Biology » Animal Systems » Circulatory Systems In Animals

The Cardiac Cycle Continued

Continued from the previous lesson:

Optional Investigation: Investigating Heart Rates Before, During and After Strenuous Exercise


To investigate your heart rate before, during and after strenuous exercise


  • stopwatch
  • pen and paper for recording


  1. Work in pairs on the field and ensure you have a stop watch.
  2. One partner performs the experiment and the other records the results. Partners then swap roles.
  3. Take the resting pulse rate before exercising.
  4. One partner runs quickly around the field twice.
  5. Immediately after the run take his/her pulse.
  6. Continue to take his pulse every minute for 5 minutes.
  7. Record the results and plot a graph using the data pertaining to you.


  • Graph should have Time (minutes) on horizontal axis and Pulse rate (beats per min) on the vertical axis.
  • Both axes must go up in equal intervals along the entire length.
  • Resting pulse is shown as a dotted line parallel to the horizontal axis.
  • Graph should start ON resting pulse and go up, then gradually back down to resting pulse rate.


Record your results here:

TimeHeart rate (beats/minute)
Before exercise (resting) 
\(\text{0}\) \(\text{min}\)(immediately after exercise) 
\(\text{1}\) \(\text{min}\) (after exercise) 
\(\text{2}\) \(\text{min}\) 
\(\text{3}\) \(\text{min}\) 
\(\text{4}\) \(\text{min}\) 
\(\text{5}\) \(\text{min}\) 

Draw a line graph to illustrate your results. Show the resting pulse rate as a separate dotted line on the axis.


Write your conclusion.

Sample Conclusion

Pulse rate increases when exercise is done, then gradually returns to resting pulse after the exercise.


  1. Write a hypothesis for this investigation.
  2. Write down the independent variable.
  3. Write down the dependent variable.
  4. Name ONE factor that must be kept constant during this investigation.
  5. Write down TWO ways in which the accuracy of this investigation can be improved.
  6. What conclusions can be made about your cardiovascular fitness?
  7. Explain why the heart rate increases during exercise.

Sample Answers

  1. Pulse rate during exercise will be higher than resting pulse rate.
  2. There are TWO independent variables. The main one is Resting, Doing Exercise and Recovering (or Type of Activity), but time can also be seen as a secondary independent variable.
  3. The dependent variable is Pulse Rate.
  4. There are several variables that need to be controlled. Some are:
    1. Pulse must be taken before and immediately after exercise.
    2. Pulse must be taken exactly at one minute intervals during recovery.
    3. Always take pulse as 30 seconds x 2 or over a full minute.
  5. Several things may be done. Some are:
    1. Keep measuring pulse rate until it returns to resting rate – this may take longer than 5 min in some learners.
    2. Use a heart rate monitor for greater accuracy with pulse rates.
    3. Control more variables in order to get similar groups of people – all the same age, same gender, same fitness level, same mass approximately etc.
  6. The conclusions MUST be based on the results obtained and will probably also indicate relative fitness levels – fit individuals tend to recover faster after exercise. It must also be linked to the original hypothesis and state whether this hypothesis is accepted or rejected. 
  7. Heart rate increases due to the higher rate of cell respiration that is required to provide the necessary energy during running. The cells demand MORE oxygen and release MORE carbon dioxide than normal, so breathing and heart rate both speed up to deliver the greater amount of \(\text{O}_{2}\) and remove the greater amount of \(\text{CO}_{2}\) formed.

Stroke Volume

The stroke volume is the amount of blood pumped through the heart during each cardiac cycle. The stroke volume can change depending on the needs of the body. During exercise, muscles need more oxygen and glucose in order to produce energy in the form of ATP. Therefore the heart increases its stroke volume and stroke rate to meet this demand. This is a temporary change to maintain homeostasis, and after exercise the heart rate and stroke volume return to normal.

When a person exercises regularly, and is fit, the heart undergoes certain long-term adaptations. The heart muscle gets stronger, and expels more blood with each contraction. There is therefore a greater stroke volume with each heartbeat. Since the heart expels more blood with each stroke, the heart has to beat less often in order to maintain the same volume of blood flow. Therefore, fit people often have lower resting heart rates.


Cardiac output is the volume of blood that is pumped by the heart in one-minute. Cardiac output is equal to the stroke volume (SV) multiplied by the heart rate (HR).

Blood Pressure

Blood pressure refers to the force that the blood exerts on the blood vessel walls. Blood pressure is determined by the size of the blood vessels and ensures that blood flows to all the parts of the body. Normal blood pressure is 120/80 (120 over 80) measured in units of mercury (mm Hg). The 120 represents the systolic pressure, which is when the ventricles contract. The 80 represents the diastolic pressure, which is when general diastole occurs.

Blood pressure can be increased by smoking, stress, adrenalin surges, water retention, high cholesterol, obesity and lack of exercise. High blood pressure (hypertension) is dangerous and increases the risk of an aneurysm, stroke or heart attack. Low blood pressure (hypotension) can lead to light-headedness and fainting because of insufficient blood supply to the brain.

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