Cardiac output is defined as the volume of blood pumped by the heart per minute. It is the product of which two variables?

• A. Heart rate and stroke volume
• B. Heart rate and end-diastolic volume
• C. Stroke volume and end systolic volume
• D. Heart rate and cardiac reserve

Answer: (A)

Cardiac output is the volume of blood the heart pumps each minute, and it depends on two things: how often the heart beats (heart rate) and how much blood is pushed out with each beat (stroke volume). Stroke volume is the amount ejected per beat and is calculated as the difference between end-diastolic volume (the filling volume) and end-systolic volume (the leftover volume after contraction). Multiply heart rate by stroke volume to get cardiac output (CO = HR × SV). End-diastolic and end-systolic volumes are involved in determining how much is ejected per beat, not directly multiplied to form CO. Cardiac reserve refers to the capacity to increase CO above rest, not the basic definition. So the product of heart rate and stroke volume best explains cardiac output.