Investigating Heart Rate

Introduction

Resting heart rate, maximum heart rate, target heart rate—what do all these numbers mean? What is the importance of these numbers and what criteria are used to determine them? What factors influence each of these signs of cardiac health? The purpose of this activity is to learn how these indicators are measured or determined and to investigate the effects of physical activity and additional weights on the heart rate.

Concepts

• Cardiology
• Heart rate

Background

A closed circulatory system is one where blood never leaves the system of blood vessels and the heart. In animals with a fourchambered heart, the blood follows two distinct circuits as it passes through the body. The systemic circuit carries oxygenated blood from the left side of the heart, through the arteries and arterioles, and finally to the capillaries, where oxygen is delivered to the cells via diffusion. The deoxygenated blood then travels back through venules and the larger veins to the vena cava before being returned to the right atrium of the heart. In the pulmonary circuit, deoxygenated blood travels from the heart through the pulmonary arteries to the lungs, where oxygen diffuses into the blood. The blood then enters the pulmonary veins before returning to the heart. Note: Arteries carry blood away from the heart and veins carry blood to the heart, regardless of whether the blood is oxygenated or deoxygenated.

In order for muscle tissue to receive more oxygen during physical exertion, the heart increases its contraction rate. A heart rate, also called the cardiac rate or pulse, is measured in beats per minute (bpm). The more times the heart contracts (beats) within one minute, the faster the heart rate. Many physiological factors or conditions influence heart rate—some of the most important include age, gender, heart disease, stress, thyroid problems, anemia, stimulants, depressants and other medications.

The maximum rate that a heart can beat is generally the same for people within the same age group. The maximum cardiac rate for an individual is calculated by subtracting the person’s age from 220. Individuals who are in good physical condition can deliver more oxygen to their muscles before reaching the maximum cardiac rate than can individuals in poor condition. People who are physically fit also pump a greater volume of blood with each contraction during physical exertion. As a result, a physically fit person’s heart does not need to beat as fast to deliver the same amount of oxygenated blood to the muscle tissues. An adult athlete in peak fitness may have a resting heart rate of 50–57 bpm, compared to a resting heart rate of 70–76 bpm for an average adult male. During physical exertion, a person who is in poor physical condition reaches the maximum cardiac rate at a lower work level than a person of comparable age who is in better shape. During physical exertion, the goal is not to reach the maximum cardiac rate but rather the target heart rate. The target heart rate is actually a range of heart rates corresponding to the maximum cardiac rate multiplied by 50–85%. The target heart rate is considered the optimum value for each age group to achieve physical conditioning and strengthening of the circulatory system (see Table 1).

Materials

Safety Precautions

Isopropyl alcohol is a flammable liquid and a moderate fire risk; slightly toxic by ingestion and inhalation. Wear chemical splash goggles and avoid sources of ignition when handling isopropyl alcohol. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Isopropyl alcohol may be disposed of by evaporation according to Flinn Suggested Disposal Method #18a. Used cotton balls may be disposed of in the regular trash according to Flinn Suggested Disposal Method #26a.

Procedure

1. Obtain three volunteers who are willing to perform the following physical tasks—jumping jacks for one minute, followed by rest for two minutes and then jumping jacks again with a 10-lb weight pack for one minute.
2. Each volunteer should select a second individual who will be the data collector, measuring the volunteer’s heart rate after each interval.
3. Saturate two cotton balls with isopropyl alcohol. Use one cotton ball to clean the bell and diaphragm of the stethoscope and the second to clean the earpieces of the stethoscope. Discard cotton balls in the regular trash.
4. Each of the three volunteers should remain quietly seated for two minutes before their resting heart rate is measured.
5. The data collector should place the stethoscope over the volunteer’s heart and count the number of beats for 20 seconds.
6. Record the number of resting heartbeats in 20 seconds for each volunteer on the Investigating Heart Rate worksheet. Calculate what the heart rate would be in beats per minute and enter this in the table as well.
7. Each volunteer will perform jumping jacks for 60 seconds.
8. After 60 seconds measure the volunteer’s heart rate again as described in steps 5 and 6. Record the results in the “Physical Exercise” column of the worksheet.
9. Allow the volunteer to sit and rest for two minutes.
10. Using a scale, weigh 10 lbs worth of books. Add this weight of books to each volunteer’s back pack.
11. Wearing the 10-lb back pack, each volunteer should perform jumping jacks for 60 seconds.
12. Measure the number of heartbeats for each volunteer as described in steps 5 and 6. Record the results in the “Physical Exercise with Weights” column of the worksheet.

Student Worksheet PDF

10999_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the activity as written at least seven times: 100 mL of 70% isopropyl alcohol, 50 cotton balls, and 3 stethoscopes.
• Be sensitive when recruiting volunteers to perform this demonstration. Make sure volunteers do not have any health issues where this activity may cause physical stress.
• Paperback or soft cover books are preferred over hardcover books as they will be more comfortable for the volunteers to carry while performing the physical activity.

Sample Data

Answers to Questions

1. Calculate the average resting heart rate and the heart rates after physical exercise and after physical exercise with weights for the three volunteers.

Average resting heart rate: (75 + 60 + 72)/3 = 69 beats per minute

Average heart rate after physical exercise: (132 + 105 + 144)/3 = 127 beats per minute

Average heart rate after physical exercise with weights: (156 + 120 + 162)/3 = 146 beats per minute

2. Determine the maximum heart rate for the three volunteers based on their average age, and calculate the target heart rate range. How did the average heart rate of the volunteers after physical exercise compare with the target heart rate for a person of that age?

Each volunteer is 16 years old. Their maximum target heart beat is 220 bpm – 16 bpm (their age) = 204 bpm. Therefore 50–85% of their maximum heart rate is their target heart rate. (204 bpm × 0.50%) to (204 × 0.85%) = 102 bpm – 173 bpm.

(132 + 106 + 144) / 3 = The average of all three volunteers is 127 bpm which is within the target heart rate range.

3. Determine the change in average heart rate for the three individuals after regular physical exercise compared to after physical exercise with weights.

Volunteer 1 = 156 bpm – 132 bpm = 24 bpm
Volunteer 2 = 120 bpm – 106 bpm = 14 bpm
Volunteer 3 = 162 bpm – 144 bpm = 18 bpm

(24 + 14 + 18) / 3 = 19 The average change is an increase of 19 bpm.

4. What does the average change in heart rate obtained in question 3 indicate about the amount of work the heart must do as weight is added? What is the implication of this result for the effect of body weight on cardiac health?

Since the average heart rate increased when additional weight was borne indicates that the heart must work harder to do the same activity for the same amount of time.

5. Weight lifting can cause a person’s weight to increase by building muscle. a) How does building muscle improve cardiac health—be specific in terms of oxygen delivery and heart rate. b) What other benefits does building muscle have on the body as a whole?

With an increase in weight the heart rate does need to increase so that the proper tissues receive the correct amount of oxygen.

If the amount of weight gained as fat is severe enough to be considered obesity it can lead to many complications. These complications include heart disease, high blood pressure, sleep apnea, stroke and type 2 diabetes.

Conversely, as muscle mass increases you will be able to work harder and longer without becoming fatigued. You will also increase joint flexibility and bone density.

References

American Heart Association. Target Heart Rates. http://www.americanheart.org/presenter.jhtml?identifier=4736 (accessed January 14, 2010).