3.3 - Cardiac Output
What is cardiovascular fitness?
Cardiovascular fitness describes a person's ability to keep exercising for long periods without becoming overly tired. This is sometimes called stamina, and it plays a key role in activities that require sustained effort, such as running or cycling. This type of fitness relies on how well the body can move oxygen from the lungs through the blood to the muscles that are working, and how effectively those muscles can use that oxygen.
Key factors in cardiovascular fitness
- Oxygen transport - The lungs take in oxygen, which the blood carries to muscles; better transport means you can exercise longer.
- Muscle efficiency - Muscles need to use the delivered oxygen well to produce energy without quick fatigue.
Understanding heart rate
Heart rate refers to the number of times the heart beats per minute, measured in beats per minute (bpm). This happens when the ventricles (the lower chambers of the heart) contract to push blood out. Heart rate gives a snapshot of how hard the cardiovascular system is working, especially during exercise.
Measuring heart rate
You can check heart rate by feeling for a pulse at certain points on the body.
Common pulse points:
- Carotid artery - Located in the neck, just below the jaw.
- Radial artery - Found on the wrist, on the thumb side.
During exercise, heart rate rises to meet the body's higher demand for oxygen, and monitoring it helps track fitness levels.
Resting heart rate and variations
Resting heart rate is the beats per minute when a person is at rest. For teenagers, it typically ranges from 50 to 90 bpm, while for adults it is usually 60 to 80 bpm.
Factors affecting resting heart rate:
- Age - Younger people often have higher rates, which decrease with age.
- Gender - Females may have slightly higher rates than males.
- Size - Larger body size can influence the rate due to greater blood volume needs.
Elite athletes might have a resting heart rate as low as 40 bpm, compared to 70 bpm for someone with average fitness of the same age. This lower rate shows a more efficient heart.
Maximum heart rate and training zones
Maximum heart rate is the highest rate your heart can reach safely during intense exercise. A simple way to estimate it is:
Formula for maximum heart rate:
For fitness improvements, aim to exercise at 55–85% of this maximum. This zone helps build cardiovascular endurance without overstraining.
Heart rate recovery as a fitness indicator
After exercise, a fitter person's heart rate returns to resting levels more quickly. This recovery time measures how well the cardiovascular system adapts, and faster recovery is a benefit of regular training.
Stroke volume in cardiovascular function
Stroke volume is the amount of blood pumped out by each ventricle of the heart in a single beat, measured in millilitres (ml). For someone with average fitness, this is around 70 ml per beat, but athletes can reach about 90 ml due to training adaptations.
How exercise affects stroke volume
Regular physical activity boosts stroke volume through changes in the heart. The heart muscle undergoes hypertrophy, which means an increase in size. This allows the heart chambers to hold and pump more blood per beat.
Effects of hypertrophy:
- Stronger contractions - The heart pushes out blood more forcefully.
- Better blood retention - Chambers expand to hold more blood before each pump.
As a result, the heart works more efficiently, delivering more oxygen-rich blood to muscles with fewer beats.
Cardiac output and its importance
Cardiac output is the total volume of blood the heart pumps out per minute, typically measured in litres per minute. It shows how much blood, and thus oxygen, is circulating to support activity. Higher cardiac output means better oxygen supply to working muscles, which is vital for endurance.
Formula for cardiac output
Where:
- Heart rate (HR) = Beats per minute (bpm)
- Stroke volume (SV) = Blood per beat (ml)
- Cardiac output (CO) = Litres per minute
This calculation combines how fast the heart beats with how much blood it pumps each time.
Worked example - Calculating cardiac output
A moderately fit person has a heart rate of 80 bpm and a stroke volume of 75 ml during light exercise. Calculate their cardiac output.
Step 1: Identify the values
- Heart rate (HR) = 80 bpm
- Stroke volume (SV) = 75 ml
Step 2: Apply the formula
Step 3: Perform the calculation
First, multiply: 80 × 75 = 6,000 ml per minute
Convert to litres: 6,000 ml = 6 litres per minute
Step 4: Interpretation
This cardiac output of 6 litres per minute means the heart is pumping enough blood to support moderate activity, delivering oxygen to muscles effectively.
Benefits of regular exercise on cardiovascular components
Consistent training brings long-term improvements to the cardiovascular system, making it more efficient overall. These changes help in sports and daily life, reducing fatigue and enhancing performance.
Key improvements from exercise
- Lower resting heart rate - The heart becomes stronger, so it doesn't need to beat as often at rest.
- Increased stroke volume - Through hypertrophy, more blood is pumped per beat, improving efficiency.
- Higher cardiac output - Combining better heart rate control and stroke volume allows for greater blood flow, up to 35 litres per minute in elite endurance athletes.
- Faster recovery - Heart rate returns to normal quicker after exercise, indicating better fitness.
- Enhanced oxygen delivery - More blood means more oxygen to muscles, delaying tiredness and boosting endurance.
Athletes often track heart rate during training to set intensity levels, ensuring they build these benefits without overtraining.