Investigating Heart Rates

Applied Science: BTEC Level 3 National Extended Diploma, topic 4.5.12

The effect of different substances on heart rates can be investigated using Daphnia crustaceans.

Daphnia are small, transparent crustaceans that can be used to investigate the effect of different chemicals on heart rates.
Because they are transparent, it is easy to count the heart rate of Daphnia.

To conduct this experiment, six Daphnia are placed in beakers of water and cooled.
Cooling helps lower the heart rate, making it easier to count.
The heart rate of all six Daphnia are measured prior to being placed into the beakers.
Three Daphnia are then placed in beakers with equal volumes of pond water. This is the control group.
Three Daphnia are placed in beakers with equal volumes of pond water + 0.5% caffeine. This is the experimental group.

All six Daphnia are then left for 30 minutes.
After 30 minutes the heart rate of each Daphnia is counted for 30 seconds.
All conditions apart from the addition of caffeine should remain constant throughout the experiment.
These conditions include temperature, glucose concentration, the volume of pond water, and size of the beaker.

The results are shown in the table. Some important points to note are:
The Daphnia in just pond water are the control group. This is to ensure any change in heart rate is due to the addition of caffeine.
Three Daphnia are used in each group as it allows the mean heart rate to be calculated.
This allows the detection of any anomalous results.
- An anomalous result does not fit the pattern of the other results. This may be due to an experimental error.

As you can see from the results, adding caffeine increases the heart rate of Daphnia.
This is because caffeine is positively chronotropic.
Positive chronotropes increase the heart rate.
Caffeine does this by inhibiting specific receptors in the heart. Stimulation of these receptors normally decreases heart rates.
So, when caffeine inhibits these receptors, heart rate increases.

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