Investigating Heart Rate (AQA AS Biology): Revision Note

Investigating heart rate

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

  • the effect of exercise on human pulse rate

  • the effect of caffeine on the heart rate of water flea, known as Daphnia

Investigating the effect of exercise intensity on human heart rate

Apparatus

• Heart rate monitor

• Space in which to carry out a chosen form of exercise, e.g. step-ups on stairs

• One group of healthy humans

Method

1. Use the heart rate monitor to record the heart rate of an individual while at rest

2. Carry out a chosen form of exercise at low intensity for a set time period

   • E.g. gentle step-ups on stairs for 30 seconds

3. Use the heart rate monitor to record the heart rate of the individual immediately after exercise

4. Allow 5 minutes of recovery time

5. Repeat the exercise at a different level of intensity

   • E.g. for a longer time period / faster step ups / a more intensive type of exercise

6. Repeat 1-5 at another higher level of intensity

7. Repeat 1-6 with several individuals and calculate a group average for different levels of exercise

Limitations

• There needs to be a way of quantifying exercise intensity, otherwise individuals may not all be exercising at the same level

• Heart rate will begin to slow down immediately after exercise, so it should always be taken at the same point for each repeat

• Participants may become increasingly tired over the course of the experiment, so heart rate may not just reflect one level of intensity

Investigating the effect of caffeine on Daphnia heart rate

• Daphnia, also known as water fleas, are small aquatic invertebrates

• They are suitable for investigating heart date due to having transparent bodies; their internal organs, such as the heart, can be observed using a light microscope

• It is possible to investigate the impact of caffeine on the heart rate of Daphnia by placing them in a caffeine solution on a microscope slide and counting their heartbeats

Apparatus

• Light microscope

• Cavity slide

• Daphnia

• Pipette

• Caffeine solutions at a range of concentrations

• Distilled water

• Stop watch

Method

1. Prepare five different concentrations of caffeine solution and a control solution of distilled water

   • The serial dilution technique could be used here

2. Add some pond water into the well of a cavity slide and add three drops of distilled water 

3. Select an individual Daphnia and use a pipette to carefully transfer it to the cavity slide

   • You can also use a Petri dish if you do not have access to a cavity slide

4. Place the cavity slide onto the stage of a microscope and observe the animal under low power

   • The beating heart is located on the dorsal side just above the gut and in front of the brood pouch

5. Use a stopwatch to time 20 seconds, and count the number of heart beats

   • The heart beat of Daphnia is very rapid, so you can count the beats by making dots on a piece of paper

6. Count the dots and express heart rate as number of beats per minute

   • Multiply by three to convert beats per 20 seconds into beats per 60 seconds

7. Return the Daphnia to the stock culture

8. Repeat steps 3-7 with at least 2 other Daphnia individuals

9. Repeat steps 3-8 with different caffeine concentration solutions

Variations

• You can also investigate the effect of:

  • temperature 

  • other chemicals, such as alcohol (1% ethanol solution)

Ethical considerations

• Although they are simple organisms that may not 'suffer' in the same way as animals with more developed nervous systems, Daphnia still deserve respect

  • They cannot give consent to be studied

  • They cannot express pain

• Care can be taken to minimise potential harm by:

  • handling animals gently

  • keeping examination periods as short as possible

  • returning animals promptly to the holding tank after being examined

  • avoiding extreme experimental conditions, e.g. extremes of temperature or strong caffeine solutions

Calculating cardiac output

• Cardiac output (CO) is:

the total volume of blood pumped by the heart per unit of time

• The CO of an individual can be calculated using values for heart rate and stroke volume

  • Heart rate = number of times the heart beats per minute, or the number of cardiac cycles per minute

  • Stroke volume = the volume of blood pumped out of the left ventricle during one cardiac cycle

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 = heart rate ÷ cardiac output