IB Biology HLTransport in Animals & PlantsPaper 1 & 2~9 min read
Measuring Pulse Rate
A pulse is not blood arriving. It is the artery wall itself bulging outwards as a wave of pressure passes through it. Find a spot where an artery runs close to the skin over a bone, and you can count what the heart is doing from the outside.
📚 What you need to know
Ventricular contraction forces a large volume of blood into the arteries, which expand to take it. That expansion is the pulse.
A pulse is easiest to feel where an artery lies close to the skin or passes over a bone.
The radial artery (wrist, below the base of the thumb) and the carotid artery (neck, just below the jaw) are the usual places.
Place two fingers on the artery and gently compress it. Never use your thumb – it has a pulse of its own and gives an inaccurate count.
Count for 60 seconds, or count for 30 seconds and multiply by 2.
Digital devices – data loggers, smartwatches, fitness bands – scan blood flow through the radial artery to give a reading.
Exercise raises the pulse rate; how fast it returns to resting afterwards is a useful measure of fitness.
Where to find it
Both are arteries. You cannot take a pulse from a vein, because the pressure wave has been lost by then.
🧩 Taking a pulse
Sit still for a few minutes first if you want a true resting rate.
Place the tips of two fingers on the radial or carotid artery.
Compress gently until you feel a regular beat.
Count the beats for 60 seconds, watching a clock or timer.
If you count for a shorter time, scale up: multiply a 30 second count by 2, or a 15 second count by 4.
Repeat and take a mean, because one count can easily be miscounted.
The rule about the thumb catches people out every year. Your thumb contains its own artery close to the surface, so you end up feeling your own pulse and counting a mixture of two rhythms. Use the index and middle fingers.
What exercise does to it
During exercise, muscles respire faster. They need more oxygen and glucose delivered and more carbon dioxide taken away, so the heart beats faster and pumps a larger volume with each beat. Pulse rate rises within seconds of starting.
When you stop, the rate does not drop straight back. It falls quickly at first, then more slowly, because the body is still repaying the oxygen it borrowed and clearing the carbon dioxide and lactate that built up.
Recovery time – how long the trace takes to reach the dashed line again – is a better fitness measure than the peak rate.
Before you use a person as a subject. Ask whether they have any heart or breathing condition, keep the exercise moderate, let them stop at any point, and use their data anonymously. Consent and safety are part of the mark scheme in practical questions.
Worked examples
WE 1
Scaling up a short count
A student counts 22 pulses in 15 seconds. Calculate the pulse rate in beats per minute, and suggest one reason why counting for 60 seconds would be better. (3 marks)
Step 1: how many 15 second blocks in a minute
60 ÷ 15 = 4
Step 2: multiply22 × 4 = 88Step 3: the reason
Any miscount in a short period is multiplied by four, so a single missed beat changes the answer by 4 beats per minute; counting for a full minute reduces this error.
88 beats per minutestate the unit – beats per minute, not just 88
WE 2
Reading a recovery graph
Using the graph above, describe what happens to the pulse rate from the start of exercise until 14 minutes. (3 marks)
Point 1: during exercise
From 2 minutes the rate rises steeply from 72 to about 150 beats per minute, then levels off while exercise continues.
Point 2: immediately after
When exercise stops at 7 minutes the rate falls rapidly, dropping by around 50 beats per minute in the first two minutes.
Point 3: later
The fall then slows, and by 14 minutes the rate is close to but still slightly above the resting value of 72.
Steep rise, plateau, rapid fall, then a slow return“describe” wants the shape plus figures from the graph; do not explain unless asked
WE 3
Why the rate rises
Explain why pulse rate increases during exercise. (3 marks)
Point 1: the demand
Muscles respire faster, so they need more oxygen and glucose delivered.
Point 2: the waste
They also produce more carbon dioxide, which must be carried away to the lungs.
Point 3: the response
A faster heart rate increases the volume of blood delivered per minute, so supply and removal keep up with demand.
More respiration means more delivery and more removal neededmention both directions – delivering oxygen and removing carbon dioxide
💡 Exam tips
Say pulse rate is measured in beats per minute and always convert to that unit.
Name the artery you would use: radial or carotid.
Mention two fingers, gentle pressure, not the thumb if asked to describe the method.
Improve reliability with repeats and a mean; improve accuracy with a longer counting time or a digital sensor.
For fitness comparisons, use recovery time as well as resting rate.
Name your variables: independent (exercise intensity or time), dependent (pulse rate), controlled (same person, same exercise, same conditions).
⚠ Common mistakes
Using the thumb. It has its own pulse and ruins the count.
Pressing too hard. Heavy pressure closes the artery and the pulse vanishes.
Forgetting to scale to a minute. A 30 second count must be doubled.
Saying the pulse is blood arriving in the wrist. It is the artery wall expanding as a pressure wave passes.
Trying to take a pulse from a vein. Venous blood is at low, steady pressure with no pulse.
Ignoring consent and safety in practical questions involving exercise.
Up next: Coronary Heart Disease – what happens when the arteries supplying the heart itself start to narrow.
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