IB Biology SLTopic 2 — Transport in Animals & PlantsPaper 1 & 2Practical skill~9 min read
Measuring Pulse Rate (Skills)
A pulse is not blood sloshing past your fingers. It is the artery wall itself bouncing outwards. Once that clicks, you understand why you can only feel it in certain places and why the technique matters.
📚 What you need to know
The ventricles contract and force a large volume of blood into the arteries, which expand to take it. That expansion is the pulse.
You can feel it where an artery is close to the skin and passes over a bone to press against.
The two used in school practicals are the radial artery (wrist) and the carotid artery (neck).
Use two fingers and press gently. Never use your thumb — it has a pulse of its own.
Count for 60 seconds, or 30 seconds and double it. Pulse rate is in beats per minute (bpm).
Digital devices (data loggers, smartwatches, fitness bands) read pulse by scanning blood flow, usually at the wrist.
What you are actually feeling
When the ventricles contract, they push a big slug of blood into arteries that are already full. The blood has to go somewhere, so the artery wall stretches outwards for a moment, then recoils. That stretch travels along the artery as a wave.
Put your finger on an artery near the surface and you feel that wave arrive. One wave equals one heartbeat, which is why pulse rate and heart rate give you the same number.
🤔 Why only at certain spots?
To feel the wall move, you need to squash the artery slightly against something firm. If the artery is buried under muscle and fat, your finger pressure spreads out and you feel nothing.
So the good spots have two things at once: the artery lies near the skin, and there is a bone directly behind it to press against. The wrist gives you both, which is why the radial artery is the standard choice.
The wrist is the safer choice for classwork. Heavy pressure on the neck can slow the heart down and make someone feel faint.
The method, step by step
🧩 Taking a pulse properly
Sit still for a couple of minutes first if you want a resting rate. Walking to your seat and measuring straight away gives you a raised value.
Place two fingers (index and middle) on the radial or carotid artery.
Compress gently until you feel the beat. Too hard and you squash the artery shut; too soft and you feel nothing.
Count every beat for 60 seconds, using a timer you can see without moving your hand.
Repeat and take a mean. One reading is a data point, not a result.
Never use your thumb. The thumb has a large artery running into it, so you end up counting your own pulse instead of theirs — or worse, a confusing mixture of both.
Real resting traces have around 60 to 80 beats in this window. Six are drawn here so you can see the shape of each beat.
Short counts and why they cost you accuracy
Counting for 30 seconds and doubling is allowed and saves time. Counting for 15 seconds and multiplying by four is quicker still. But there is a hidden cost, and examiners love asking about it.
Counting time
What you do
Effect of miscounting by one beat
60 seconds
Use the number as it is
Answer is out by 1 bpm
30 seconds
Multiply by 2
Answer is out by 2 bpm
15 seconds
Multiply by 4
Answer is out by 4 bpm
10 seconds
Multiply by 6
Answer is out by 6 bpm
Any error you make gets multiplied by the same number your count does. That is the whole argument in one sentence, and it is worth a mark whenever a question asks you to evaluate the method.
Making the results trustworthy
Repeat and take a mean. Three readings smooth out the odd miscount.
Control what you can. Same person, same arm, same time of day, seated and rested before the measurement.
Remember what raises a pulse. Exercise, caffeine, stress, illness, temperature, even standing up. If you are testing exercise, you need everything else kept the same.
Use a data logger if reliability matters. It removes human miscounting, though it can still be thrown off by a loose strap or movement.
Worked examples
WORKED EXAMPLE
A student counts 21 beats in 15 seconds. Calculate the pulse rate in beats per minute. [1 mark]
How many 15-second blocks fit in a minute?60 ÷ 15 = 4Scale the count up21 × 4 = 84Pulse rate = 84 bpmAlways write the units. “84” on its own can be marked wrong.
WORKED EXAMPLE
Resting readings for one student were 70, 74 and 72 bpm. After five minutes of exercise they were 106, 110 and 108 bpm. Calculate the mean at rest, the mean after exercise, and the percentage increase. [3 marks]
Step 1: mean at rest(70 + 74 + 72) ÷ 3 = 216 ÷ 3 = 72 bpmStep 2: mean after exercise(106 + 110 + 108) ÷ 3 = 324 ÷ 3 = 108 bpmStep 3: percentage increaseincrease = 108 − 72 = 36 bpm(36 ÷ 72) × 100 = 50%72 bpm at rest, 108 bpm after exercise, a 50% increasePercentage change always divides by the ORIGINAL value, not the new one.
WORKED EXAMPLE
A student measures their pulse using their thumb and gets a rate that keeps changing. Explain the problem and suggest an improvement. [2 marks]
Name the problem
The thumb has its own artery and therefore its own pulse, so the student may be counting a mixture of two pulses.
Give the fix
Use the index and middle fingers on the radial artery instead, pressing gently, and repeat the count three times to find a mean.
Thumb pulse interferes — use two fingers and repeat for a mean
💡 Exam tip
Write bpm or beats per minute after every pulse figure.
If asked to improve reliability, say repeat and calculate a mean — and say what you would keep the same.
If asked to improve accuracy, say count for the full 60 seconds or use a data logger.
Reliability is about repeats agreeing. Accuracy is about being close to the true value. Use whichever word the question uses.
Percentage change divides by the starting value. Getting this backwards is the single most common calculation slip in this topic.
If a question asks why the pulse rises during exercise, link it to more oxygen and glucose delivered to muscles, and faster removal of carbon dioxide.
⚠ Common mix-up
Using the thumb. Guaranteed unreliable reading.
Pressing too hard. You close the artery and feel nothing, then assume you are in the wrong place.
Thinking you feel blood flowing. You feel the artery wall stretching and recoiling.
Saying a pulse can be felt in a vein. It cannot — pressure there is low and steady, with no pulse wave.
Measuring straight after walking into the room and calling it a resting pulse.
Confusing “repeat the measurement” with “test more people”. Repeats improve reliability; more people improve how representative the data is.
Up next: Coronary Heart Disease (Skills) — what happens when an artery slowly blocks up, and how to judge whether a study really proves that something causes it.
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