Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 791: What is the maximum voluntary ventilation (MVV) and how does it relate to respiratory function?

A. Amount of air expired in one minute at rest
B. Maximum amount of air that can be inspired and expired in one minute (Correct Answer)
C. Maximum amount of air that can be inspired per breath
D. Maximum amount of air remaining in lung after forced expiration

Explanation: ***Maximum amount of air that can be inspired and expired in one minute*** - The **Maximum Voluntary Ventilation (MVV)** measures the maximum volume of air a person can breathe in and out during a 12-second period, extrapolated to one minute. - It reflects the overall function of the **respiratory muscles**, **airway patency**, and lung compliance, indicating the patient's ventilatory reserve. *Amount of air expired in one minute at rest* - This describes the **minute ventilation** at rest, which is typically much lower than the MVV and does not reflect maximal respiratory capacity. - It is calculated as **tidal volume** multiplied by the respiratory rate during quiet breathing. *Maximum amount of air that can be inspired per breath* - This sounds similar to **inspiratory capacity** or **inspiratory reserve volume**, which are single-breath measurements, not a measurement over one minute. - Inspiratory capacity is the maximum amount of air that can be inspired after a normal expiration. *Maximum amount of air remaining in lung after forced expiration* - This describes the **residual volume**, which is the volume of air remaining in the lungs after a maximal exhalation. - Residual volume is crucial for keeping the **alveoli patent** and preventing lung collapse, but it does not represent a ventilation capacity.

Question 792: Which of the following is a feature of pulmonary oxygen toxicity?

A. Decreased mucociliary transport in airways
B. Increased capillary endothelial permeability
C. Inhibition of phagocytosis function of alveolar macrophages
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - All three listed features are well-established manifestations of **pulmonary oxygen toxicity** - **Oxygen free radicals** generated during prolonged exposure to high O₂ concentrations cause synergistic damage affecting multiple cellular and physiological processes - The combination of these effects leads to significant **lung injury** and respiratory dysfunction **Why each option is correct:** **Increased capillary endothelial permeability:** - Oxygen free radicals directly damage endothelial cells, disrupting tight junctions - Results in **pulmonary edema** and impaired gas exchange - One of the earliest manifestations of O₂ toxicity **Decreased mucociliary transport in airways:** - High O₂ concentrations impair ciliated epithelial cell function - Alters mucus viscosity and composition - Reduces clearance of inhaled particles and pathogens, increasing risk of **respiratory infections** **Inhibition of phagocytosis function of alveolar macrophages:** - Alveolar macrophages are highly susceptible to oxidative stress - Impaired ability to phagocytose pathogens and cellular debris - Compromises the **lung's immune defense** and promotes inflammation

Question 793: Diffusion related to O2 transport across respiratory membrane is an example of?

A. Simple diffusion (Correct Answer)
B. Facilitated diffusion
C. Active diffusion
D. Osmotic diffusion

Explanation: ***Simple diffusion*** - Oxygen crosses the **respiratory membrane** (alveolar and capillary walls) directly through the lipid bilayer, driven by its **partial pressure gradient**. - This process does not require protein carriers or metabolic energy, fitting the definition of **simple diffusion**. *Facilitated diffusion* - This type of diffusion requires a **specific protein carrier** to transport molecules across the membrane. - While it does not require metabolic energy, oxygen transport across the respiratory membrane is efficient enough via simple diffusion due to its small size and lipid solubility. *Active diffusion* - This term is a **misnomer**; diffusion is by definition a passive process. - **Active transport** involves moving molecules against their concentration gradient, which requires metabolic energy (ATP). *Osmotic diffusion* - **Osmosis** specifically refers to the diffusion of **water** across a selectively permeable membrane. - It does not describe the movement of gases like oxygen.

Question 794: Which of the following statements about the O2-Hb dissociation curve is correct?

A. It demonstrates cooperative binding. (Correct Answer)
B. The curve is a straight line.
C. It is 100% saturated at a PO2 of 100 mmHg.
D. A hemoglobin molecule can carry 4 molecules of O2.

Explanation: ***It demonstrates cooperative binding.*** - **Cooperative binding** describes how the binding of one oxygen molecule to hemoglobin increases the affinity of the remaining binding sites for oxygen. - This property gives the O2-Hb dissociation curve its characteristic **sigmoid (S-shaped)** appearance, allowing for efficient oxygen loading in the lungs and unloading in the tissues. *The curve is a straight line.* - The O2-Hb dissociation curve is **sigmoid or S-shaped**, not a straight line, due to the phenomenon of cooperative binding. - A straight line would imply a constant affinity of hemoglobin for oxygen, which is not the case. *It is 100% saturated at a PO2 of 100 mmHg.* - Hemoglobin is typically around **97-98% saturated** at a PO2 of 100 mmHg (arterial blood). - Complete 100% saturation is rarely achieved under physiological conditions. *A hemoglobin molecule can carry 4 molecules of O2.* - While this statement is factually true (one hemoglobin molecule has **four heme groups** and can bind up to **four molecules of oxygen**), it describes the structure and oxygen-carrying capacity of hemoglobin rather than a characteristic of the dissociation **curve itself**. - The question asks about features of the O2-Hb dissociation curve, and cooperative binding is the key property that defines the curve's behavior and sigmoid shape.

Question 795: Which of the following factors does not chemically regulate respiration?

A. Partial pressure of oxygen (PO2)
B. Hydrogen ion concentration (pH)
C. Partial pressure of carbon dioxide (PCO2)
D. Systemic arterial blood pressure (BP) (Correct Answer)

Explanation: ***Systemic arterial blood pressure (BP)*** - While significant changes in blood pressure can indirectly affect respiration through other mechanisms (e.g., changes in cerebral blood flow), it is **not a direct chemical regulator** of breathing. - The control of respiration primarily involves chemoreceptors responding to blood gas levels, not baroreceptors detecting blood pressure. *Partial pressure of oxygen (PO2)* - **Peripheral chemoreceptors** (located in the carotid and aortic bodies) are highly sensitive to significant drops in **arterial PO2**. - When **PO2 falls below approximately 60 mmHg**, these chemoreceptors stimulate an increase in ventilation, serving as an important **hypoxic drive**. *Partial pressure of carbon dioxide (PCO2)* - **PCO2 is the most potent chemical regulator of respiration**, primarily acting through **central chemoreceptors** in the medulla. - An increase in arterial PCO2 leads to an increase in H+ concentration in the cerebrospinal fluid, stimulating central chemoreceptors to **increase ventilation** to expel excess CO2. *Hydrogen ion concentration (pH)* - Changes in **pH** (or hydrogen ion concentration) in the blood are closely linked to **PCO2** (via the carbonic acid-bicarbonate buffer system) and are also directly sensed by **peripheral chemoreceptors**. - A decrease in blood pH (acidemia) directly stimulates peripheral chemoreceptors to **increase ventilation**, helping to excrete CO2 and thereby raise pH.

Question 796: Functional residual capacity in normal adult is?

A. 500 ml
B. 1200 ml
C. 2400 ml (Correct Answer)
D. 3200 ml

Explanation: ***2400 ml*** - **Functional residual capacity (FRC)** is the volume of air remaining in the lungs after a normal passive exhalation. - In a healthy adult, the average FRC is approximately **2400 mL**, or 2.4 liters. *500 ml* - This volume typically represents the **tidal volume (TV)**, which is the amount of air exchanged during normal, quiet breathing. - Tidal volume is a much smaller component of lung capacity compared to FRC. *1200 ml* - This value is close to the **residual volume (RV)**, which is the amount of air remaining in the lungs after a maximal forceful exhalation. - FRC is the sum of expiratory reserve volume and residual volume, thus larger than RV alone. *3200 ml* - This value is closer to the **inspiratory capacity (IC)**, which is the maximum volume of air that can be inspired after a normal expiration. - Alternatively, it could be closer to the **vital capacity (VC)**, which is typically around 4500-5000 mL in healthy adults, making 3200 mL still too low for VC and too high for FRC.

Question 797: Closing volume is related to which of the following?

A. Tidal volume
B. Residual volume (Correct Answer)
C. Vital capacity
D. None of the options

Explanation: ***Residual volume*** - **Closing volume (CV)** is the lung volume at which the smallest airways in dependent lung regions begin to close during expiration. - CV is measured as the volume of gas expired from the beginning of airway closure (phase IV of the single-breath nitrogen test) down to **residual volume (RV)**. - **Closing Capacity (CC) = Closing Volume (CV) + Residual Volume (RV)**, demonstrating their direct mathematical relationship. - When CC exceeds FRC (functional residual capacity), airways close during normal tidal breathing, leading to gas trapping and V/Q mismatch. - CV increases with age, smoking, and obstructive lung diseases, encroaching on the expiratory reserve volume and eventually affecting tidal breathing. *Tidal volume* - **Tidal volume (TV)** is the volume of air inhaled or exhaled during normal, quiet breathing (approximately 500 mL in adults). - TV is not used in the measurement or definition of closing volume. - While increased CV can cause airway closure *during* tidal breathing (when CC > FRC), TV itself is not mathematically or definitionally related to CV. *Vital capacity* - **Vital capacity (VC)** is the maximum volume of air that can be exhaled after maximal inspiration (VC = IRV + TV + ERV). - VC is a measure of overall ventilatory capacity but does not specifically relate to the point at which airways begin to close. - CV represents a small fraction of the total lung volumes and is specifically about airway mechanics, not maximal breathing capacity. *None of the options* - This is incorrect because **residual volume** has a direct mathematical relationship with closing volume through the equation CC = CV + RV.

Question 798: Distending capacity of lung (maximum change in volume during inspiration) is maximum at?

A. Apex of the lung
B. Base of the lung (Correct Answer)
C. Mid region of the lung
D. Lower lobe of the lung

Explanation: ***Base of the lung*** - Due to **gravitational forces**, the negative intrapleural pressure is less negative at the base compared to the apex, meaning the alveoli at the base are less stretched at rest. - This **lesser distension at rest** allows the alveoli at the base to have a greater capacity to expand and distend during inspiration, leading to better ventilation. *Apex of the lung* - The **more negative intrapleural pressure** at the apex causes alveoli to be more distended (larger) at functional residual capacity (FRC). - These already stretched alveoli have **less capacity to further distend** during inspiration, leading to less ventilation compared to the base. *Mid region of the lung* - The distending capacity in the mid-region is **intermediate** between the apex and the base. - It is **not the maximum** because the gravitational gradient of intrapleural pressure still allows the base to have a greater change in volume. *Lower lobe of the lung* - While the base of the lung is part of the lower lobe, referring specifically to the "lower lobe" is still **less precise** than the "base." - The specific term "base" refers to the region with the **largest distending capacity** due to the physiological pressure gradient.

Question 799: Which of the following does not stimulate central chemoreceptors?

A. None of the options stimulate
B. Increased PCO2
C. Hypoxia (Correct Answer)
D. Hydrogen ion concentration in CSF

Explanation: ***Hypoxia*** - Central chemoreceptors are primarily sensitive to **PCO2** and **hydrogen ion concentration** in the CSF and are not significantly stimulated by hypoxia. - Peripheral chemoreceptors (located in the carotid and aortic bodies) are the main sensors for **hypoxia**. *Increased PCO2* - An increase in **PCO2** in the arterial blood readily diffuses across the blood-brain barrier into the **cerebrospinal fluid (CSF)**. - In the CSF, CO2 is converted to **carbonic acid**, which dissociates into hydrogen ions, directly stimulating central chemoreceptors. *Hydrogen ion concentration in CSF* - Central chemoreceptors are directly stimulated by an increase in the **hydrogen ion concentration** in the CSF. - This increased acidity is typically a result of elevated CO2 levels diffusing into the CSF. *None of the options stimulate* - This option is incorrect because both **increased PCO2** and **hydrogen ion concentration in the CSF** are potent stimulators of central chemoreceptors. - Central chemoreceptors are crucial for regulating ventilation in response to changes in blood gases.

Question 800: What is the primary function of the human respiratory system?

A. Gas exchange (Correct Answer)
B. Nutrient absorption
C. Hormone regulation
D. Waste elimination

Explanation: ***Gas exchange*** - The primary function of the respiratory system is to facilitate the exchange of gases (oxygen and carbon dioxide) between the air and the blood. - This process occurs mainly in the alveoli of the lungs, where oxygen diffuses into the bloodstream and carbon dioxide diffuses out. *Nutrient absorption* - Nutrient absorption is the primary function of the digestive system, not the respiratory system. - The digestive system breaks down food into molecules that can be absorbed into the bloodstream. *Hormone regulation* - Hormone regulation is primarily controlled by the endocrine system, which produces and secretes hormones to regulate various bodily functions. - While some hormones can affect respiratory rate, hormone regulation is not the respiratory system's primary function. *Waste elimination* - The primary organ for waste elimination from the blood is the kidney, as part of the urinary system, which excretes metabolic waste products. - The respiratory system eliminates carbon dioxide (a metabolic waste product), but this is considered part of gas exchange rather than general waste elimination.

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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