Cardiovascular System — MCQs

Q: A 28-year-old marathon runner undergoes maximal exercise testing. At peak exercise, her oxygen consumption reaches 3600 mL/min, cardiac output is 24 L/min, and arterial oxygen content is 200 mL O2/L blood. Apply the Fick equation to determine her mixed venous oxygen content at peak exercise.

50 mL O2/L blood. ***50 mL O2/L blood*** - According to the **Fick Principle**, oxygen consumption (VO2) equals cardiac output (CO) multiplied by the **arteriovenous oxygen difference** (CaO2 - CvO2); rearranging the formula gives **CvO2 = CaO2 - (VO2 / CO)**. - Calculating the values provided: **200 - (3600 / 24) = 200 - 150 = 50 mL O2/L**, illustrating the significantly increased **oxygen extraction** during peak exercise. *100 mL O2/L blood* - This value results from an incorrect calculation or an assumption of a smaller **arteriovenous oxygen difference** of only 100 mL/L. - At peak exercise in an athlete, the **tissues extract** more than 50% of the oxygen, making this value too high. *125 mL O2/L blood* - This value would imply a very narrow **a-v O2 difference** of 75 mL/L, which is more characteristic of moderate rather than maximal exercise. - It fails to account for the high **metabolic demand** and high VO2 (3600 mL/min) seen in a marathon runner. *75 mL O2/L blood* - This value suggests an oxygen extraction of 125 mL/L, which is lower than the **150 mL/L** actually occurring based on the provided data. - While closer to the correct physiological range, it does not satisfy the mathematical requirement of the **Fick equation** parameters provided. *150 mL O2/L blood* - This value represents the **arteriovenous oxygen difference** (VO2/CO) rather than the **mixed venous oxygen content**. - Selecting this option indicates a confusion between the amount of oxygen **extracted by tissues** and the amount remaining in the venous blood.

Practice by Chapter

Cardiac Electrophysiology

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Cardiac Cycle

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Cardiac Output and Its Regulation

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Regional Circulations

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Cardiovascular Responses to Exercise and Stress

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