Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 861: Where is the primary respiratory regulatory centre located?

A. Pneumotaxic center
B. Apneustic center
C. Dorsal medulla
D. Ventral medulla (Correct Answer)

Explanation: ***Ventral medulla*** - The **ventral respiratory group (VRG)**, located in the **ventral medulla**, contains both inspiratory and expiratory neurons crucial for generating the basic rhythm of breathing, especially during active respiration. - It plays a significant role in providing the **inspiratory drive** and coordinating the activity of inspiratory and expiratory muscles. *Dorsal medulla* - The **dorsal respiratory group (DRG)**, located in the **dorsal medulla**, primarily controls **inspiration** during quiet breathing. - It receives input from peripheral chemoreceptors and mechanoreceptors, modifying breathing according to metabolic demands. *Apneustic center* - The **apneustic center** is located in the **lower pons** and provides stimulatory signals to the inspiratory neurons of the DRG. - It helps to prolong inspiration, contributing to a longer, deeper breath, and is opposed by the pneumotaxic center. *Pneumotaxic center* - The **pneumotaxic center** is located in the **upper pons** and primarily inhibits inspiration, preventing over-inflation of the lungs. - It fine- tunes the breathing rhythm by sending inhibitory signals to the inspiratory neurons in the DRG, thus promoting a regular breathing pattern.

Question 862: Which of the following statements about pulmonary surfactant is correct?

A. Maintain alveolar integrity (Correct Answer)
B. Secreted by type I pneumocytes
C. A structural protein in epithelial cells
D. A component of mucus

Explanation: **Maintain alveolar integrity** - Pulmonary surfactant **reduces surface tension** at the air-liquid interface within the alveoli, preventing their collapse during expiration. - This function is crucial for maintaining **alveolar stability** and efficient gas exchange. *Secreted by type I pneumocytes* - Pulmonary surfactant is primarily secreted by **type II pneumocytes**, also known as great alveolar cells, not type I pneumocytes. - **Type I pneumocytes** are responsible for gas exchange due to their thin, flat structure. *A structural protein in epithelial cells* - Pulmonary surfactant is a complex mixture of **lipids (primarily phospholipids)** and proteins, not solely a structural protein. - Its primary role is functional (reducing surface tension), not structural support for epithelial cells. *A component of mucus* - Pulmonary surfactant is an independent substance found within the alveolar lining, distinct from the **mucus** produced by goblet cells in the airways. - Mucus primarily functions in trapping foreign particles and is found in larger airways.

Question 863: What is the primary regulator for the central chemoreceptor?

A. Partial pressure of oxygen (PaO2)
B. Carbon dioxide (CO2) (Correct Answer)
C. Bicarbonate ions (HCO3-)
D. Hydrogen ions (H+)

Explanation: ***Carbon dioxide (CO2)*** - CO2 is the **primary regulator** of central chemoreceptors in the medulla, serving as the key physiological variable that drives respiratory control. - CO2 easily diffuses across the **blood-brain barrier** into the cerebrospinal fluid (CSF), where it reacts with water to form carbonic acid (H2CO3). - The carbonic acid dissociates into **H+ and HCO3-**, and the resulting increase in H+ concentration (decreased pH) directly stimulates the central chemoreceptors. - Clinically, we monitor and regulate **arterial PCO2** levels, making CO2 the primary chemical regulator of ventilation under normal conditions. *Hydrogen ions (H+)* - While H+ is the **direct molecular stimulus** that activates central chemoreceptors, it is not the primary regulator in physiological terms. - H+ ions do not readily cross the blood-brain barrier, so changes in blood H+ have minimal direct effect on central chemoreceptors. - The H+ that stimulates these receptors is generated **locally in the CSF** from CO2 diffusion and hydration, making CO2 the upstream regulator. *Partial pressure of oxygen (PaO2)* - PaO2 is the primary stimulus for **peripheral chemoreceptors** (carotid and aortic bodies), not central chemoreceptors. - Central chemoreceptors are relatively **insensitive to changes in PaO2** unless oxygen levels are severely reduced and directly affecting brain metabolism. *Bicarbonate ions (HCO3-)* - HCO3- is a product of the CO2 hydration reaction in the CSF but acts primarily as a **buffer** against pH changes. - Bicarbonate levels adapt slowly over time (chronic compensation) but are not the acute regulator of ventilation or the primary stimulus for central chemoreceptors.

Question 864: In newborns, the paradoxical response where lung inflation stimulates further inspiration instead of the normal inhibitory response is explained by

A. Hering-Breuer inflation reflex
B. Hering-Breuer deflation reflex
C. Head's paradoxical reflex (Correct Answer)
D. J-reflex

Explanation: ***Head's paradoxical reflex*** - This reflex is characterized by an **inspiratory effort** in response to lung inflation, which is especially prominent in **newborns and infants**. - It aids in **maintaining lung inflation** during periods of potential lung collapse or reduced compliance. *Hering-Breuer inflation reflex* - This reflex is typically **inhibitory**, meaning lung inflation **inhibits inspiration** and prolongs exhalation. - It is mediated by **stretch receptors in the airways** and helps prevent overinflation of the lungs. *Hering-Breuer deflation reflex* - This reflex is stimulated by **lung deflation**, leading to an **increase in inspiratory effort**. - It is important in situations of lung collapse, such as **pneumothorax**, to increase ventilation. *J-reflex* - The J-reflex is mediated by **juxtacapillary receptors** (J receptors) in the alveolar walls. - It is typically activated by events such as **pulmonary edema** or **embolism**, leading to rapid shallow breathing and bradycardia.

Question 865: Oxygen dissociation curve shifts to the right in:

A. Hypothermia
B. Hypercarbia (Correct Answer)
C. Metabolic alkalosis
D. Fetal hemoglobin (HbF) presence

Explanation: ***Hypercarbia*** - Increased arterial partial pressure of carbon dioxide (**PaCO2**) leads to a decrease in pH (*acidosis*), which **reduces hemoglobin's affinity for oxygen**. - This reduced affinity facilitates oxygen release to the tissues, shifting the **oxygen dissociation curve to the right** (Bohr effect). *Hypothermia* - **Decreased body temperature** causes an increase in hemoglobin's affinity for oxygen, making it harder for oxygen to be released to tissues. - This effect shifts the **oxygen dissociation curve to the left**. *Fetal hemoglobin (HbF) presence* - **Fetal hemoglobin (HbF)** has a higher affinity for oxygen compared to adult hemoglobin (HbA). - This higher affinity helps in oxygen transfer from the mother to the fetus and shifts the **oxygen dissociation curve to the left**. *Metabolic alkalosis* - **Metabolic alkalosis** is characterized by an increase in blood pH, which enhances hemoglobin's affinity for oxygen. - This increased affinity makes it more difficult for oxygen to be unloaded in the tissues and shifts the **oxygen dissociation curve to the left**.

Question 866: A neonate while suckling milk can breathe without difficulty due to:

A. High larynx (Correct Answer)
B. A smaller tongue
C. A smaller pharynx
D. The soft palate

Explanation: ***High larynx*** - A **high larynx** in neonates positions the epiglottis to interdigitate with the soft palate, effectively separating the **airflow channel** from the **foodway** during swallowing. - This anatomical arrangement allows for simultaneous **breathing and suckling** without aspiration, facilitating continuous feeding. *A smaller tongue* - While neonates do have proportionally different oral anatomy, a **smaller tongue** alone does not fully explain the ability to breathe while suckling. - The interaction of multiple structures, including the larynx and soft palate, is key to this function. *A smaller pharynx* - The pharynx in neonates is indeed relatively smaller than in adults, but this reduction in size doesn't directly enable concurrent **breathing and suckling**. - Its primary role in this context is as a pathway for both air and food, which needs to be precisely managed by other structures. *The soft palate* - The **soft palate** plays a crucial role by creating a seal with the epiglottis, but it is the elevated position of the larynx that allows this complete separation. - Without the **high larynx**, the soft palate alone could not adequately prevent aspiration during simultaneous breathing and feeding.

Question 867: Which of the following factors contributes to increased airway resistance?

A. Forced expiration
B. Denser air
C. High lung volume
D. Low lung volume (Correct Answer)

Explanation: ***Low lung volume*** - At **low lung volumes**, the radial traction on the airways by the surrounding lung tissue is significantly reduced, leading to **narrowing of the small airways**. - This **decreased airway caliber** directly increases resistance to airflow, making breathing more difficult. - This is the **most physiologically significant factor** affecting airway resistance in normal breathing and clinical conditions. *Forced expiration* - While **forced expiration** can transiently increase airway resistance in certain conditions (e.g., in patients with obstructive lung disease due to dynamic airway compression), it is not a fundamental factor that increases resistance in healthy airways. - The primary mechanism of increased resistance during forced expiration in disease states is due to the **collapse of compliant airways** under positive intrathoracic pressure. *Denser air* - Breathing **denser air** (e.g., at sea level vs. high altitude, or in hyperbaric conditions) does increase resistance, particularly in **turbulent flow** conditions in larger airways. - However, this effect is **relatively minor** compared to the dramatic changes in resistance caused by lung volume variations. - In clinical practice and normal physiology, **lung volume is the predominant variable** affecting airway resistance. *High lung volume* - At **high lung volumes**, the airways are pulled open by increased radial traction from the surrounding lung parenchyma, which actually **decreases airway resistance**. - This wider airway diameter facilitates easier airflow, thereby reducing the effort required for ventilation.

Question 868: Midpontine section with bilateral vagotomy causes

A. Deep and slow breathing
B. Apneustic breathing (Correct Answer)
C. Irregular and gasping breathing
D. Cheyne-Stokes breathing

Explanation: ***Apneustic breathing (Correct)*** - This pattern is characterized by a **prolonged inspiratory gasp** followed by a brief, insufficient expiratory effort. - A lesion in the **midpons**, coupled with **bilateral vagotomy**, removes inhibitory inputs from both the vagus nerve (Hering-Breuer reflex) and the pneumotaxic center, leading to **unopposed apneustic center activity** with prolonged depth of inspiration. - This is the classic result when both the pneumotaxic center control and vagal stretch receptor feedback are eliminated. *Deep and slow breathing (Incorrect)* - This pattern is typically seen in conditions like **Kussmaul breathing** due to metabolic acidosis, where the body compensates by increasing tidal volume and slightly reducing respiratory rate. - It does not involve the characteristic prolonged inspiratory hold seen with pontine lesions and vagotomy. *Irregular and gasping breathing (Incorrect)* - This description is more consistent with **agonal breathing** or ataxic breathing (Biot's breathing), often associated with severe damage to the **medulla oblongata** or terminal brainstem failure. - It reflects a complete disorganization of respiratory rhythmicity rather than a specific prolonged inspiratory hold. *Cheyne-Stokes breathing (Incorrect)* - Characterized by a **cyclic pattern** of crescendo-decrescendo breathing (waxing and waning tidal volumes) separated by periods of apnea. - This pattern typically results from damage to the **cerebral hemispheres** or **diencephalon**, leading to altered ventilatory control in response to CO2 levels.

Question 869: Which of the following methods is most commonly used to measure anatomical dead space?

A. Bohr equation
B. Xenon dilution technique
C. Spirometry
D. Single breath nitrogen test (Correct Answer)

Explanation: ***Single breath nitrogen test*** - This method is widely used for measuring **anatomical dead space**, particularly the **Fowler's method**. - It involves analyzing the **nitrogen washout curve** after a single breath of 100% oxygen, with the anatomical dead space corresponding to the volume of gas exhaled before the nitrogen concentration begins to rise. *Bohr equation* - The Bohr equation is used to calculate **physiological dead space**, which includes both anatomical and alveolar dead space. - While it incorporates anatomical dead space, it doesn't directly measure it in isolation but rather focuses on the **effective ventilation** in gas exchange. *Xenon dilution technique* - This technique is primarily employed to assess **lung volumes** and **regional ventilation**, not specifically anatomical dead space. - It typically involves the inhalation of a small amount of **radioactive xenon** gas. *Spirometry* - Spirometry measures **lung volumes** and **airflow rates** such as forced vital capacity (FVC) and forced expiratory volume in one second (FEV1). - It does not directly measure dead space but provides information on **ventilatory function** and disease states affecting airflow.

Question 870: Normal intrapleural pressure during the start of inspiration is ________ mm of Hg.

A. -2
B. -5 (Correct Answer)
C. 0
D. -7

Explanation: ***Correct Option: -5 mm Hg*** - At the **start of inspiration** (resting state/end of passive expiration), the normal intrapleural pressure is **-5 mm Hg** - This represents the baseline negative pressure that keeps the lungs inflated against the chest wall - As inspiration proceeds, the diaphragm contracts and this pressure becomes MORE negative (drops to -7 to -8 mm Hg), creating the pressure gradient that draws air into the lungs *Incorrect Option: -2 mm Hg* - A pressure of **-2 mm Hg** is LESS negative than the resting value - This would actually represent a pressure moving TOWARD atmospheric pressure, which would cause exhalation, not inspiration - This value is not physiologically accurate for any normal phase of the respiratory cycle *Incorrect Option: 0 mm Hg* - A pressure of **0 mm Hg** indicates intrapleural pressure equal to atmospheric pressure - This occurs in pathological conditions like **pneumothorax**, where air enters the pleural space - This is not a normal physiological value during respiration *Incorrect Option: -7 mm Hg* - An intrapleural pressure of **-7 mm Hg** occurs DURING active inspiration when the diaphragm contracts - This represents the more negative pressure that develops as the thoracic cavity expands - This is not the pressure at the START/beginning of inspiration, but rather during the inspiratory phase

Practice by Chapter

Mechanics of Breathing

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Pulmonary Ventilation

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Pulmonary Circulation

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Gas Exchange in the Lungs

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Oxygen and Carbon Dioxide Transport

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Control of Breathing

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Respiratory Adjustments in Health and Disease

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High Altitude Physiology

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Diving Physiology

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Respiratory Function Tests

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