# 6.1.12 Investigating Variables that Affect Heart Rate

### Investigating Variables that Affect 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 humans
• Serious 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
• Weight
• Height
• Temperature
• Diet
• Dehydration
• When designing experiments investigating a single factor, it is essential to try and control the other variables

#### Investigating the effect of caffeine on the heart rate of humans

Apparatus

• Heart rate monitor
• Caffeine solution of a known concentration
• Measuring cylinders
• Cups
• One group of healthy humans

Method

• Use the heart rate monitor to record the heart rates of each individual while at rest
• These results can be used to calculate the mean heart rate of the group as well as the range of resting heart rates for the group prior to caffeine consumption
• Use the measuring cylinder to pour 200ml of the caffeine solution into a cup (one for each individual)
• Ask each individual to consume the drink
• Wait 15 minutes
• Use the heart rate monitor to record the heart rates of each individual while at rest
• These results can be used to calculate the mean heart rate of the group as well as the range of resting heart rates for the group after caffeine consumption
• It is important that the individuals taking part are not moving and all sat down so that these variables are controlled
• 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
• Present the results in a graph or table form

Limitations

• Care needs to be taken when selecting the caffeine concentration
• Too much caffeine can be dangerous, while too little may show no effect
• It is very difficult to ensure that individuals within a sample group are sufficiently similar
• By using the same group for the before and after measurements this is not as much of an issue
• If multiple groups were used, with some consuming caffeine and some not, then this would be a major issue and care would need to be taken to find individuals with a similar life history (same sex, age, weight and height etc.)

#### Exam Tip

Exams may ask about the safety and ethical considerations when studying the heart rates of humans. An experiment should be designed so that the safety of any of the subjects is never at risk, for example they should not be pushed to exercise at such an intensity that could cause a heart attack.

### Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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