anatomic Dead Space $\approx$ Physiological Dead Space. In lung diseases (like COPD), Physiological Dead Space increases because of "Alveolar Dead Space" (ventilated but non-perfused alveoli). * **Positioning:** Dead space is higher when standing than when supine.

Q: Which of the following physiological conditions will lead to pulmonary vasodilation?

Baroreceptor stimulation. **Explanation:** The pulmonary circulation is unique because its primary function is gas exchange, and its vascular resistance is regulated differently compared to the systemic circulation. **1. Why Baroreceptor Stimulation is Correct:** The pulmonary vasculature is innervated by the autonomic nervous system. Stimulation of the **arterial baroreceptors** (located in the carotid sinus and aortic arch) in response to high systemic blood pressure triggers a reflex that increases parasympathetic activity and decreases sympathetic tone. This leads to **pulmonary vasodilation**, helping to accommodate volume and prevent pulmonary hypertension. **2. Why the Other Options are Incorrect:** * **Hypercapnia (A) and Chemoreceptor Stimulation (D):** In the lungs, hypercapnia (high $CO_2$) and acidosis (often sensed by chemoreceptors) act as potent **vasoconstrictors**. This is part of the body’s mechanism to divert blood flow away from poorly ventilated areas (where $CO_2$ is high) toward better-ventilated areas to optimize gas exchange. * **Decreased $PaCO_2$ (B):** While hypocapnia (low $CO_2$) generally causes vasodilation in the systemic and cerebral circulation, its effect on the pulmonary vasculature is relatively weak compared to the profound **Hypoxic Pulmonary Vasoconstriction (HPV)** triggered by low $O_2$. **Clinical Pearls for NEET-PG:** * **Hypoxic Pulmonary Vasoconstriction (HPV):** This is the most important local control mechanism. Unlike systemic vessels (which dilate during hypoxia), pulmonary vessels **constrict** in response to low alveolar $PO_2$. * **Nitric Oxide (NO):** The most potent endogenous pulmonary vasodilator; often used therapeutically in persistent pulmonary hypertension of the newborn (PPHN). * **Zone of West:** Remember that pulmonary vascular resistance is lowest at **Functional Residual Capacity (FRC)**.

Question 1

The normal intrapleural pressure at the start of inspiration is 
                            approximately cm of H2O?

Accepted Answer

-5.0

Answer

-7.5

Answer

-2.0

Answer

-0.5

Question 2

The basal regions of the upright human lung are normally better ventilated than the upper regions because:

Accepted Answer

The lower regions have a small resting volume and a relatively large increase in volume

Answer

Airway resistance of the upper regions is higher than of the lower regions

Answer

There is less surfactant in the upper regions

Answer

The blood flow to the lower regions is higher

Question 3

Excessive tidal volume load is prevented by activation of which of the following receptors?

Accepted Answer

Bronchial stretch receptors

Answer

J receptor

Answer

Thoracic muscle spindle

Answer

Aerial baroreceptor

Question 4

Which of the following neurons predominantly fire during forceful expiration?

Accepted Answer

Ventral group of neurons

Answer

Dorsal group of neurons

Answer

Pneumotaxic center

Answer

Chemoreceptors

Question 5

Early in an attack of asthma, which of the following sets of arterial blood values is most likely?

Accepted Answer

Low PO2 and low PCO2

Answer

Normal PO2 and normal PCO2

Answer

Normal PO2 and low PCO2

Answer

Normal PO2 and elevated PCO2

Question 6

What will be the effect on respiration if a transaction is made between the pons and medulla?

Accepted Answer

Irregular and gasping

Answer

Apnea

Answer

No effect

Answer

Slow and deep

Question 7

What happens in decreased lung compliance?

Accepted Answer

Increased respiratory rate and decreased tidal volume

Answer

Increased respiratory rate and increased tidal volume

Answer

Decreased respiratory rate and increased tidal volume

Answer

Decreased respiratory rate and decreased tidal volume

Question 8

Anatomic dead space in a non-smoker under normal conditions is:

Accepted Answer

2.2 cc/kg

Answer

5.1 cc/kg

Answer

3.1 cc/kg

Answer

1.5 cc/kg

Question 9

Which of the following physiological conditions will lead to pulmonary vasodilation?

Accepted Answer

Baroreceptor stimulation

Answer

Hypercapnia

Answer

Decreased PaCO2

Answer

Chemoreceptor stimulation

Question 10

At which point is the concentration of CO2 the least?

Accepted Answer

Anatomical dead space at the end of inspiration

Answer

Anatomical dead space at the end of expiration

Answer

Alveoli at the end of inspiration

Answer

Alveoli at the end of expiration

Respiratory System — MCQs

Respiratory System — MCQs

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