Physiology 1
Fall 2007
Name:
Homeostasis
Introduction:
Homeostasis may be thought of as the central theme in physiology. Nearly all body systems
function to maintain a state of dynamic constancy within the internal environment, a state known
as homeostasis. The mechanism of maintaining this state of homeostasis typically involves
negative feedback control of effectors (muscles & glands).
The maintenance of homeostasis is a requirement for life. When homeostasis is disturbed,
illness may result and the disturbance must be corrected or the survival of the organism may
be at risk. In fact, the symptoms and risk factors associated with many diseases are due to
uncorrected disturbances in homeostasis. Under normal circumstances all disturbances are
quickly corrected. When the disturbance is detected (by a sensor) this information is
conducted to an integration center or integrator for comparison to homeostatic values and
corrective instructions are delivered to effectors as necessary.
The effector typically produces changes that oppose the disturbance that triggered the
response. Often, the response effectively prevents the disturbance from producing more than a
slight deviation from the set point value. Slight deviations from the set-point are the norm
though. Homeostasis is thus a state of dynamic, rather than absolute, constancy (dynamic
equilibrium).
Since the typical response to a disturbance in homeostasis results in the production of
changes that oppose the initial disturbance, this response is called “negative feedback”.
Negative feedback systems vary in complexity. Some are simple, utilizing only one effector.
These negative feedback systems can be compared to a thermostat controlled heater in a
home. When the thermostat detects a room temperature below the set point value the heater is
turned on and the room warms up. When the temperature reaches the set point value the heater
is turned off. Simple systems like this system however, are unable to correct for temperature
deviations above the set point. Cooling of the room when the temperature exceeds the set point
value is uncontrolled or dependent on passive cooling.
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More complex negative feedback systems often involve two antagonistic effectors in the same
system. This might be comparable to a home with a thermostat controlled heater and air
conditioner. When the thermostat detects a room temperature below the set point value the
heater is turned on. When the thermostat detects a room temperature above the set point value
the heater is turned off and the air conditioner is turned on and the room cools down. Deviations
from the set point in either direction are actively opposed by the two antagonistic effectors.
Lab Exercises
Exercise A: Heart Rate & Negative Feedback Control
Homeostasis, the dynamic equilibrium of the internal environment, can be evaluated by
measuring and recording physiological values such as heart rate, body temperature, blood pH
etc... over a period of time. Due to the dynamic nature of homeostasis, fluctuations in measured
values are normally seen during the measurement time period.
In this procedure we are going to measure heart rate which is determined by two
antagonistic effectors. Sympathetic nerves stimulate an increase in heart rate while
parasympathetic nerves produce an inhibitory effect that slows the heart rate.
Procedure
Work in pairs. Take turns in serving as the subject in this procedure. You will be determining
heart rate by palpating (feeling) the radial pulse at the wrist or the carotid pulse in the neck.
To determine the pulse gently press your index and middle fingers (not your thumb) against the
radial artery in the subject’s wrist or the carotid artery in the neck until you feel a pulse.
Count the number of pulses in a 15 second interval, record this number in the data table
provided. (see below)
Wait 15 seconds and take the subject’s pulse during the next 15 seconds. Repeat this
procedure, taking a pulse during every other 15 second interval for a 5-minute period. A total of
ten measurements should be obtained. Record your data in the table below.
Since heart rate is expressed as beats or pulses per minute, the values you recorded will need
to be corrected by multiplying each pulse count by 4 (each 15 second measurement represents
1/4 of the heart beats in a minute). Record the corrected heart rates (expressed as beats per
minute or bpm) in the data table below.
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Graph the corrected heart rates (expressed as bpm) by placing a dot at the point corresponding
to the heart rate for each measurement interval, then connect the dots. Note: you will first need
to establish the scale on the y axis. (Here is one way to do this: write the lowest heart rate value
at the bottom of the scale (by the L) and the highest value at the top of the scale (by the H).
Divide the difference between the high and low value by 6 to determine the value of each
interval on the Y axis.)
H
Beats
per
Minute
L
0
1
2
3
4
5
6
Measurements
7
8
9
10
Exercise B: Calculation of Normal Homeostatic Values
Normal homeostatic values are based on a statistical average among normal healthy people.
Because people tend to differ, no single value (average or not) could apply to every person. As
a result “normal values” are often expressed over a range encompassing the measurements of
most normal healthy people. Even with this adjustment, perfectly healthy people may fall outside
of published normal ranges. Keep in mind, normal values are based on statistical averages and
are subject to the limitations of statistical analysis. Also, the determination of what constitutes a
healthy individual from whom the normal range is determined is subject to interpretation.
For example, endurance athletes typically have a low heart rate while healthy moderately active
individuals often have a much higher heart rate. A normal range of heart rates based on both
types of individuals would differ dramatically from normal values determined from a population
of athletes alone. Thus the makeup of the population and how you determine which individuals
are healthy can significantly effect the “normal range” that is established.
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CLINICAL SIGNIFICANCE: The principles of homeostasis and established “normal”
ranges are a mainstay in healthcare. Measurement of body temperature, blood
pressure, heart rate, and many other variables, provide clues to the state of the internal
environment and the health of the patient. If any measured values deviate significantly
from the normal range that suggests that homeostasis is not being maintained. Not only
does a deviation from a normal range suggest a breakdown in homeostasis but the type
of variation can provide evidence leading to identification of an illness and its eventual
resolution.
Procedure
Every student in the class will now determine his or her average heart rate (pulse rate) from the
previous data. This can be accomplished by taking an average of the ten heart rates determined
previously. To calculate your average, add the ten heart rate values recorded above and divide
the total by 10. Record your average below and on the transparency for the rest of the class to
review. You should also indicate whether you exercise on a regular basis (at least three times a
week) or not.
Your average pulse rate: beats per minute: ____________
Do you exercise on a regular basis: ______________
When the last student has recorded her/his average heart rate on the instructor computer, copy
this information in the data table provided below. Make notations distinguishing each student
into one of two groups: those who exercise on a regular basis (at least three times a week) and
those who do not.
Class pulse-rate averages:
Mean
Heart
Rate
Exercise
(Y/N)
Mean
Heart
Rate
Exercise
(Y/N)
Mean
Heart
Rate
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Exercise
(Y/N)
Mean
Heart
Rate
Exercise
(Y/N)
Calculate the overall class average pulse rate.
Total # of students: ________
Overall class average pulse rate: _____________
Tally the number of students into exercise and non-exercise categories. Also, calculate the
percentage of students in the class who are within each category and record this percentage
below.
Distribution of class pulse rate averages (all students):
Pulse rate
(beats /
minute)
Entire Class
Number of
Students
Exercise Group
Percentage
of Total
Number of
Students
Percentage
of Total
Non-Exercise Group
Number of
Students
Percentage
of Total
Over 100
90-100
80-90
70-80
60-70
50-60
Under 50
Determine the average pulse rate and range of values for the two categories. Enter this
information in the given spaces below.
Averages for exercise and non-exercise groups from distribution above:
Exercise group
Non-exercise group
Range of pulse rates
Average of pulse rates
How do the Exercise and Non-exercise averages compare to the total class average?
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Exercise C: Homeostatic Recovery after a Disturbance
In this procedure you will determine the pulse rate changes during the recovery phase following
exercise. Since data will be pooled for analysis it is important that all subjects perform the same
amount of exercise at the same intensity. The exercise will consist of stepping up and down on
a step stool for two minutes. Use the following chart to determine the appropriate step height
for your height.
YOUR HEIGHT
STEP HEIGHT
Less than 5'3"
13 inches
5'4"-5'9"
16 inches
Greater than 5'9”
19 inches
Procedure
Select the appropriate step stool and complete the exercise by stepping up and down for 2
minutes. Complete one step cycle in 2 seconds or 30 steps a minute (one step involves
stepping both feet up onto the step stool and back to the floor). Try to be as consistent and
accurate as possible in your pace for the full 2 minutes.
When the two minutes are up, immediately sit down and have someone start recording your
pulse. Count the number of pulses in the 15 second interval immediately following completion of
exercise, wait 15 seconds and take the subjects pulse during the next 15 seconds. Repeat this
procedure, taking a pulse during every other 15 second interval, for a 5-minute period. A total of
ten measurements will be obtained. Record your data in the table on the next page.
Since heart rate is expressed as beats or pulses per minute the values you recorded will need
to be corrected by multiplying each pulse count by 4 (each 15 second measurement represents
1/4 of the heart beats in a minute). Record the corrected heart rates (expressed as beats per
minute or bpm) in the data table provided. You now have a record of heart rate once every 30
seconds for 5 minutes. All this is exactly what you have done in exercise A.
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Graph the corrected heart rates (expressed as bpm) in your report by placing a dot at the point
corresponding to the heart rate for each measurement interval, then connect the dots. Note: you
will first need to establish the scale on the y axis. ( See experiment A)
H
Beats
per
Minute
L
0
1
2
3
4
5
6
Measurements
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7
8
9
10
Questions
What does Homeostasis mean?
What is the main goal of Homeostasis?
Which of these characteristics of life helps maintain homeostasis when environmental
conditions change?
A) growth and development
B) metabolism
C) organization
D) reproduction
E) responsiveness
Suppose that a temperature controlled home contained two thermostat controlled antagonistic
effectors: a heater and a cooler. Using a drawing (graph), show how this system could operate
to maintain a constant temperature.
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Explain why your graph of pulse-rate measurements suggests the presence of negative
feedback control mechanisms. With reference to the description of the effects of sympathetic
and parasympathetic nerves on heart rate, draw a flow chart to show how these antagonistic
effectors maintain dynamic constancy of the resting pulse rate.
Mary has finished eating a large meal. Her blood is being flooded with sugar from this meal.
How does her body prevent her blood sugar from going too high?
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