


5 of 10

4. Supply and demand

Picture 4.1 Taking your pulse.

Even when the body is resting the cells use oxygen and produce carbon dioxide. The heart rate is low at around 70 beats per minute (beats/minute).

As the level of activity increases the muscles start to work harder. The heart rate rises to supply extra oxygen and glucose to muscle cells to fuel this increased respiration and remove the extra carbon dioxide it produces. Sensory nerves monitor the levels of these gases in the blood and signal to the brain so that it can adjust the heart rate.

Picture 4.2 Click here to see the effects of exercise on heart rate.

Exercise

An average person can maintain their pulse at around 130 beats/minute to cope with activity for short periods of time. Being overweight, or doing very little exercise, means that the body is less able to cope with the demands of physical activity.

Elite marathon runners may have resting heart rates as low as 40-50 beats/minute because training has made their cardiovascular system extremely efficient. During a race they can run for long periods of time without their pulse rising much over 110 beats/minute.

Picture 4.3 Graph of heart rates for question 5.

Question 5

The graph shows how three student's heart rates changed before, during and after exercise.

a) What measurements would you take before the exercise and why?

b) How would you make sure that it was a fair comparison between the exercise each person performed?

c) At what time was the exercise started and when did it finish?

d) Student C's pulse increased slightly before the exercise started. Why was this?

e) In what other ways would the body respond to exercise?

f) Whose pulse dips below his/her resting level as they recover from exercise? Suggest why this could happen?

g) Who does not recover fully from the exercise during the experiment? Suggest why this is the case.