Respiratory System — MCQs
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Which of the following factors does not chemically regulate respiration?
Which of the following does not stimulate central chemoreceptors?
Functional residual capacity in normal adult is?
Diffusion related to O2 transport across respiratory membrane is an example of?
What is the primary graphical difference between the Hb-O2 dissociation curve and the Hb-CO curve?
Distending capacity of lung (maximum change in volume during inspiration) is maximum at?
What is the maximum voluntary ventilation (MVV) and how does it relate to respiratory function?
Where does the respiratory exchange of gases begin?
Which of the following statements is true about the Bohr effect?
What is the primary function of the human respiratory system?
Respiratory System Indian Medical PG Practice Questions and MCQs
Question 791: 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 792: 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 793: 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 794: 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 795: What is the primary graphical difference between the Hb-O2 dissociation curve and the Hb-CO curve?
- A. CO has a higher affinity for hemoglobin than oxygen.
- B. The Hb-CO curve is shifted to the left compared to the Hb-O2 curve. (Correct Answer)
- C. None of the options.
- D. CO binding prevents normal oxygen release despite high oxygen saturation
Explanation: ***The Hb-CO curve is shifted to the left compared to the Hb-O2 curve.*** - A leftward shift of the dissociation curve indicates a **higher affinity** of hemoglobin for the binding molecule, meaning a lower partial pressure is needed to achieve a given saturation. - This shift visually represents the fact that **carbon monoxide (CO) binds to hemoglobin with much greater affinity than oxygen**, making it harder for oxygen to bind and be released. *CO has a higher affinity for hemoglobin than oxygen.* - While this statement is true and crucial to understanding the difference, it describes the *reason* for the curve shift rather than the direct visual representation of the difference in the curves themselves. - The phrasing "primary difference between the... curves" refers to their graphical distinction, which is the leftward shift. *None of the options.* - This option is incorrect because there is a primary difference between the two curves, which is well-described by one of the other choices. - The distinct binding characteristics of CO and O2 to hemoglobin lead to clear graphical differences. *CO binding prevents normal oxygen release despite high oxygen saturation* - This statement is a consequence of CO binding to hemoglobin and its effect on oxygen transport, but it's not the primary graphical difference between the two dissociation curves. - While CO binding *does* impede oxygen release, the *shift* of the curve visually represents the altered binding dynamics.
Question 796: 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 797: 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 798: Where does the respiratory exchange of gases begin?
- A. Bronchi
- B. Alveoli
- C. Bronchiole (Correct Answer)
- D. Tissue level
Explanation: ***Correct: Bronchiole (Respiratory Bronchioles)*** - The **respiratory zone** where gas exchange begins starts at the **respiratory bronchioles** - Respiratory bronchioles have **occasional alveoli** budding from their walls, marking the first site where oxygen and carbon dioxide exchange occurs - This represents the **transition** from the conducting zone (which only transports air) to the respiratory zone (where gas exchange happens) - According to standard respiratory physiology, the respiratory zone includes: respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli *Incorrect: Alveoli* - While **alveoli** are the **primary and most efficient** site of gas exchange due to their enormous surface area (~70 m²) and thin walls - They are NOT where gas exchange "begins" - gas exchange has already started in the respiratory bronchioles - Alveoli represent the terminal and most developed part of the respiratory zone where the majority of gas exchange occurs *Incorrect: Bronchi* - **Bronchi** are part of the **conducting zone** of the respiratory system - Their walls are too thick and they lack alveoli, so **no gas exchange** occurs here - Their function is to conduct air to and from the lungs, with mucus and cilia helping to filter particles *Incorrect: Tissue level* - **Tissue level** gas exchange refers to **internal/systemic respiration** - the exchange of gases between blood and body tissues - This occurs in systemic capillaries throughout the body, NOT in the lungs - The question asks about where respiratory exchange begins in the lungs (external respiration), not tissue-level gas exchange
Question 799: Which of the following statements is true about the Bohr effect?
- A. All are true
- B. Left shift of Hb-02 dissociation curve
- C. It is due to H+.
- D. Decrease affinity of O2 by increase PCO2 (Correct Answer)
Explanation: ***Decrease affinity of O2 by increase PCO2*** - The **Bohr effect** describes how an increase in **PCO2** (carbon dioxide partial pressure) or a decrease in pH (more acidic environment) reduces hemoglobin's affinity for oxygen. - This is the most complete statement, as increased PCO2 leads to increased H+ (via CO2 + H2O → H2CO3 → H+ + HCO3-), which binds to hemoglobin and reduces oxygen affinity. - This facilitates oxygen release to metabolically active tissues producing more CO2 and H+. - Causes a **right shift** of the oxygen-hemoglobin dissociation curve. *Left shift of Hb-O2 dissociation curve* - A **left shift** indicates *increased* affinity of hemoglobin for oxygen (seen with decreased PCO2, increased pH, decreased temperature, or decreased 2,3-BPG). - The Bohr effect causes a **right shift**, not a left shift, signifying *decreased* oxygen affinity and promoting oxygen release to tissues. - This option is the opposite of what occurs in the Bohr effect. *It is due to H+.* - While **H+ ions** are indeed the molecular mechanism of the Bohr effect (H+ binds to histidine residues on hemoglobin, stabilizing the deoxygenated T-state), this statement alone is incomplete. - It doesn't mention the physiological trigger (increased PCO2) or the functional consequence (decreased O2 affinity). - The first option is more comprehensive and better describes the complete Bohr effect phenomenon. *All are true* - This is incorrect because the statement about a **left shift** is definitively false. - The Bohr effect produces a *right shift*, not a left shift, of the Hb-O2 dissociation curve.
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.
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