Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 891: The oxygen-hemoglobin dissociation curve is sigmoid because

A. Binding of one oxygen molecule decreases the affinity of binding other O2 molecules
B. Oxygen affinity of Hemoglobin decreases when the pH of blood falls
C. Binding of oxygen to Hemoglobin reduces the affinity of Hb for CO2
D. Binding of one oxygen molecule increases the affinity of binding other O2 molecules (Correct Answer)

Explanation: ***Binding of one oxygen molecule increases the affinity of binding other O2 molecules*** - The **sigmoid shape** of the oxygen-hemoglobin dissociation curve reflects the cooperative binding of oxygen. When one oxygen molecule binds to a heme unit in hemoglobin, it causes a conformational change that increases the affinity of the remaining heme units for oxygen. - This **cooperative binding** means that at low partial pressures of oxygen, very little oxygen binds to hemoglobin. However, once a few oxygen molecules bind, subsequent binding occurs much more readily and steeply, leading to the characteristic 'S' shape. *Binding of one oxygen molecule decreases the affinity of binding other O2 molecules* - This statement is incorrect as it describes **negative cooperativity**, which is the opposite of what occurs with oxygen and hemoglobin. - Decreased affinity after initial binding would lead to a **hyperbolic (rectangular)** curve rather than a sigmoid one, similar to myoglobin's oxygen binding curve. *Oxygen affinity of Hemoglobin decreases when the pH of blood falls* - This describes the **Bohr effect**, where a decrease in pH (acidosis) or an increase in CO2 shifts the curve to the right, indicating reduced oxygen affinity and enhanced oxygen release to tissues. - While this is an important physiological phenomenon, it explains the **shift** of the curve rather than its inherent **sigmoid shape**. *Binding of oxygen to Hemoglobin reduces the affinity of Hb for CO2* - This phenomenon is known as the **Haldane effect**, where oxygen binding promotes the release of CO2 from hemoglobin in the lungs. - The Haldane effect is another crucial aspect of hemoglobin function but does not explain the **sigmoid shape** of the oxygen-hemoglobin dissociation curve itself.

Question 892: Which of the following statements about peripheral chemoreceptors is true?

A. Sensitive to hypoxia (Correct Answer)
B. Insensitive to oxygen levels
C. Primarily sensitive to PCO2
D. Sensitive to pH changes, but not primarily responsive to hypoxia

Explanation: ***Sensitive to hypoxia*** - **Peripheral chemoreceptors**, located in the **carotid bodies** and **aortic arch**, are highly sensitive to decreases in arterial **PO2 (hypoxia)**. - This response is crucial for initiating increased ventilation to correct low oxygen levels. *Primarily sensitive to PCO2* - While peripheral chemoreceptors do respond to high **PCO2**, the **central chemoreceptors** in the brainstem are the primary and most powerful regulators of ventilation in response to changes in **arterial PCO2**. - The response of peripheral chemoreceptors to PCO2 is secondary to their hypoxia-sensing role. *Insensitive to oxygen levels* - This statement is incorrect as peripheral chemoreceptors are the main physiological sensors for **arterial PO2** and are acutely responsive to hypoxia. - Their sensitivity to falling PO2 becomes significant when arterial PO2 drops below 60 mmHg. *Sensitive to pH changes, but not primarily responsive to hypoxia* - Peripheral chemoreceptors are indeed sensitive to changes in **pH**, particularly metabolic acidosis, largely due to their sensitivity to the resulting changes in PCO2 and direct pH effects. - However, they are also **primarily responsive to hypoxia**, making the latter part of the statement incorrect.

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