Respiratory System Practice Questions

Q: What is the normal pulmonary capillary pressure?

10 mm Hg. **Explanation:** The pulmonary circulation is a **low-pressure, low-resistance system** compared to the systemic circulation. The normal mean pulmonary capillary pressure is approximately **10 mm Hg** (ranging between 7–12 mm Hg). This low pressure is physiological necessity to prevent the filtration of fluid into the alveoli, ensuring efficient gas exchange. **Why 10 mm Hg is correct:** In the lungs, the Starling forces are balanced such that the capillary hydrostatic pressure (10 mm Hg) is significantly lower than the plasma colloid osmotic pressure (~28 mm Hg). This "safety factor" keeps the alveoli dry by favoring the absorption of fluid into the interstitium/capillaries rather than filtration into the air spaces. **Analysis of Incorrect Options:** * **A (5 mm Hg):** This is closer to the normal **Left Atrial Pressure** (mean ~2–5 mm Hg). While low, it does not represent the pressure within the capillary bed itself. * **C (20 mm Hg):** This value is too high for a normal capillary. A mean pulmonary capillary pressure above 18–20 mm Hg typically indicates pulmonary venous congestion or early **pulmonary edema**. * **D (32 mm Hg):** This is the average hydrostatic pressure at the arterial end of a **systemic capillary**. If pulmonary capillary pressure reached this level, massive pulmonary edema would occur instantly. **High-Yield NEET-PG Pearls:** * **Mean Pulmonary Artery Pressure (mPAP):** Normal is ~15 mm Hg. Pulmonary hypertension is defined as mPAP >20 mm Hg at rest. * **West Zones of the Lung:** Pulmonary capillary pressure varies by position; it is lowest at the apex (Zone 1) and highest at the base (Zone 3) due to gravity. * **PCWP (Pulmonary Capillary Wedge Pressure):** Measured via a Swan-Ganz catheter, it is a clinical surrogate for left atrial pressure and pulmonary capillary pressure. Normal is 6–12 mm Hg.

Q: What is true regarding the respiratory center?

It is inhibited during swallowing.. **Explanation:** The respiratory center, located in the brainstem, is responsible for the rhythmic generation of breathing. **Why Option B is Correct:** The respiratory center is **inhibited during swallowing** (deglutition). This is a protective reflex known as **"Deglutition Apnea."** When the bolus of food passes through the pharynx, the respiratory center in the medulla is inhibited to prevent the aspiration of food into the trachea. This ensures that the airway is protected while the upper esophageal sphincter opens. **Why Other Options are Incorrect:** * **Option A:** The central chemoreceptors in the medulla are primarily sensitive to changes in **H+ concentration and PaCO2**, not PaO2. A fall in PaO2 (hypoxia) is sensed by **peripheral chemoreceptors** (carotid and aortic bodies), which then send signals to the respiratory center. * **Option C:** While the respiratory and cardiac centers are anatomically close in the medulla and show functional coordination (e.g., Sinus Arrhythmia), they are distinct functional units. The question asks for a fundamental physiological property; inhibition during swallowing is a more definitive functional characteristic. * **Option D:** The respiratory centers are situated in the **Medulla** (Dorsal and Ventral Respiratory Groups) and the **Pons** (Pneumotaxic and Apneustic centers), not the midbrain. **High-Yield Pearls for NEET-PG:** * **Pneumotaxic Center:** Located in the upper pons; acts as a "switch-off" point for inspiration, thereby increasing respiratory rate. * **Apneustic Center:** Located in the lower pons; prolongs inspiration (apneusis). * **Hering-Breuer Reflex:** A protective reflex that prevents over-inflation of the lungs via stretch receptors and the Vagus nerve. * **Central Chemoreceptors:** Located on the ventral surface of the medulla; they do **not** respond to hypoxia.

Q: Partial pressure of carbon dioxide (CO2) is lowest in which of the following?

Expired air. ### Explanation The partial pressure of carbon dioxide ($PCO_2$) in the respiratory system is determined by the mixing of atmospheric air with alveolar air. **1. Why Expired Air is Correct:** Expired air (mixed expired air) is a combination of **Alveolar air** (where $PCO_2$ is ~40 mmHg) and **Anatomic Dead Space air** (where $PCO_2$ is ~0 mmHg). Because the dead space air dilutes the CO2 coming from the alveoli, the total $PCO_2$ in expired air drops to approximately **27–32 mmHg**. This makes it the lowest among the given options. **2. Analysis of Incorrect Options:** * **Venous Blood:** Contains the highest $PCO_2$ (~46 mmHg) as it carries CO2 produced by tissue metabolism back to the lungs. * **Arterial Blood:** After gas exchange in the lungs, arterial $PCO_2$ equilibrates with alveolar air, resulting in a value of approximately **40 mmHg**. * **End-Tidal Air:** This represents the very last portion of air expired, which comes entirely from the alveoli. Therefore, End-tidal $PCO_2$ ($EtCO_2$) is roughly **40 mmHg**, reflecting arterial $PCO_2$. **3. High-Yield NEET-PG Pearls:** * **Hierarchy of $PCO_2$:** Venous blood (46) > Arterial blood (40) = Alveolar air (40) = End-tidal air (40) > Expired air (27-32) > Atmospheric air (0.3). * **Dead Space Calculation:** The difference between arterial $PCO_2$ ($PaCO_2$) and expired $PCO_2$ ($PeCO_2$) is used in the **Bohr Equation** to calculate physiological dead space. * **Clinical Correlation:** In a healthy individual, $EtCO_2$ is a reliable non-invasive surrogate for $PaCO_2$. However, in lung diseases (increased V/Q mismatch), the gradient between $PaCO_2$ and $EtCO_2$ increases.

Q: Carbon monoxide poisoning causes which of the following?

Oxygen dissociation curve shifts to the left. **Explanation:** **Why Option B is Correct:** Carbon Monoxide (CO) has an affinity for hemoglobin approximately **210–240 times greater** than that of oxygen. When CO binds to one of the four heme sites (forming carboxyhemoglobin), it induces a conformational change in the hemoglobin molecule. This change increases the affinity of the remaining heme sites for oxygen. Consequently, oxygen binds more tightly and is not easily released to the tissues. This **decreased P50** and increased affinity manifest as a **Leftward Shift** of the Oxygen Dissociation Curve (ODC). **Why Other Options are Incorrect:** * **A. Hypoxic Hypoxia:** CO poisoning causes **Anemic Hypoxia**. The arterial partial pressure of oxygen ($PaO_2$) remains normal, but the total oxygen-carrying capacity of the blood is reduced because CO occupies hemoglobin binding sites. * **C. Cyanosis:** Cyanosis requires a high concentration of deoxygenated hemoglobin (>5g/dL). In CO poisoning, carboxyhemoglobin is **cherry-red** in color. Therefore, patients typically present with a "cherry-red" appearance rather than the bluish tint of cyanosis. * **D. Diffusion Capacity ($DL_{CO}$):** While CO is used to *measure* diffusion capacity, the poisoning itself does not decrease the lung's intrinsic ability to transfer gases across the alveolar-capillary membrane. **High-Yield Clinical Pearls for NEET-PG:** * **The Double Whammy:** CO poisoning is lethal because it simultaneously reduces oxygen loading (anemic hypoxia) and impairs oxygen unloading (left shift). * **Pulse Oximetry ($SpO_2$):** Standard pulse oximeters cannot distinguish between oxyhemoglobin and carboxyhemoglobin, often giving **falsely normal** readings. * **Treatment:** 100% Hyperbaric Oxygen (HBO) is the treatment of choice to reduce the half-life of carboxyhemoglobin.

Q: Stagnant hypoxia is seen in which of the following conditions?

Shock. **Explanation:** **Stagnant Hypoxia** (also known as hypokinetic hypoxia) occurs when there is a **decrease in the velocity of blood flow**, leading to inadequate delivery of oxygen to the tissues despite normal arterial $PO_2$ and oxygen content. **Why Shock is Correct:** In **Shock** (and Congestive Heart Failure), the cardiac output falls significantly. This results in a slow, sluggish circulation. Because the blood stays in the capillaries longer, tissues extract more oxygen than usual, leading to a high arteriovenous oxygen difference. However, the overall delivery rate is insufficient to meet metabolic demands, resulting in stagnant hypoxia. **Analysis of Incorrect Options:** * **COPD (Chronic Obstructive Pulmonary Disease):** This causes **Hypoxic Hypoxia**. The primary defect is inadequate oxygenation of blood in the lungs due to ventilation-perfusion mismatch or alveolar hypoventilation, leading to low arterial $PO_2$. * **Anemia:** This causes **Anemic Hypoxia**. The arterial $PO_2$ is normal, but the total oxygen-carrying capacity of the blood is reduced due to low hemoglobin levels. * **CO (Carbon Monoxide) Poisoning:** This is also a form of **Anemic Hypoxia**. CO binds to hemoglobin with high affinity, preventing oxygen binding and shifting the oxygen-dissociation curve to the left, hindering oxygen release to tissues. **High-Yield NEET-PG Pearls:** 1. **Arterial $PO_2$** is **Normal** in Anemic, Stagnant, and Histotoxic hypoxia; it is **Low** only in Hypoxic hypoxia. 2. **Cyanosis** is most prominent in Stagnant hypoxia due to the excessive buildup of reduced hemoglobin in the stagnant capillary beds. 3. **Histotoxic Hypoxia** (e.g., Cyanide poisoning) occurs when tissues cannot utilize oxygen despite normal delivery (due to inhibition of Cytochrome Oxidase).

Q: Which of the following is responsible for the movement of O2 from the alveoli into the blood in the pulmonary capillaries?

Passive diffusion. ### Explanation **Correct Answer: D. Passive diffusion** The exchange of gases (O2 and CO2) across the alveolar-capillary membrane occurs exclusively via **passive diffusion**. This process is governed by **Fick’s Law**, which states that the rate of gas transfer is proportional to the surface area and the partial pressure gradient, and inversely proportional to the thickness of the membrane. Oxygen moves from the alveoli (where $P_{A}O_2$ is ~104 mmHg) into the pulmonary capillary blood (where $P_{v}O_2$ is ~40 mmHg) simply because it moves down its **concentration/partial pressure gradient**. No cellular energy (ATP) or carrier proteins are required for this movement. **Why other options are incorrect:** * **A. Active transport:** This requires ATP to move substances against a gradient. Gas exchange does not consume energy and follows a downward pressure gradient. * **B. Filtration:** This is the movement of water and solutes across a membrane due to hydrostatic or osmotic pressure (common in the Glomerulus), not applicable to gas exchange. * **C. Facilitated diffusion:** This requires specific carrier proteins (e.g., GLUT transporters for glucose). Oxygen molecules are small and lipid-soluble, allowing them to pass directly through the phospholipid bilayer of the respiratory membrane without carriers. **High-Yield Clinical Pearls for NEET-PG:** * **Diffusion Limitation vs. Perfusion Limitation:** Under normal resting conditions, O2 transfer is **perfusion-limited** (equilibrium is reached 1/3rd of the way along the capillary). In states of fibrosis or intense exercise, it can become **diffusion-limited**. * **Diffusion Capacity ($D_L$):** Carbon Monoxide (CO) is used to measure the diffusing capacity of the lung ($D_LCO$) because it is strictly diffusion-limited. * **Solubility:** CO2 is **20 times more soluble** than O2; therefore, it diffuses much faster despite a smaller pressure gradient.

Question 1

Which of the following is used to measure the resistance in small airways?

Accepted Answer

Maximum mid expiratory flow rate

Answer

Vital capacity

Answer

FEV1

Answer

Closing volume

Question 2

What is the normal pulmonary capillary pressure?

Accepted Answer

10 mm Hg

Answer

5 mm Hg

Answer

20 mm Hg

Answer

32 mm Hg

Question 3

Select the best statement regarding the respiratory centers.

Accepted Answer

The pneumotaxic center can override the function of the inspiratory centers.

Answer

The normal rhythmic pattern of breathing originates from neurons in the dorsal respiratory group.

Answer

During quiet breathing, expiratory neurons are inhibited.

Answer

The apneustic center promotes inspiration.

Question 4

Shifting of the oxygen dissociation curve to the right indicates what?

Accepted Answer

The affinity is less and more O2 is released to the tissue.

Answer

Affinity is more and less oxygen is released to tissues.

Answer

Amount of oxygen transported is less.

Answer

More carbon dioxide is transported.

Question 5

What is true regarding the respiratory center?

Accepted Answer

It is inhibited during swallowing.

Answer

It is directly stimulated by a fall in PaO2.

Answer

It is connected with the cardiac center.

Answer

It is situated in the midbrain.

Question 6

Partial pressure of carbon dioxide (CO2) is lowest in which of the following?

Accepted Answer

Expired air

Answer

Arterial blood

Answer

End-tidal air

Answer

Venous blood

Question 7

Alveolar hypoventilation is observed in which of the following conditions?

Accepted Answer

Guillain-Barré syndrome

Answer

Acute asthma

Answer

Bronchiectasis

Answer

CREST syndrome

Question 8

Carbon monoxide poisoning causes which of the following?

Accepted Answer

Oxygen dissociation curve shifts to the left

Answer

Hypoxic hypoxia

Answer

Cyanosis

Answer

Diffusion capacity of lungs decreases

Question 9

Stagnant hypoxia is seen in which of the following conditions?

Accepted Answer

Shock

Answer

COPD

Answer

Anemia

Answer

CO poisoning

Question 10

Which of the following is responsible for the movement of O2 from the alveoli into the blood in the pulmonary capillaries?

Accepted Answer

Passive diffusion

Answer

Active transport

Answer

Filtration

Answer

Facilitated diffusion

Respiratory System — MCQs

Respiratory System — MCQs

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