1.1.6 Investigating Heart Rate

• There are a wide range of factors that influence the heart rate of an organism
• Experiments can be designed to investigate the effect of a named variable on an organism's heart rate
• These experiments are commonly done using invertebrates, e.g. Daphnia, as well as vertebrates, e.g. humans
  • Care needs to be taken during these experiments to ensure that no test subjects are harmed
• Some of the factors that can influence heart rate include
  • Drugs
  • Caffeine
  • Alcohol
  • Sex i.e. male or female
  • Weight
  • Height
  • Temperature
  • Diet
  • Dehydration
• When designing experiments investigating a single factor, it is essential to control other variables
• Heart rate investigations can be used to study the relationship between heart structure and function

• 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 heart beats

Daphnia internal organs can be viewed through a light microscope

Apparatus

• Light microscope
• Cavity slide
• Culture of 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 a large 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 5 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)

Results

• To analyse your results it is best to draw a graph
  • Take an average of the heart rate repeats for each caffeine concentration
  • Plot average heart rate (y axis) against caffeine concentration (x axis)
  • The graph should show a positive correlation; as caffeine concentration increases, heart rate increases

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
• Some people believe it is more ethical to experiment with invertebrates than vertebrates as they have less sophisticated nervous systems and may not feel pain in the same way 
• Care can be taken to minimise potential harm by
  • Animals should be handled gently
  • Examination periods should be kept as short as possible
  • Animals should be returned promptly to the holding tank after being examined
    • This is in line with ethical approaches that are appropriate to field work where pond animals are returned to their habitat after observations have been made
  • Extreme ranges of the variables being tested should be avoided, e.g. extremes of temperature or strong caffeine solutions