6-1-13-heart-rate

Control of heart rate

• While the initiation of the heartbeat is myogenic, there are times when heart rate needs to change from its baseline rate

  • E.g. during exercise

The autonomic nervous system

• Heart rate is controlled by the cardioregulatory centre in the medulla of the brain, which is connected to the sinoatrial node (SAN) by nerves of the autonomic nervous system

• The autonomic nervous system controls unconscious activities, such as heart rate, breathing rate and digestion; it has two branches:

  • the sympathetic branch is involved with ‘fight or flight’, e.g.:

    • increasing heart rate

    • increasing breathing rate

    • diverting blood flow to muscles

  • the parasympathetic branch is involved with ‘rest and digest’, e.g.:

    • slowing heart rate

    • slowing breathing rate

    • digesting food

Chemoreceptors and pressure receptors

• Whether the medulla sends nerve impulses via a sympathetic or parasympathetic nerve depends on information from receptors in the blood vessels, e.g.:

  • chemoreceptors detect the carbon dioxide concentration of the blood

    • CO₂ concentration affects blood pH, which is also detected by chemoreceptors

  • pressure receptors detect blood pressure

Investigating factors affecting heart rate

• It is possible to investigate the effect of a named variable on an organism’s heart rate, e.g. the effect of:

  • caffeine

  • alcohol

  • temperature

  • diet

  • dehydration

Investigating the effect of caffeine on heart rate in humans

Apparatus

• Heart rate monitor, or stopwatch

• Drinking water and a type of caffeinated drink, e.g. cola, energy drink or coffee

• Measuring cylinders

• Cups

• Volunteers, with no heart conditions or caffeine sensitivity

Method

1. Divide volunteers into two groups

2. Measure resting heart rate for both groups by sitting quietly for 5 minutes before recording heart rate, e.g. by:

   • using a heart rate monitor

   • counting beats at the wrist for 30 seconds, then multiply by 2 to get beats per minute

3. Use the measuring cylinder to pour 200ml of a caffeine drink or water into a cup

4. Ask each individual to consume the drink

   • Group 1: caffeine drink

   • Group 2 (control group): the same volume of water

5. Wait 15 minutes

   • All volunteers should be resting during this time, and should not consume any other food or drink

6. Record the heart rates of each individual again

7. Repeat the heart rate measurements every 15 minutes for 2 hours

   • This will show the duration for which caffeine has an effect on the heart rate

8. Process the data, e.g.:

   • calculate the mean heart rate of each group

   • present the results in a graph form

   • calculate cardiac output

Cardiac output

• Cardiac output is found by multiplying the heart rate by the stroke volume:

Cardiac output = heart rate x stroke volume

• The equation can be rearranged to find the heart rate and stroke volume if required

  • Heart rate = cardiac output ÷ stroke volume

  • Stroke volume = cardiac output ÷ heart rate

Limitations

• It is very difficult to ensure that variables other than caffeine consumption are controlled during this type of experiment, e.g. there may be differences between individuals such as:

  • different food consumed prior to experiment

  • different fitness levels

  • different responses to caffeine

  • taking part in the experiment may have unpredictable effects on heart rate

• Using a large enough group of volunteers should help to reduce the problem described above, but this can be difficult in a school setting