Published by Australian Academy of Science
Activity 3 | The rise and rise of asthma

The effect of exercise on breathing

Note: Students suffering from respiratory or heart problems should not act as subjects for this activity.

Exercise can alter the rate of breathing and the carbon dioxide (CO₂ ) content of the exhaled air. This activity will measure the effect of exercise on breathing rate and CO₂ levels in exhaled air.

Materials (for each small group)

• 1 conical flask or small beaker;
  
• 1 100 mL measuring cylinder;
  
• 5 straws;
  
• 1 dropper bottle of sodium hydroxide (chemical formula NaOH);
  
• 1 dropper bottle of phenolphthalein;
  
• 1 stopwatch.

1. Place 100 mL of water in the conical flask and add five drops of phenolphthalein.

2. One member of the team (the subject) should sit quietly for 3 minutes.

3. After the first and third minute take the subject's pulse. (Count the pulse for 15 seconds then multiply by four to determine pulse rate per minute.)

4. After the second minute, test the CO₂ level in exhaled breath by having the subject breath through five straws into the solution in the conical flask for exactly 30 seconds. (The subject should keep the straws together so that as little air as possible escapes between them, and breath in through the nose and out through the straws. There is no need to open the mouth at all. At rest the subject should only take about three breaths in 30 seconds.)

5. Rinse the straws, and put them aside for later.

6. Add one drop of NaOH solution to the flask. Swirl the contents vigorously and observe the flask against a white background. Look for evidence of a pink colour.

7. If no pink colour appears, add another drop of NaOH solution, swirl vigorously and check colour again.

8. Keep a count of the number of drops used.

9. Continue adding drops until the solution in the conical flask turns pink and retains its colour for a least 15 seconds.

10. Record the number of drops used.

11. Wash out the flask and refill with another 100 mL of water and five drops of phenolphthalein.

    You now have two resting pulse measurements and one measurement of CO₂ levels in exhaled air when the subject is at rest.

12. The subject should now run on the spot or do star jumps for 2 minutes, working as hard as possible for the full time.

13. As soon as possible after the exercise stops and while the subject is seated, take a pulse measurement and have the subject breathe out into the conical flask.

14. Repeat the pulse measurement every 2 minutes and the CO₂ measurements at 4 minutes and 8 minutes during the 10 minutes of recovery time. (The subject should remain seated during the recovery period.)

15. Record your results in a table, then graph the data obtained from your subject.

Questions

1. If the average heart pumps about 80 mL of blood with each contraction, calculate the volume of blood pumped by the subject:
   • at rest;
     
   • immediately after vigorous exercise;
     
   • 10 minutes after completing exercise.
     

   (Your answers should be in mL/minute.)

2. Calculate the percentage increase in heart rate from rest to immediately after exercise.

Teachers notes

Students suffering from respiratory or heart problems should not act as subjects for this exercise.

Preparation

• It may be helpful to provide two conical flasks (or beakers) per group of students.

• If the titration is taking a long time, another CO₂ sample can be taken in the second flask.

• Five straws are the maximum needed per subject.

  Since straws vary in diameter, you should experiment with yours to determine how many are needed to give an even delivery of exhaled air into the phenolphthalein solution. (Using multiple straws to exhale through minimises bubbling.)

• Ask subjects to practise breathing out through the straws before beginning the activity.

• If you have more than one subject per group, you will need a new set of straws for each new subject.

• Phenolphthalein solution: dissolve 1 gram of solid phenolphthalein in 500 mL methylated spirits.

• NaOH solution: 0.01 M (0.4 grams NaOH made to 1 litre with water).

• Students should measure the pulse rate by using the tips of the fingers just on the outside of the major vein on the thumb side of the front side of the subject's wrist. Ask students to practise this before beginning the exercise.

Summary of data collection

At the end of the experiment students should have:

• heart rate measurements taken 3 minutes before exercise started and 1 minute before exercise started;

• heart rate measurements taken at 2-minute intervals during a 10-minute recovery period;

• CO₂ measurements taken 2 minutes before exercise started;

• CO₂ measurements taken at 4-minute intervals during an 8-minute recovery period.

CO₂ concentration is measured by titration. When CO₂ dissolves in water it forms a 'weak' acid (carbonic acid). By measuring the amount of NaOH needed to neutralise this acidic solution, a relative measure of the amount of CO₂ is obtained – the more NaOH needed, the more CO₂ has been exhaled.