Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 831: What is the air remaining in the lung after normal expiration?

A. Tidal Volume (TV)
B. Residual Volume (RV)
C. Functional Residual Capacity (FRC) (Correct Answer)
D. Vital Capacity (VC)

Explanation: ***Functional Residual Capacity (FRC)*** - **FRC** represents the volume of air remaining in the lungs after a **normal expiration**. - It is the sum of the **expiratory reserve volume (ERV)** and the **residual volume (RV)**. *Tidal Volume (TV)* - **TV** is the volume of air inspired or expired with a **normal breath**. - It does not represent the total air remaining in the lungs after expiration. *Residual Volume (RV)* - **RV** is the volume of air remaining in the lungs after a **maximal expiration**. - It is a component of FRC but does not fully describe the air remaining after a *normal* expiration. *Vital Capacity (VC)* - **VC** is the maximum volume of air that can be exhaled after a **maximal inspiration**. - It represents the maximum amount of air that can be exchanged with a single breath, not the air remaining after normal expiration.

Question 832: Central chemoreceptors are most sensitive to which of the following changes in blood?

A. PO2
B. HCO3-
C. pH
D. PCO2 (Correct Answer)

Explanation: ***PCO2*** - Central chemoreceptors, located in the **medulla oblongata**, are exquisitely sensitive to changes in the **partial pressure of carbon dioxide (PCO2)** in the arterial blood. - An increase in blood PCO2 readily crosses the **blood-brain barrier** to the cerebrospinal fluid (CSF), where it is converted to carbonic acid and then to H+ and HCO3-. The resulting **drop in CSF pH** directly stimulates these chemoreceptors, leading to increased ventilation. *PO2* - While **peripheral chemoreceptors** (carotid and aortic bodies) are sensitive to changes in **PO2**, particularly when it drops significantly (below 60 mmHg), central chemoreceptors are not. - The primary role of central chemoreceptors is to monitor and respond to changes in CO2 and pH, rather than oxygen levels. *pH* - Central chemoreceptors are indirectly sensitive to **pH changes** in the cerebrospinal fluid (CSF), which result from blood PCO2 changes. - However, they are not directly or primarily sensitive to changes in **blood pH** because hydrogen ions do not readily cross the blood-brain barrier. *HCO3-* - Bicarbonate ions (**HCO3-**) are important in buffering pH, but central chemoreceptors do not directly sense bicarbonate levels. - Changes in HCO3- indirectly affect pH, and it is the resultant **H+ concentration** in the CSF, derived from CO2, that primarily stimulates central chemoreceptors.

Question 833: Which of the following is used for the diagnosis of asthma?

A. Measurement of tidal volume
B. End expiratory flow rate
C. Total lung capacity
D. FEV1 (Correct Answer)

Explanation: ***FEV1*** - **Forced expiratory volume in 1 second (FEV1)** is the gold standard spirometric parameter for asthma diagnosis - Key diagnostic criteria include: - Reduced **FEV1/FVC ratio** (<0.70 or <0.75-0.80 in adults) - **Bronchodilator reversibility**: ≥12% and ≥200 mL increase in FEV1 after inhaled short-acting β2-agonist - This reversibility distinguishes asthma from fixed obstructive diseases like COPD - Serial **peak expiratory flow (PEF)** monitoring can also demonstrate variability characteristic of asthma *Measurement of tidal volume* - **Tidal volume** measures the amount of air inhaled or exhaled during normal breathing (typically ~500 mL at rest) - Not a diagnostic parameter for asthma as it doesn't assess **airway obstruction** or **hyperresponsiveness** - May be reduced during acute exacerbations but lacks specificity for asthma diagnosis *End expiratory flow rate* - Not a standard diagnostic parameter for asthma - While **mid-expiratory flow rates** (FEF25-75%) and **peak expiratory flow (PEF)** are assessed, **FEV1** remains the primary diagnostic measure - FEV1 provides better reproducibility and standardization for diagnosis *Total lung capacity* - **Total lung capacity (TLC)** represents total lung volume after maximal inhalation - May be normal or increased in asthma due to **air trapping** and hyperinflation - Not used as a primary diagnostic criterion as asthma diagnosis focuses on demonstrating **reversible airflow limitation**, not lung volumes

Question 834: Where are glomus cells primarily located?

A. Bladder
B. Brain
C. Kidney
D. Carotid and aortic bodies (Correct Answer)

Explanation: ***Carotid and aortic bodies*** - **Glomus cells**, also known as **chemoreceptors**, are primarily located in the **carotid bodies** at the bifurcation of the common carotid artery and in the **aortic bodies** near the aortic arch. - These cells are crucial for monitoring blood oxygen, carbon dioxide, and pH levels, playing a vital role in the body's **respiratory and cardiovascular regulation**. *Bladder* - The bladder’s primary function is to store urine, and it contains specialized cells for distension and contraction, but not **glomus cells** involved in chemoreception. - While the bladder does have nerve endings, they are mainly concerned with detecting stretch and facilitating micturition, not monitoring blood gas levels. *Brain* - The brain contains various specialized cells, including neurons and glial cells, which are responsible for its complex functions. - Although the brain has centers that respond to blood gas changes (e.g., in the medulla), the primary **peripheral chemoreceptors (glomus cells)** are not located within the brain tissue itself. *Kidney* - The kidneys are involved in filtering blood, regulating blood pressure, and producing hormones, containing specialized cells like **juxtaglomerular cells** and podocytes. - However, they do not contain **glomus cells** as a primary site for sensing blood gas levels.

Question 835: Which of the following statements about lung compliance is NOT true?

A. Measured by intrapleural pressure at different lung volumes. (Correct Answer)
B. Decreased at the height of inspiration.
C. Increased in emphysema.
D. Increased by surfactant.

Explanation: ***Measured by intrapleural pressure at different lung volumes.*** - Lung compliance is measured by the **change in lung volume (ΔV)** divided by the **change in transpulmonary pressure (ΔP)**, which is the difference between alveolar and intrapleural pressure. - While intrapleural pressure is a component of transpulmonary pressure, compliance is not measured solely by intrapleural pressure at different lung volumes. *Increased in emphysema.* - This statement is **true**. Emphysema involves the destruction of **elastic fibers** in the lung tissue. - Loss of elastic recoil leads to an **increase in compliance**, meaning the lungs are easier to distend but collapse more readily. *Decreased at the height of inspiration.* - This statement is **true**. At high lung volumes (height of inspiration), the **elastic limit** of the lung tissue is approached. - The lungs become **stiffer** and less compliant, requiring a greater pressure change for a given volume change. *Increased by surfactant.* - This statement is **true**. Surfactant reduces **surface tension** in the alveoli. - By lowering surface tension, surfactant prevents alveolar collapse and **increases overall lung compliance**, making it easier to inflate the lungs.

Question 836: What does the Hering-Breuer reflex decrease during respiration?

A. Duration of inspiration (Correct Answer)
B. Depth of inspiration
C. Depth of expiration
D. Duration of expiration

Explanation: ***Duration of inspiration*** - The **Hering-Breuer reflex** is a protective reflex that prevents overinflation of the lungs by inhibiting further inspiration once the lungs are adequately stretched. - Activation of **stretch receptors** in the bronchial walls sends signals via the vagus nerve to the brainstem, which then inhibits the inspiratory neurons, thus **shortening the inspiratory phase**. *Duration of expiration* - The Hering-Breuer reflex primarily affects inspiration and does not directly shorten the duration of expiration. - Expiration is typically a passive process at rest, driven by the elastic recoil of the lungs, and its duration is not the main target of this reflex. *Depth of inspiration* - While the reflex ultimately limits the **volume of inspired air**, its primary action is to *terminate* inspiration, thus affecting its duration rather than directly reducing the force or 'depth' of each breath. - The **depth of inspiration** is more related to the strength of inspiratory muscle contraction and central respiratory drive. *Depth of expiration* - The Hering-Breuer reflex does not influence the depth of expiration. - Expiration is largely passive, and the depth of expiration is typically not regulated by this reflex unless breathing becomes forced.

Question 837: Which of the following contains the PRIMARY central chemoreceptors responsible for detecting CO2 and pH changes in cerebrospinal fluid?

A. Medulla (Correct Answer)
B. Baroreceptors in carotid sinus
C. Peripheral chemoreceptors in carotid bodies
D. All of the above

Explanation: ***Medulla*** - The **medulla oblongata** in the brainstem houses the primary central chemoreceptors. - These chemoreceptors are located on the **ventral surface of the medulla** and are highly sensitive to changes in the **pH of the cerebrospinal fluid (CSF)**, which is indirectly affected by the partial pressure of carbon dioxide (PCO2) in arterial blood. - CO2 diffuses across the blood-brain barrier, combines with water to form H+ ions, which directly stimulate these central chemoreceptors. *Baroreceptors in carotid sinus* - **Baroreceptors** primarily detect changes in **arterial blood pressure**, not CO2 or pH levels. - They are located in the carotid sinus and aortic arch and are involved in cardiovascular reflexes, not direct chemoreception for respiratory drive. *Peripheral chemoreceptors in carotid bodies* - **Peripheral chemoreceptors** in the carotid bodies (and aortic bodies) detect changes in **arterial blood O2, CO2, and pH**. - However, they are **peripheral**, not central chemoreceptors, and are the primary detectors of **hypoxemia**. - They contribute to respiratory drive but are secondary to central chemoreceptors for CO2 detection. *All of the above* - This option is incorrect because only the **medulla** contains the primary central chemoreceptors for CO2 and pH detection in CSF. - Baroreceptors detect blood pressure, and peripheral chemoreceptors are not central chemoreceptors.

Question 838: Peripheral and central chemoreceptors may both contribute to the increased ventilation that occurs as a result of which of the following?

A. A decrease in arterial oxygen content
B. A decrease in arterial blood pressure
C. An increase in arterial carbon dioxide tension (Correct Answer)
D. A decrease in arterial oxygen tension

Explanation: ***An increase in arterial carbon dioxide tension*** - An increase in **arterial PCO2** (hypercapnia) leads to a rapid decrease in the **pH of the cerebrospinal fluid (CSF)**, which strongly stimulates **central chemoreceptors** in the medulla. - While overwhelmingly driven by central chemoreceptors, a significant increase in **arterial PCO2** also causes a slight decrease in **arterial pH**, which can additionally stimulate **peripheral chemoreceptors** in the carotid and aortic bodies, leading to increased ventilation. *A decrease in arterial oxygen content* - A decrease in **arterial oxygen content** (e.g., due to anemia or carbon monoxide poisoning) without a significant drop in **arterial PO2** primarily affects oxygen delivery to tissues. - It does not directly stimulate peripheral chemoreceptors, which are sensitive to **PO2**, not content, nor does it affect central chemoreceptors directly to increase ventilation in this manner. *A decrease in arterial blood pressure* - A decrease in **arterial blood pressure** is sensed by **baroreceptors** and primarily triggers cardiovascular reflexes (e.g., increased heart rate and vasoconstriction) to restore blood pressure. - It does not directly stimulate peripheral or central chemoreceptors to significantly increase ventilation unless severe hypoperfusion leads to significant changes in arterial blood gases. *A decrease in arterial oxygen tension* - A decrease in **arterial oxygen tension (PO2)**, especially when it falls below approximately 60 mmHg, acts as a potent stimulus for **peripheral chemoreceptors**. - However, **central chemoreceptors** are primarily sensitive to **PCO2** and CSF pH, and a decrease in **arterial PO2** alone has little direct effect on their activity.

Question 839: What is the Bohr effect in relation to hemoglobin's affinity for oxygen?

A. Decrease in CO2 affinity of hemoglobin when the pH of blood falls
B. Decrease in O2 affinity of hemoglobin when the pH of blood rises
C. Decrease in O2 affinity of hemoglobin when the pH of blood falls (Correct Answer)
D. Decrease in CO2 affinity of hemoglobin when the pH of blood rises

Explanation: ***Decrease in O2 affinity of hemoglobin when the pH of blood falls*** - The **Bohr effect** describes how **hemoglobin's affinity for oxygen decreases** in acidic environments (lower pH), leading to increased oxygen release to tissues. - This physiological response is crucial in active tissues, where increased metabolism produces more **carbon dioxide** and **lactic acid**, lowering the local pH. *Decrease in CO2 affinity of hemoglobin when the pH of blood falls* - This statement incorrectly relates the Bohr effect to **CO2 affinity** and its change with pH in this manner. - The Bohr effect primarily concerns oxygen affinity, not CO2 affinity; CO2 and H+ directly influence oxygen binding. *Decrease in O2 affinity of hemoglobin when the pH of blood rises* - An **increase in pH** (alkaline environment) would, in fact, **increase hemoglobin's affinity for oxygen**, promoting oxygen uptake in the lungs. - This describes the opposite of the Bohr effect, which is about oxygen release in acidic conditions. *Decrease in CO2 affinity of hemoglobin when the pH of blood rises* - While pH changes do affect CO2 transport, this statement does not accurately describe the Bohr effect. - The **Haldane effect** is more relevant to the relationship between oxygenation status and hemoglobin's CO2 affinity.

Question 840: What is the difference between the amount of Oxygen consumed and Carbon Dioxide produced per minute at rest?

A. 20 ml/min
B. 50 ml/min (Correct Answer)
C. 75 ml/min
D. 100 ml/min

Explanation: ***50 ml/min*** - The body typically consumes about **250 ml/min of oxygen** at rest and produces approximately **200 ml/min of carbon dioxide**. - The difference between oxygen consumed and carbon dioxide produced is therefore **50 ml/min** (250 - 200 = 50). - This difference exists because the **respiratory quotient (RQ)** is approximately **0.8** (200/250), meaning less CO2 is produced than O2 consumed on a molar basis. *20 ml/min* - This value is **too low** and underestimates the physiological difference between oxygen consumption and carbon dioxide production. - With typical O2 consumption of 250 ml/min and RQ of 0.8, the difference cannot be this small. *75 ml/min* - This value represents an **overestimation** of the difference between oxygen consumption and carbon dioxide production under normal resting conditions. - This would imply an RQ of approximately 0.7, which is lower than the typical mixed diet RQ of 0.8. *100 ml/min* - This value is a significant **overestimation** of the physiological difference. - This would suggest an RQ of 0.6, which is not physiologically normal for resting conditions on a mixed diet.

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