BIOPAC LESSON 9 HOMEOSTASIS

Homeostasis is the state of dynamic constancy of the internal environment and is maintained by
negative feedback mechanisms. In a negative feedback mechanism, a sensor detects a
change in the internal environment, and then activates an effector to cause a change in the
reverse direction. This returns the environment toward (and oftentimes past) its normal or
average value, called the set point. Fluctuations above and below the set point normally occur,
even in values that appear to be unchanging, like your resting heart rate. The amount of
fluctuation can be measured by the range (difference between highest and lowest values) and
the standard deviation (how much the data vary from the mean).

The scientific method is a series of steps used by scientists (and those of us who want to be
more scientific) to explore and learn about the world. The scientific method can be applied
when an answer is not yet known, but can be discovered through testing. As a first step in the
scientific method, the scientist makes observations about a situation or problem and looks in
the literature for information about the topic. The scientist then forms an idea about what might
be causing or affecting a situation. This is called a hypothesis. In order for a hypothesis to be
usable, it has to be testable. That is, experiments have to be performed that can determine if
the hypothesis is correct.

These experiments need to be properly developed in order to be helpful. Ideally, an experiment
should test only one factor (called the independent variable) at a time, using the correct
equipment. Measurements should be made carefully and repeated to achieve a consistent

The data is then interpreted and analyzed to determine if there is a relationship between the
variable and the outcome. Eventually, statistics are used to help determine whether the
relationship is strong enough to be significant. The scientist forms conclusions about the
results and whether the hypothesis is correct or not. When he or she is satisfied, another
variable can be chosen for testing and the process continues until the scientist feels enough
work has been done to evaluate the problem. Final observations and conclusions are
organized and presented to other scientists for them to verify or reject.

Assign duties to each member of your group. The assignments should change each lab.
       Director – Reads instructions, directs other members, delegates responsibility.
       Technician – Operates the computer and connects the equipment.
       Recorder – Takes notes, records hypotheses, tests performed and data obtained.
       Subject – Volunteers for the experiment; is responsible for safety and accuracy.

1. Examine the process of homeostasis, measuring the heart rate at rest.
2. Determine data statistics, like average, range, standard deviation of the heart rate at rest.
3. Learn how to estimate values from the data, then compare to calculated values obtained by the
BIOPAC software.
4. Explore and practice the scientific method in a student-led scientific inquiry.


Plug the SS5L respiration transducer into channel 1 of the
BIOPAC MP30 unit (the box located below the computer’s
monitor). Plug the SS2L electrode lead cable into channel 2 and
the SS3L GSR (Galvanic Skin Response) transducer into channel
3. These do not need to be screwed in, but take care with
inserting and removing any plug-ins. Position the respiration
transducer’s band snugly around the chest, just below the armpits. Tighten the velcro band
barely enough to slightly pull. It can go on over the shirt and will stretch every time the subject
breathes. Attach the adhesive electrode pads to the arm and
legs as diagrammed and snap on the electrode leads. Fill the
concavity in both of the finger electrodes of the GSR
transducer with electrode gel. Fasten the Velcro around the
fingers, as diagrammed, so that the gel filled portion is over
the fingertips.

Click on Calibrate and the computer will automatically
calibrate the signals for 10 seconds while the subject sits quietly. Ignoring on-line instructions
that are contrary, click on Record and record for 3 minutes while
the subject breathes normally. Click Suspend. You will see
electrical potentials from the skin (GSR), respiratory
movements, and heart rate in bar graph form. This graph
calculates the number of beats per minute at each point along
the recording. Note how the heart rate goes up and down. We
will only use the bar graph representation of heart rate for this
lesson. The instructor will check your observations for accuracy.

What do you think is causing the heart rate to go up and down?
State this as a hypothesis. Can you test this hypothesis? If not,
choose a hypothesis that can be tested (refer to SUPP- BP4 for more detail) and continue to
the next part.

Click Resume and relax. Notice that a triangle shaped marker has been placed above the
data. (Add as many markers as you want by pressing F9, once or more than once, during
recording. You can also add markers when not recording by left clicking in the markers
channel.) In the blank channel above the markers you should type in a label for each section of
data. Perform testing for your hypothesis and record the results, looking for the predominant
change to the heart rate, which will occur within a few seconds typically. Repeat the testing to
verify the change and record. Conclude whether your hypothesis was correct or not (in other
words, was this variable what caused the heart rate to go up or down?). If time allows, test up
to 3 variables, record the results, and form conclusions. Click on Done and go to Review
Current Data File.

a. Use the cursors on the bottom of the screen to scroll to the 3 minute resting heart rate.
Estimate the minimum, maximum, and mean heart rate by looking at the heart rate scale on the
right margin of the screen. Record this visual estimate in the laboratory report. Need a better
view? Click on the magnifying glass icon in the lower right hand corner and draw a box around

a representative part of the recording, about 1 minute in length. Do this by drop and dragging
the magnification glass across the data in a diagonal fashion. This will draw a box around the
data and when you release the mouse button, the screen will magnify the selected area so you
can perform measurements more easily. If you make a mistake zooming, choose Zoom
previous from the Display menu and start over.

b. Would grids help? Click on File, then Display Preferences, then Grids, and then Show
Grids and the recording will look like graph paper.

c. Now have the computer make measurements by selecting (highlighting) a representative
section of the recording. To do this, click on the I Beam icon. Drag and drop it through the
data you want to highlight. Look for 4 sets of windows located above the data. Each set has 2
buttons. Use the left button to select the data channel from which you want to measure and
use the right button to display a drop-down menu of measurements. Select Ch 41 Heart Rate
(since this is the channel you want to measure) and the following measurement selections for
the 4 windows: minimum, maximum, mean, and standard deviation. You know what max, min,
and mean are (or look them up). The standard deviation measures how much the signal varies
above or below the mean. Larger the standard deviation = more variable signal.
d. Record these calculated values for resting heart rate.
e. Record your hypotheses, results, and conclusions in the table.
e. Print several representative portions of your data. The BIOPAC software will print only what
data shows on the computer monitor, so scroll to the appropriate section and resize it if
necessary. On the resting heart rate printout, draw a line estimating set point and label this.
Show the heart rate change of at least one of the hypotheses in Part II.

                                    LABORATORY REPORT

Name_____________________________________                   Lab Section______________

(Attach a copy of representative portions of your graphical data.)

Minimum Heart Rate           Estimated Value____________ Calculated_____________
Maximum Heart Rate           Estimated Value____________ Calculated_____________
Mean Heart Rate              Estimated Value____________ Calculated_____________
Range (Find the difference of the calculated Min to Max) _____________________________
Standard Deviation (Record calculated value) ______________________________________

1. List several hypotheses from Part II, whether the heart rate  or , and whether your
hypothesis was correct or not.
Hypothesis                                                    Results: HR  or    Correct/Incorrect?

What is your overall conclusion about what is causing the heart rate to go up and down in this

2. Record the standard deviation from each group: ______________________________
Record your standard deviation ______ Is it large compared to the others? ___________
If the heart rate never changed what would the standard deviation be? _______________

3. Define:
       a. Homeostasis

         b. Sensor

         c. Effector

         d. Set Point

4. Use body temperature as an example of homeostasis and draw a flow diagram showing the
causes and effects that help to maintain the temperature constant. Identify the set point,
sensors and effectors involved. (Make 1 diagram, including both systems - cooling and


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