Cardiac Output Equals Heart Rate Times Stroke Volume
Medi stands beside a glowing holographic heart model in a sports medicine lab, adjusting sliders labeled Heart Rate and Stroke Volume while a digital display shows cardiac output in liters per minute updating in real time.
- Calculate cardiac output using the formula CO = HR × SV.
- Explain how the body independently adjusts heart rate and stroke volume to meet changing oxygen demands.
- Identify the roles of the sympathetic and parasympathetic nervous systems in regulating heart rate.
- Predict how stroke volume changes in response to venous return, contractility, and afterload.
- Compare cardiac output at rest versus during vigorous exercise using realistic numerical values.
Key terms
- Cardiac output
- The volume of blood the heart pumps per minute, equal to heart rate multiplied by stroke volume.
- Stroke volume
- The volume of blood ejected by a ventricle in a single beat, set by preload, contractility, and afterload.
- Preload
- The degree of ventricular stretch at end of filling, determined by venous return before contraction begins.
- Frank-Starling mechanism
- The intrinsic property whereby greater ventricular filling produces a more forceful contraction and larger stroke volume.
- Afterload
- The arterial pressure the ventricle must overcome to eject blood; higher afterload reduces stroke volume.
Two Independent Levers on Output
Cardiac output rises during exercise through two separately controlled levers. Heart rate is set by the balance of autonomic input at the SA node: withdrawing parasympathetic (vagal) tone lifts the brake first, then sympathetic norepinephrine and circulating epinephrine push rate higher. Stroke volume is set by preload, contractility, and afterload. Because these levers are independent, the body can fine-tune output — raising rate without changing stroke volume, or boosting stroke volume via contractility while rate holds. Understanding which lever dominates a given scenario is the key to interpreting CO changes quantitatively.
The Frank-Starling Counterbalance
A faster heart rate shortens diastole, giving the ventricle less time to fill — which by itself would lower stroke volume. During exercise this is offset by sharply increased venous return from the skeletal-muscle pump and venoconstriction, raising preload. Greater preload stretches cardiac muscle to a more optimal sarcomere length, so contraction is stronger (Frank-Starling). Sympathetic stimulation simultaneously raises contractility independent of filling. Together these effects let stroke volume rise even as filling time falls, which is why elite athletes can push cardiac output past 20 L/min.
Worked examples
Calculate cardiac output for an athlete at peak effort.
- Identify the values: HR = 190 bpm and SV = 130 mL.
- Apply CO = HR × SV = 190 × 130 = 24,700 mL/min.
- Convert to liters by dividing by 1,000: 24,700 ÷ 1,000 = 24.7 L/min.
Answer: CO = 24.7 L/min, roughly a fivefold increase over a resting 4.9 L/min.
Determine stroke volume given cardiac output and heart rate.
- Start from CO = HR × SV and rearrange to SV = CO ÷ HR.
- Convert CO to mL/min: 7.2 L/min × 1,000 = 7,200 mL/min.
- Divide by heart rate: 7,200 ÷ 80 = 90 mL.
Answer: Stroke volume = 90 mL per beat.
Activity
Set the heart rate and stroke volume sliders to match each scenario, calculate cardiac output, then identify which lever — HR or SV — is the dominant contributor to the change.
Practice
A patient has a heart rate of 90 bpm and a stroke volume of 55 mL; calculate the cardiac output in liters per minute.
Predict how a sudden rise in aortic pressure (increased afterload) affects stroke volume and cardiac output, and explain why.
Common mistakes to avoid
- A faster heart rate always increases cardiac output.Very high rates shorten filling time so much that stroke volume falls, which can lower cardiac output despite the faster rate.
- The parasympathetic system speeds the heart during exercise.Parasympathetic (vagal) tone slows the heart; exercise raises rate by withdrawing it and adding sympathetic stimulation.
Check your understanding
A student's resting heart rate is 65 bpm and stroke volume is 75 mL. What is their cardiac output in liters per minute?
During a sprint, cardiac output rises sharply. Which statement BEST explains the contribution of stroke volume to this increase?
A patient's cardiac output is measured at 6 L/min with a heart rate of 100 bpm. What is their stroke volume?
Which factor, when increased, will DECREASE stroke volume?
Recap
Cardiac output equals heart rate times stroke volume, and the body adjusts each lever independently: autonomic balance sets heart rate while preload, contractility, and afterload set stroke volume. The Frank-Starling mechanism and sympathetic drive let stroke volume rise even when faster rates shorten filling.
Reflect
When CO must rise quickly, why might the body prefer adjusting heart rate before stroke volume?