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Cardiac output is the volume of blood pumped by one ventricle per minute: CO = HR × SV. At rest, a typical heart rate is about 75 beats per minute (range 60–100) and a typical stroke volume about 70 mL, giving CO ≈ 5.25 L/min.

In exercise, heart rate in healthy young people may rise to about 150 bpm and stroke volume to about 130 mL, so CO rises to about 19.5 L/min, 4–5 times the resting value. Maximum heart rate can be estimated as 220 − age. Trained athletes typically have very low resting heart rates.

Muscles run on ATP. Creatine phosphate powers the first few seconds of contraction (about 15 seconds' worth). Glycolysis breaks down glucose without oxygen and yields a little ATP; aerobic respiration breaks down glucose in the presence of oxygen to carbon dioxide, water and about 36 ATP per glucose, compared with 4 from glycolysis.

Experiment: Heart-rate recovery after step exercise

1. Sit quietly for 3 minutes, then measure your resting heart rate: count your pulse for 30 s and double it. Repeat once and take the mean.
2. Step up and down on the step at a steady pace you can keep (up-up-down-down) for 2 minutes.
3. Immediately sit down and measure your pulse (30 s × 2). Repeat every minute for 5 minutes.
4. Plot heart rate (bpm) against time after exercise and mark your resting value with a horizontal line.
5. Calculate the percentage increase from rest to the first reading, and the recovery time to within 10 bpm of resting.
6. Estimate cardiac output at rest and at your first reading, assuming SV = 70 mL at rest. Evaluate: what is the uncertainty if you can miscount by 1 beat in 30 s?

Time	Pulse in 30 s	Heart rate (bpm)	Notes
Rest
0 min after
1 min after
2 min after
3 min after
4 min after
5 min after

Questions

1. A pupil's resting HR is 72 bpm and SV 70 mL. Just after exercise HR is 144 bpm and SV 110 mL. Calculate CO at rest and after exercise, and the factor by which it increased.

2. Describe your recovery graph and state your recovery time.

3. Explain why breathing stays faster for some minutes after exercise has stopped.

4. You count pulses for 15 s and multiply by 4. If you can miscount by 1 beat, what is the uncertainty in bpm? Suggest one way to reduce it.

Sources for every fact on this sheet: echoesof.science/school/exercise/sources/

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