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Cardiac output (CO) is a crucial physiological measurement that indicates the volume of blood the heart pumps per minute. The formula for calculating cardiac output incorporates two key components: heart rate (HR) and stroke volume (SV).

The correct definition is CO = HR x SV. This indicates that cardiac output is directly proportional to both the number of heartbeats per minute (heart rate) and the volume of blood ejected with each heartbeat (stroke volume). Therefore, if either the heart rate or stroke volume increases, cardiac output will increase accordingly. This relationship is vital for understanding hemodynamics and the body's ability to meet its metabolic demands.

In contrast, the other options do not accurately represent the relationship between heart rate, stroke volume, and cardiac output based on established cardiovascular physiology principles. For example, simple addition, division, or subtraction of these components does not reflect the physiological interactions that occur during the cardiac cycle. Understanding this formula is essential for assessing cardiovascular health and guiding treatment in various clinical scenarios.