Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 841: 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 842: Which of the following defines vital capacity?

A. Air in lung after normal expiration
B. Maximum air in lung after end of maximal inspiration
C. Maximum air that can be expirated after maximum inspiration (Correct Answer)
D. Maximum air that can be expirated after normal expiration

Explanation: ***Maximum air that can be expirated after maximum inspiration*** - **Vital capacity (VC)** is the maximum volume of air that can be exhaled after a maximal inspiration. - It represents the sum of **tidal volume (TV)**, **inspiratory reserve volume (IRV)**, and **expiratory reserve volume (ERV)**. - VC = TV + IRV + ERV *Air in lung after normal expiration* - This describes the **functional residual capacity (FRC)**, which is the volume of air remaining in the lungs after a normal passive exhalation. - FRC = ERV + RV (expiratory reserve volume + residual volume). *Maximum air that can be expirated after normal expiration* - This refers to the **expiratory reserve volume (ERV)**, the additional amount of air that can be exhaled forcibly after a normal passive exhalation. - It is the extra air expelled beyond tidal volume during forced expiration. *Maximum air in lung after end of maximal inspiration* - This definition corresponds to **total lung capacity (TLC)**, which is the maximum volume of air the lungs can hold after a maximal inspiration. - TLC = VC + RV (vital capacity + residual volume).

Question 843: 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.

Question 844: True about normal expiration is:

A. In expiration pleural pressure is equal to alveolar pressure
B. Muscles that elevate the chest cage are classified as muscles of expiration
C. At the end of normal expiration, the air in the lung is FRC
D. Chest wall has a tendency to move outward which is balanced by inward recoil of alveoli (Correct Answer)

Explanation: ***Chest wall has a tendency to move outward which is balanced by inward recoil of alveoli*** - At **Functional Residual Capacity (FRC)**, the outward recoil of the **chest wall** balances the inward elastic recoil of the **lungs**, resulting in no net force and a stable lung volume. - This equilibrium point represents the resting volume of the respiratory system when respiratory muscles are relaxed during **normal expiration**. - This statement directly describes the **mechanism** of normal expiration—the passive process driven by balanced recoil forces. *At the end of normal expiration, the air in the lung is FRC* - While **technically true** that FRC is the volume remaining after normal expiration, this option describes the **endpoint volume** rather than the process of normal expiration itself. - The question asks what is true **about normal expiration** (the process), not what is true **at the end** of expiration (the outcome). - The correct answer better addresses the mechanism and forces involved during the expiratory process. *In expiration pleural pressure is equal to alveolar pressure* - **INCORRECT**: Pleural pressure is **always negative** relative to alveolar pressure (typically -5 to -8 cm H₂O at FRC). - During **normal expiration**, pleural pressure becomes *less negative* as lung volume decreases, but **never equals** alveolar pressure. - If pleural pressure equaled alveolar pressure, the lungs would collapse (pneumothorax). *Muscles that elevate the chest cage are classified as muscles of expiration* - **INCORRECT**: Muscles that **elevate the chest cage**, such as the **external intercostals** and **diaphragm**, are primarily involved in **inspiration**. - **Normal expiration** is a *passive process* driven by the elastic recoil of the lungs and chest wall, **not requiring muscle contraction**.

Question 845: What is the estimated PaO2 after giving FiO2 at 0.5 in a normal person?

A. > 200 mmHg (Correct Answer)
B. < 100 mmHg
C. 150–200 mmHg
D. 100–150 mmHg

Explanation: ***> 200 mmHg*** - In a **normal healthy person** breathing FiO2 of 0.5 (50% oxygen), the expected **PaO2** is typically **250-300 mmHg**. - Using the **alveolar gas equation**: PAO2 = FiO2(PB - PH2O) - PaCO2/RQ = 0.5(760 - 47) - 40/0.8 ≈ **306 mmHg** - The normal **A-a gradient** is 5-15 mmHg, so PaO2 = 306 - 10 ≈ **296 mmHg** - **Clinical rule of thumb**: PaO2 ≈ 5 × FiO2% = 5 × 50 = **250 mmHg** (approximation accounting for physiological shunt) - Therefore, the expected range is clearly **> 200 mmHg** in a normal individual *150–200 mmHg* - This range would indicate **mild oxygenation impairment** or increased shunt fraction - While adequate for tissue oxygenation, this is **lower than expected** for a normal person on 50% oxygen - May suggest underlying **mild V/Q mismatch** or early pulmonary dysfunction *100–150 mmHg* - This represents **moderate impairment** in oxygen transfer - Indicates significant **pulmonary pathology** such as pneumonia, ARDS, or substantial shunt - Not consistent with normal lung function on FiO2 0.5 *< 100 mmHg* - This represents **severe hypoxemia** despite supplemental oxygen - Indicates **critical pulmonary dysfunction** with large shunt or severe V/Q mismatch - Requires immediate intervention and is never expected in a healthy individual on 50% oxygen

Question 846: The chloride shift occurs rapidly and is essentially complete within:

A. 1 second (Correct Answer)
B. 2 seconds
C. 5 seconds
D. 60 seconds

Explanation: ***1 second*** - The **chloride shift**, an exchange of bicarbonate and chloride ions across the red blood cell membrane, is a very rapid process. - This rapid kinetics ensures efficient **CO2 transport** from tissues to the lungs. *2 seconds* - While seemingly a short duration, **2 seconds** is generally considered longer than the actual time frame for the completion of the chloride shift. - The physiological need for immediate CO2 buffering necessitates a faster mechanism. *5 seconds* - A duration of **5 seconds** would imply a slower rate of gas exchange than physiologically required for efficient CO2 transport. - Such a delay could lead to transient but significant alterations in **blood pH**. *60 seconds* - **60 seconds** (1 minute) is far too long for a process critical to immediate blood gas regulation. - If the chloride shift took this long, it would severely impair the body's ability to excrete **CO2** and maintain acid-base balance.

Question 847: What is the physiological effect of very high positive end-expiratory pressure in a patient with respiratory distress?

A. Increased blood pressure
B. Decreased body temperature
C. Decreased blood pressure (Correct Answer)
D. Increased body temperature

Explanation: ***Decreased blood pressure*** - Very high **positive end-expiratory pressure (PEEP)** increases intrathoracic pressure, which in turn reduces **venous return** to the heart. - This decreased preload leads to a **reduction in cardiac output**, ultimately causing **hypotension** (decreased blood pressure). - This is a well-recognized hemodynamic complication of excessive PEEP in mechanical ventilation. *Increased blood pressure* - High PEEP typically lowers, rather than increases, blood pressure due to its effects on **venous return** and **cardiac output**. - The elevated intrathoracic pressure acts as a barrier to venous return, reducing preload and thus blood pressure. *Decreased body temperature* - **PEEP** primarily affects cardiovascular and respiratory physiology, not **thermoregulation**. - Body temperature changes are usually related to systemic inflammation, infection, or environmental factors, not directly to PEEP settings. *Increased body temperature* - Similar to decreased body temperature, **PEEP** does not directly regulate body temperature. - An elevated body temperature (fever) would suggest an underlying **infection** or **inflammatory process**, which might be present in a patient with respiratory distress but is not a direct physiological effect of high PEEP.

Question 848: Which test is primarily used to assess gas diffusion capacity in the lungs?

A. DLCO (Correct Answer)
B. Spirometry
C. Both DLCO and Spirometry
D. None of the options

Explanation: ***DLCO*** - **DLCO (Diffusing Capacity of the Lungs for Carbon Monoxide)** specifically measures the transfer of gas from the alveoli to the red blood cells, directly assessing the **gas diffusion capacity** of the lungs. - It is crucial for identifying interstitial lung diseases, emphysema, or other conditions affecting the **alveolar-capillary membrane**. *Spirometry* - **Spirometry** primarily assesses **lung volumes and airflow rates**, such as FEV1 and FVC, to diagnose obstructive or restrictive ventilatory defects. - It does not directly measure the efficiency of **gas exchange** across the alveolar-capillary membrane. *Both DLCO and Spirometry* - While both are important in pulmonary function testing, they measure different aspects of **lung function**. DLCO specifically measures **diffusion capacity**, while spirometry measures **airflow and lung volumes**. - Therefore, they are not primarily used for the *same* assessment. *None of the options* - DLCO is indeed the primary test for assessing **gas diffusion capacity** in the lungs. - This option is incorrect because a correct answer is provided.

Question 849: All of the following factors influence the hemoglobin dissociation curve, except.

A. Temperature
B. 2–3 DPG levels
C. Plasma sodium concentration (Correct Answer)
D. CO2 tension

Explanation: ***Plasma sodium concentration*** - While essential for **osmolality** and **electrolyte balance**, plasma sodium concentration does not directly influence the binding affinity of hemoglobin for oxygen. - Changes in sodium concentration primarily affect fluid shifts and neural function, not the **hemoglobin dissociation curve**. *CO2 tension* - An increase in **PCO2** (hypercapnia) leads to a **rightward shift** of the hemoglobin dissociation curve, indicating decreased oxygen affinity. - This effect, known as the **Bohr effect**, facilitates oxygen release in tissues with high metabolic activity. *Temperature* - An increase in **body temperature** causes a **rightward shift** in the hemoglobin dissociation curve, leading to reduced oxygen affinity. - This is beneficial during exercise or fever, as it promotes oxygen unloading to active tissues. *2–3 DPG levels* - **2,3-bisphosphoglycerate (2,3-BPG)** binds to deoxygenated hemoglobin, stabilizing its T-state and reducing its affinity for oxygen, thus shifting the curve to the **right**. - During chronic hypoxia or anemia, 2,3-BPG levels increase to enhance oxygen delivery to tissues.

Question 850: Which of the following is not a normal stimulus for resting ventilation?

A. Stretch receptors
B. J receptors (Correct Answer)
C. PCO2
D. PO2

Explanation: ***J receptors*** - **J receptors** (juxtacapillary receptors) are located in the alveolar walls and are primarily stimulated by **pulmonary edema**, inflammation, or vascular congestion. - Their stimulation typically causes rapid, shallow breathing, but they are **completely inactive during normal, resting ventilation**. - These receptors only become active under pathological conditions, making them **not a normal stimulus for resting breathing**. *Stretch receptors* - **Pulmonary stretch receptors** in the airways respond to lung distension, mediating the **Hering-Breuer reflex** which helps regulate breathing depth and rate. - These receptors are **active during normal tidal breathing** and contribute to the rhythmic pattern of respiration at rest. *PO2* - **Peripheral chemoreceptors** (carotid and aortic bodies) monitor **arterial PO2** and do have **tonic baseline activity** at normal PO2 levels (~95-100 mmHg). - While their contribution is **minimal at normal oxygen levels**, they are present and functioning, making them technically a (weak) normal stimulus. - They become a **major stimulus only when PO2 drops below 60 mmHg** (hypoxemia). *PCO2* - **PCO2** (specifically, the H+ concentration in the cerebrospinal fluid derived from PCO2) is the **most potent and immediate normal stimulus** for resting ventilation. - **Central chemoreceptors** in the medulla are extremely sensitive to changes in CSF pH, directly regulating breathing to maintain arterial PCO2 within a narrow range (~40 mmHg).

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