Respiratory System Practice Questions

Q: What is the partial pressure of oxygen in the expired air?

116 mm Hg. **Explanation:** The partial pressure of oxygen ($PO_2$) in **expired air** is approximately **116 mm Hg**. This value is a result of the mixing of two distinct air volumes during exhalation: 1. **Dead Space Air:** This air remains in the conducting zones (trachea, bronchi) and does not participate in gas exchange. Its $PO_2$ is similar to humidified inspired air (~149–150 mm Hg). 2. **Alveolar Air:** This air has undergone gas exchange, resulting in a lower $PO_2$ of approximately 100 mm Hg. When these two volumes mix during expiration, the resulting $PO_2$ (116 mm Hg) is higher than alveolar air but lower than atmospheric air. **Analysis of Incorrect Options:** * **Option B (158 mm Hg):** This is the $PO_2$ of **atmospheric (dry) air** at sea level. Once air is inspired and humidified in the upper airways, the $PO_2$ drops to ~149 mm Hg due to the addition of water vapor pressure. * **Option C (100 mm Hg):** This is the $PO_2$ of **alveolar air**. It is lower than inspired air because oxygen is constantly diffusing into the pulmonary capillaries. * **Option D (0.3 mm Hg):** This is the partial pressure of **Carbon Dioxide ($PCO_2$)** in atmospheric air. **NEET-PG High-Yield Pearls:** * **$PCO_2$ in Expired Air:** Approximately **27–32 mm Hg** (lower than alveolar $PCO_2$ of 40 mm Hg due to dilution with dead space air). * **Water Vapor Pressure:** At body temperature (37°C), it is always **47 mm Hg**. * **Alveolar Gas Equation:** $PAO_2 = FiO_2(P_{atm} - PH_2O) - (PaCO_2 / R)$. This is crucial for calculating the A-a gradient.

Q: A healthy 30-year-old male runs on the treadmill for 30 minutes. Which of the following muscles are primarily used during expiration?

Internal intercostals and abdominal recti. **Explanation:** The key to answering this question lies in distinguishing between **quiet breathing** and **forced (active) breathing**. 1. **Why Option C is Correct:** In a healthy individual at rest, expiration is a **passive process** resulting from the elastic recoil of the lungs. However, during exercise (like running on a treadmill), ventilation increases, and expiration becomes an **active process**. * **Abdominal Recti:** These are the most important muscles for forced expiration. They contract to push the abdominal contents upward against the diaphragm, forcefully decreasing the thoracic volume. * **Internal Intercostals:** These muscles pull the rib cage downward and inward (depressing the ribs), further decreasing the thoracic dimensions. 2. **Analysis of Incorrect Options:** * **Option A & B (Diaphragm):** The diaphragm is the primary muscle of **inspiration**. While it relaxes during expiration, it does not "drive" the expiratory phase. * **Option A & D (Sternocleidomastoid):** This is an **accessory muscle of inspiration**. It helps lift the sternum upward during deep or labored breathing (e.g., respiratory distress) to increase thoracic volume. 3. **NEET-PG High-Yield Pearls:** * **Primary Muscle of Quiet Inspiration:** Diaphragm (contributes ~75% of air movement). * **Accessory Muscles of Inspiration:** Sternocleidomastoid (lifts sternum), Scalene (lifts first two ribs), and External Intercostals. * **Active Expiration:** Occurs during exercise, coughing, sneezing, or in obstructive pathologies like COPD/Asthma. * **Bucket Handle Movement:** Mediated by external intercostals (increases transverse diameter). * **Pump Handle Movement:** Mediated by the sternum/upper ribs (increases anteroposterior diameter).

Q: All of the following muscles are involved in inspiration except?

Internal intercostal muscle. **Explanation:** The mechanics of breathing depend on the coordinated contraction of specific muscle groups to alter thoracic volume. **1. Why the Correct Answer is Right:** The **Internal Intercostal muscles** (specifically the interosseous portion) are primarily **muscles of expiration**. When they contract, they pull the ribs downward and inward, decreasing the transverse and anteroposterior diameters of the thorax. This increases intrapulmonary pressure, forcing air out of the lungs. Note: The *external* intercostals are inspiratory, while *internal* are expiratory (Mnemonic: **Ex**ternal = **In**spiration; **In**ternal = **Ex**piration). **2. Why the Other Options are Wrong:** * **Diaphragm (Option C):** This is the **primary muscle of inspiration**, responsible for about 75% of air movement during quiet breathing. Its contraction increases the vertical dimension of the thoracic cavity. * **Scalene (Option A) & Sternocleidomastoid (Option D):** These are **accessory muscles of inspiration**. The scalene muscles elevate the first two ribs, while the sternocleidomastoid elevates the sternum. They are typically recruited during deep breathing or respiratory distress to further expand the thoracic cage. **Clinical Pearls for NEET-PG:** * **Quiet Breathing:** Inspiration is an **active** process (requires muscle contraction), while quiet expiration is a **passive** process (due to elastic recoil of the lungs). * **Forced Expiration:** This is an **active** process involving the **Abdominal muscles** (Rectus abdominis, Obliques) and Internal intercostals. * **Nerve Supply:** The Diaphragm is supplied by the Phrenic nerve (C3, C4, C5). "C3, 4, 5 keep the diaphragm alive."

Q: Which of the following conditions can cause hypoxemia?

All of the above. **Explanation:** Hypoxemia is defined as a decrease in the partial pressure of oxygen in arterial blood ($PaO_2$). It is caused by five primary physiological mechanisms: **Hypoventilation, V/Q Mismatch, Right-to-Left Shunt, Diffusion Impairment, and Low Inspired Oxygen ($FiO_2$).** * **Hypoventilation (Option A):** When the rate of alveolar ventilation decreases, $CO_2$ accumulates in the alveoli. According to the Alveolar Gas Equation, as $PACO_2$ rises, $PAO_2$ must fall, leading to hypoxemia. * **Myasthenia Gravis (Option B):** This is a neuromuscular junction disorder that leads to weakness of the respiratory muscles (diaphragm and intercostals). This results in **Type II Respiratory Failure** characterized by hypoventilation, thereby causing hypoxemia. * **Pulmonary Emboli (Option C):** An embolus obstructs blood flow to a portion of the lung. This creates areas that are ventilated but not perfused, leading to a **Ventilation-Perfusion (V/Q) Mismatch** (specifically, an increase in physiological dead space). This is one of the most common clinical causes of hypoxemia. **High-Yield Clinical Pearls for NEET-PG:** 1. **A-a Gradient:** This is the key to differentiating causes. The A-a gradient is **normal** in Hypoventilation and Low $FiO_2$, but **increased** in V/Q Mismatch, Shunt, and Diffusion impairment. 2. **Oxygen Response:** Hypoxemia caused by a **Right-to-Left Shunt** is the only type that does not significantly improve with 100% supplemental oxygen. 3. **V/Q Ratios:** In a standing position, both ventilation and perfusion are highest at the **base** of the lung, but the V/Q ratio is highest at the **apex**.

Q: What change occurs in blood as it passes through systemic capillaries, EXCEPT for one of the following?

Shift of oxygen dissociation curve to the left. ### Explanation In systemic capillaries, blood undergoes specific physiological changes as it exchanges gases and solutes with tissues. **Why Option C is the Correct Answer (The Exception):** As blood passes through systemic capillaries, it picks up **CO₂** and **H⁺ ions** (metabolic byproducts). This increase in $PCO_2$ and decrease in pH causes a **Rightward Shift** of the Oxygen Dissociation Curve (ODC), known as the **Bohr Effect**. A right shift decreases hemoglobin's affinity for oxygen, facilitating oxygen unloading to the tissues. Therefore, a "shift to the left" is incorrect in this context. **Analysis of Incorrect Options:** * **A. Increase in hematocrit:** As $CO_2$ enters RBCs, it is converted to $HCO_3^-$ and $H^+$. The $HCO_3^-$ exits the cell in exchange for $Cl^-$ (**Chloride Shift/Hamburger Phenomenon**). This increase in intracellular osmotically active particles causes water to enter the RBCs, making them swell and slightly increasing the hematocrit in venous blood. * **B. pH decreases:** Tissues produce $CO_2$, which reacts with water to form carbonic acid ($H_2CO_3$), which dissociates into $H^+$ and $HCO_3^-$. This increase in $H^+$ concentration lowers the blood pH. * **D. Increase in protein content:** Due to hydrostatic pressure in the capillaries, some fluid (plasma) filters into the interstitial space, while large proteins remain in the vessel. This slight loss of fluid leads to a relative increase in the concentration of plasma proteins in the venous end. **High-Yield NEET-PG Pearls:** * **Right Shift (CADET, face Right!):** **C**O₂, **A**cidosis, **D**PG (2,3-BPG), **E**xercise, and **T**emperature all increase in systemic capillaries and shift the ODC to the right. * **Chloride Shift:** Occurs in systemic capillaries ($Cl^-$ moves into RBCs). * **Reverse Chloride Shift:** Occurs in pulmonary capillaries ($Cl^-$ moves out of RBCs).

Q: If pulmonary arterial pressure > alveolar pressure > venous pressure (Pa > Palv > Pv), under normal circumstances, which zone of the lung is being described?

Zone II. **Explanation:** The distribution of pulmonary blood flow is determined by the relationship between three pressures: **Pulmonary Arterial Pressure (Pa)**, **Alveolar Pressure (Palv)**, and **Pulmonary Venous Pressure (Pv)**. This concept is known as West’s Zones of the lung. **Why Zone II is correct:** In **Zone II (the middle zone)**, the relationship is **Pa > Palv > Pv**. Here, arterial pressure is high enough to overcome alveolar pressure, but alveolar pressure is higher than venous pressure. This creates a "Starling Resistor" or "Waterfall effect," where blood flow is determined by the difference between arterial and alveolar pressure, rather than the usual arterial-venous gradient. Flow occurs intermittently, primarily during systole. **Analysis of Incorrect Options:** * **Zone I (Palv > Pa > Pv):** Alveolar pressure exceeds arterial pressure, compressing the capillaries and resulting in no blood flow (Physiological Dead Space). This is not found under normal conditions but occurs in hemorrhage or positive pressure ventilation. * **Zone III (Pa > Pv > Palv):** Both arterial and venous pressures exceed alveolar pressure. The capillaries remain permanently open, and blood flow is continuous and highest here due to gravity. * **Zone IV:** This is a pathological zone (often seen in pulmonary edema) where interstitial pressure exceeds other pressures, reducing flow at the extreme lung bases. **High-Yield Pearls for NEET-PG:** * **Gravity Effect:** Blood flow and ventilation both increase as you move from the apex to the base, but blood flow increases more steeply. * **V/Q Ratio:** Highest at the **Apex** (~3.3) and lowest at the **Base** (~0.6). * **Postural Change:** In a supine position, the lung behaves entirely as Zone III. Zone I is typically only seen in upright individuals with low pulmonary pressures.

Question 1

In the upright position, what happens to ventilation and perfusion from the apex to the base of the lung?

Accepted Answer

Ventilation increases, perfusion increases

Answer

Ventilation decreases, perfusion decreases

Answer

Ventilation decreases, perfusion increases

Answer

Ventilation increases, perfusion decreases

Question 2

What is the partial pressure of oxygen in the expired air?

Accepted Answer

116 mm Hg

Answer

158 mm Hg

Answer

100 mm Hg

Answer

0.3 mm Hg

Question 3

A healthy 30-year-old male runs on the treadmill for 30 minutes. Which of the following muscles are primarily used during expiration?

Accepted Answer

Internal intercostals and abdominal recti

Answer

Sternocleidomastoid and Diaphragm

Answer

Diaphragm and Internal intercostals

Answer

Sternocleidomastoid

Question 4

All of the following muscles are involved in inspiration except?

Accepted Answer

Internal intercostal muscle

Answer

Scalene

Answer

Diaphragm

Answer

Sternocleidomastoid

Question 5

What is true regarding the physiology of surfactant?

Accepted Answer

SP-A plays a major role in the innate host defense of the lung.

Answer

SP-A and SP-D are hydrophobic, and SP-B and SP-D are hydrophilic.

Answer

Surfactant is secreted into the alveolar lumen as tubular myelin.

Answer

Pulmonary surfactant is composed of 80% lipids and 20% proteins.

Question 6

Regarding dead space volume in a normal individual, which of the following statements is true?

Accepted Answer

Anatomical dead space is less than physiological dead space.

Answer

Anatomical dead space is greater than physiological dead space.

Answer

Anatomical dead space is equal to physiological dead space.

Answer

Anatomical dead space is not related to physiological dead space.

Question 7

Which of the following conditions can cause hypoxemia?

Accepted Answer

All of the above

Answer

Hypoventilation

Answer

Myasthenia gravis

Answer

Pulmonary emboli

Question 8

Which parameter best denotes airway resistance?

Accepted Answer

Mid respiratory flow rates

Answer

Vital Capacity

Answer

FEV1

Answer

Total volume

Question 9

What change occurs in blood as it passes through systemic capillaries, EXCEPT for one of the following?

Accepted Answer

Shift of oxygen dissociation curve to the left

Answer

Increase in hematocrit

Answer

pH decreases

Answer

Increase in protein content

Question 10

If pulmonary arterial pressure > alveolar pressure > venous pressure (Pa > Palv > Pv), under normal circumstances, which zone of the lung is being described?

Accepted Answer

Zone II

Answer

Zone I

Answer

Zone III

Answer

Zone IV

Respiratory System — MCQs

Respiratory System — MCQs

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