What equation expresses cardiac output?

• A. Cardiac output = heart rate minus stroke volume
• B. Cardiac output = heart rate × stroke volume
• C. Cardiac output = stroke volume ÷ heart rate
• D. Cardiac output = heart rate plus stroke volume

Answer: (B)

Cardiac output is the volume of blood the heart pumps each minute. It depends on two factors: how often the heart beats (heart rate) and how much blood is ejected with each beat (stroke volume). Multiply heart rate by stroke volume to get cardiac output. This makes sense because you’re combining, per minute, the number of beats with the amount chosen to be pumped with each beat, giving a rate of blood flow. Units work out to milliliters per minute (or liters per minute): beats per minute times milliliters per beat equals milliliters per minute. For example, at 70 beats per minute and a stroke volume of 70 mL, cardiac output is about 4900 mL per minute (roughly 5 L/min). If heart rate rises while stroke volume stays the same, cardiac output increases; if stroke volume changes, it also shifts the total. Subtracting, dividing, or adding heart rate and stroke volume would not produce a flow rate.