This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Cardiac index is the ratio of? \nSelect and explain. \nA. Cardiac output and body weight, B. Cardiac output and surface area, C. Cardiac output and work of hea, D. Surface volume and surface area', 'role': 'user'}, {'content': "Answer: B \nExplanation: Cardiac index is the cardiac output corrected for the individual's size. For example, the cardiac output of a 50-kg woman will be significantly lower than that of a 90-kg man. It has been found, however, that cardiac output correlates better with body surface area than with body weight. Therefore, it is common to express the cardiac output per square meter of surface area. Under resting conditions, the cardiac index is normally approximately 3 L/min per m2. Ref: Mohrman D.E., Heller L.J. (2010). Chapter 4. Measurements of Cardiac Function. In D.E. Mohrman, L.J. Heller (Eds), Cardiovascular Physiology, 7e", 'role': 'assistant'}]