The Relationship Between Heart Rate, Cardiac Output, and Stroke Volume
Key Definitions
Before diving into the relationships, let's quickly define our key terms:
- Heart Rate (HR): Number of heartbeats per minute
- Stroke Volume (SV): Amount of blood pumped out by each ventricle per beat
- Cardiac Output (CO): Total volume of blood pumped by the heart per minute
The fundamental equation connecting these variables is: $CO = HR × SV$
At Rest
During resting conditions:
- Heart Rate: typically 60-80 beats per minute
- Stroke Volume: approximately 70-80 mL per beat
- Cardiac Output: about 5 L per minute
For a person with a resting heart rate of 70 bpm and stroke volume of 70 mL: $CO = 70 \text{ bpm} × 70 \text{ mL} = 4900 \text{ mL/min} = 4.9 \text{ L/min}$
During Exercise
Initial Response
When exercise begins:
- Heart rate increases rapidly
- Stroke volume increases due to:
- Enhanced venous return
- Stronger myocardial contractions
- More complete emptying of ventricles
Progressive Changes
As exercise intensity increases:
- Heart rate continues to rise linearly
- Stroke volume increases until about 40-60% of maximum effort
- Cardiac output increases to meet oxygen demands
Stroke volume typically plateaus during moderate exercise, while heart rate continues to increase to achieve higher cardiac output.
The Relationship Dynamic
Heart Rate and Stroke Volume
- Inverse relationship at very high intensities
- As heart rate gets extremely high:
- Less time for ventricular filling
- Potential decrease in stroke volume
Cardiac Output Response
- Increases linearly with exercise intensity
- Can reach 20-40 L/min during maximal exercise
- Achieved through combined increases in HR and SV
Many students assume stroke volume continuously increases with exercise intensity. In reality, it plateaus while heart rate continues to rise.
Training Effects
Long-term exercise adaptation leads to:
- Lower resting heart rate
- Increased stroke volume
- More efficient cardiac output
Think of the heart as a muscle - training makes it stronger and more efficient, allowing it to pump more blood with each beat while requiring fewer beats per minute.
