Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 781: Which process describes the transfer of oxygen from the lungs to the blood?

A. Filtration
B. Reabsorption
C. Diffusion (Correct Answer)
D. Active transport

Explanation: ***Diffusion*** - **Diffusion** is the primary mechanism by which **oxygen** moves from the **alveoli** in the lungs into the **pulmonary capillaries**. This movement occurs down a **concentration gradient** from an area of higher oxygen partial pressure (alveoli) to an area of lower oxygen partial pressure (blood). - Similarly, **carbon dioxide** moves from the blood into the **alveoli** by diffusion, following its own concentration gradient. *Filtration* - **Filtration** is a process by which fluids and solutes are forced through a membrane due to **hydrostatic pressure**, commonly seen in the **kidneys** for urine formation. - This mechanism is not involved in the direct transfer of gases between the lungs and the blood. *Reabsorption* - **Reabsorption** is the process by which filtered substances are taken back into the blood from the **renal tubules** in the kidneys. - It is not applicable to the gas exchange process in the respiratory system. *Active transport* - **Active transport** uses **cellular energy** (ATP) to move substances across a membrane against their **concentration gradient**. - Gas exchange in the lungs does not require active transport as it relies solely on **passive diffusion** down an electrochemical gradient.

Question 782: A patient presents with shortness of breath and cyanosis. An ABG shows a PaO2 of 55 mmHg. Which compensatory mechanism is likely to be activated?

A. Decreased cardiac output
B. Decreased erythropoietin production
C. Increased alveolar ventilation (Correct Answer)
D. Decreased alveolar ventilation

Explanation: ***Increased alveolar ventilation*** - **Hypoxemia** (PaO2 of 55 mmHg) is a powerful stimulus for the peripheral chemoreceptors, particularly the carotid bodies. - Activation of these chemoreceptors leads to an increase in the **rate and depth of breathing**, thereby increasing **alveolar ventilation** to improve oxygen uptake. *Decreased cardiac output* - In response to **hypoxemia**, the body typically tries to increase oxygen delivery to tissues by increasing **cardiac output**, not decreasing it. - A decreased cardiac output would further exacerbate tissue hypoxia. *Decreased erythropoietin production* - **Hypoxemia** stimulates the kidneys to increase the production of **erythropoietin**, not decrease it. - Increased erythropoietin production leads to an increase in **red blood cell mass**, which enhances the oxygen-carrying capacity of the blood as a long-term compensatory mechanism. *Decreased alveolar ventilation* - A **decreased alveolar ventilation** would worsen the hypoxemia by reducing the amount of oxygen reaching the alveoli and consequently the blood. - The body's immediate compensatory response to hypoxemia is to increase ventilation to counteract the low PaO2.

Question 783: Which of the following statements about Wright's spirometer is true?

A. Flow rates can be calculated
B. Gives false high values at low flow rates
C. Gives false low values at high flow rates
D. All of the options (Correct Answer)

Explanation: ***All of the options*** Wright's spirometer is a **vane-type spirometer** that measures respiratory volumes. All three statements about this device are correct: **Flow rates can be calculated:** - Wright's spirometer measures the **total volume** of air that passes through it - Flow rates can be calculated by dividing the measured volume by time **(flow = volume/time)** - This allows for determination of various flow parameters from the volume measurements **Gives false high values at low flow rates:** - At low flow rates, the mechanical **inertia of the vanes** causes them to continue rotating even after the flow has decreased - This leads to **overestimation** of the actual volume at slow flows - The device lacks sensitivity to detect when flow slows down rapidly **Gives false low values at high flow rates:** - At high flow rates, the **vanes cannot rotate fast enough** to keep up with the rapid airflow - This results in **underestimation** of the true volume at fast flows - The mechanical limitations prevent accurate capture of peak flows These characteristics make Wright's spirometer **less accurate at extreme flow rates** but still useful for measuring tidal volumes and minute ventilation in clinical settings.

Question 784: What is the definition of Functional Residual Capacity (FRC)?

A. After normal inspiration
B. After normal expiration (Correct Answer)
C. After forceful expiration
D. After forceful inspiration

Explanation: ***After normal expiration*** - **Functional Residual Capacity (FRC)** is the volume of air remaining in the lungs after a **normal, quiet exhalation** - It represents the sum of **Expiratory Reserve Volume (ERV)** and **Residual Volume (RV)** - This is the equilibrium point where the inward recoil of the lungs equals the outward recoil of the chest wall *After normal inspiration* - This would represent FRC plus the **tidal volume**, which is not a standard lung capacity measurement - The lungs are at their highest volume during a quiet breathing cycle at this point *After forceful expiration* - This describes the point at which only the **Residual Volume (RV)** remains in the lungs - All of the expiratory reserve volume has been expelled, leaving only RV - FRC exists *before* a forceful expiration, not after *After forceful inspiration* - This represents the **Total Lung Capacity (TLC)**, which is the maximum volume of air the lungs can hold - TLC = FRC + Inspiratory Capacity (IC), or RV + ERV + TV + IRV

Question 785: What happens to gas exchange when the Va/Q ratio approaches infinity?

A. Oxygen exchange is completely absent.
B. Gas exchange is impaired due to lack of blood flow. (Correct Answer)
C. Carbon dioxide exchange is completely absent.
D. Gas exchange remains normal.

Explanation: ***Gas exchange is impaired due to lack of blood flow.*** - When the **V̇A/Q̇ ratio approaches infinity**, it means **ventilation** (V̇A) is present but **perfusion** (Q̇) approaches zero. - This represents **alveolar dead space** - alveoli are ventilated but have no blood flow to participate in gas exchange. - **Both oxygen and carbon dioxide exchange are completely impaired** because there is no blood available to pick up O₂ or deliver CO₂ for elimination. - Clinical examples include **pulmonary embolism** and destroyed pulmonary vasculature. - This option correctly identifies the mechanism (lack of blood flow) and the outcome (impaired gas exchange for all gases). *Oxygen exchange is completely absent.* - While this statement is true, it is **incomplete** as it only addresses oxygen. - When perfusion is absent, **both O₂ and CO₂ exchange are equally affected**. - This option is too narrow and misses the complete physiological picture. *Carbon dioxide exchange is completely absent.* - Similar to the oxygen option, this is **incomplete** as it only mentions one gas. - Both gases require blood flow for exchange, so both are equally impaired. *Gas exchange remains normal.* - This is clearly **incorrect**. - V̇A/Q̇ → ∞ represents an extreme **ventilation-perfusion mismatch** with complete absence of perfusion. - This scenario results in severe impairment of all gas exchange.

Question 786: Respiratory acidosis is recognized primarily by an increase in which of the following?

A. PaCO2 (Correct Answer)
B. pH
C. HCO3-
D. PaO2

Explanation: ***PaCO2*** - **Respiratory acidosis** is directly caused by **hypoventilation**, leading to impaired **carbon dioxide (CO2)** elimination from the lungs. - The accumulation of **CO2** in the blood increases its partial pressure (**PaCO2**), which then reacts with water to form **carbonic acid**, lowering the blood **pH**. *PaO2* - **PaO2** (partial pressure of oxygen) usually decreases in conditions causing hypoventilation, but its increase is not the primary indicator of **respiratory acidosis**. - While both can be affected in respiratory compromise, **PaCO2** is the defining determinant of the respiratory component of acid-base balance. *HCO3-* - **HCO3-** (bicarbonate) is primarily involved in **metabolic acid-base balance** and acts as a buffer against acidity. - In **respiratory acidosis**, bicarbonate levels might increase as a compensatory mechanism (renal compensation) over time to buffer the excess acid, but an initial increase is not the primary hallmark of respiratory acidosis itself. *pH* - **pH** measures the overall acidity or alkalinity of the blood. In acidosis, the **pH** decreases. - While a low pH is present in acidosis, it is a *result* of the increased **PaCO2**, not the primary driver for recognizing respiratory acidosis.

Question 787: 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 788: 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 789: 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 790: Which of the following factors can cause an increase in pulmonary arterial pressure?

A. Histamine
B. Hypoxia (Correct Answer)
C. ANP
D. PGI2

Explanation: ***Hypoxia*** - Hypoxia causes **pulmonary vasoconstriction**, which is a unique response of the pulmonary circulation compared to systemic circulation. - This vasoconstriction increases **pulmonary vascular resistance**, directly leading to an elevation in pulmonary arterial pressure. - This phenomenon is known as **hypoxic pulmonary vasoconstriction (HPV)**, an important physiological mechanism. *Histamine* - In the pulmonary vasculature, histamine primarily causes **vasodilation**, which would *decrease* pulmonary arterial pressure. - While it can cause bronchoconstriction, its direct effect on pulmonary arterial pressure is not an increase. *ANP* - **Atrial natriuretic peptide (ANP)** primarily causes **vasodilation** and diuresis. - This effect would lead to a *reduction* in blood volume and systemic vascular resistance, thereby *decreasing* pulmonary arterial pressure. *PGI2* - **Prostacyclin (PGI2)** is a potent **vasodilator** and **platelet aggregation inhibitor**. - Its vasodilatory action would lead to a *decrease* in pulmonary vascular resistance and thus *lower* pulmonary arterial pressure.

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