2.7.3Inquiry Process
Measuring Pulse Rate- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 75 of 160
Section 1 of 5What the Pulse Is
Pressure wave
- Each contraction of the left ventricle sends blood into the aorta and produces a wave of pressure that travels along the elastic arteries.
Where to feel it
- The pulse can be felt where an artery lies close to the skin,
- such as the radial artery at the wrist or the carotid artery in the neck.
Link to heart rate
- In a healthy person the number of pulses per minute equals the number of ventricular contractions,
- so it is used to estimate heart rate.
Control
- The sinoatrial node sets the rhythm, and nerves and the hormone adrenaline speed it up or slow it down.
Section 2 of 5Investigation Method
Controlled variables include duration of exercise, intensity and the time of day.
1
The volunteer sits quietly for several minutes so a true resting rate can be recorded.
2
Press two fingers, not the thumb, lightly on the radial artery and count pulses over a fixed time such as 15 seconds.
3
Multiply the count to give beats per minute; counting for a full minute is more accurate.
4
The volunteer performs a set exercise, for example stepping on and off a step for two minutes, and the pulse is measured straight away.
5
The pulse is then taken at regular intervals, such as every minute, until it returns to the resting rate, and recovery time is noted.
Section 3 of 5Interpreting Results
Interpreting Results
Exercise
- Heart rate rises because muscles need more oxygen and glucose and produce more carbon dioxide, which the blood must carry.
Recovery time
- A fitter person typically has a lower resting rate and returns to it more quickly after exercise.
Reliability
- Repeating the trial and calculating a mean reduces random error, and using several volunteers shows whether the pattern is general.
Safety
- Volunteers should be healthy, free to stop at any time and not push beyond comfortable effort.
| Measure | Count in 15 s | Beats per min |
|---|---|---|
| Resting | 20 | 80 |
| After exercise | 34 | 136 |
Section 4 of 5Worked Example
Question
- A student counts 20 pulses in 15 s at rest and 34 pulses in 15 s straight after exercise.
- Calculate the percentage increase in heart rate.
- Resting rate = 20 × 4 = 80 beats per minute.
- Exercise rate = 34 × 4 = 136 beats per minute.
- Increase = 136 − 80 = 56 beats per minute.
- Percentage increase = 56 ÷ 80 × 100 = 70%.
Answer Heart rate increased by 70%, from 80 to 136 beats per minute.
Section 5 of 5Exam Checklist & Tips
2.7.3 – Key Points for the Exam
- Pulse The rhythmic expansion of an artery caused by each heartbeat.
- Heart rate The number of heartbeats per minute.
- Recovery time The time taken for heart rate to return to its resting value after exercise.
- Sinoatrial node The pacemaker region of the heart wall that starts each heartbeat.
Common Mistakes
- Using the thumb to take the pulse, because it has its own pulse and can give a false count.
- Multiplying a 15 second count by 15 instead of 4.
- Changing exercise duration between trials, which makes comparison unfair.
- Claiming a cause from a single volunteer without repeats.
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