Name: _____________________________________________________________ Period: ________ Date: _________
Lab - Homeostasis
Introduction
Nearly all organ systems function to maintain a controlled, stable internal environment, a state known as homeostasis. The
mechanism of maintaining homeostasis typically involves negative feedback.
The maintenance of homeostasis is a requirement for life. When homeostasis is disturbed, illness may result. Under normal
circumstances all disturbances are quickly corrected. When a sensor, or receptor, detects a disturbance caused by a stimulus,
the information is conducted to an integrator for processing, and corrective instructions are delivered to effectors (muscles
and glands) that carry out the response that contradicts the stimulus.
The effector typically produces changes that oppose the stimulus that triggered the response. Often, the response prevents
the stimulus from producing more than a slight deviation from the set point (homeostasis). 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 the heater is turned on, and the room warms up.
When the temperature reaches the set point the heater shuts off. An air conditioner works in the same way but turns on
when the temperature rises above the set point and shuts off when the temperature reaches the set point.
Pre-lab
Read the Introduction and Procedure to answer the following questions. You need NOT answer in complete sentences.
1. What may result when homeostasis is disturbed? _______________________________________________________
2. What are the functions of the sensor, integrator, and effector? ____________________________________________
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3. Why is homeostasis referred to as a state of dynamic equilibrium? ___________________________________________
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4. Describe the procedure for taking your partner’s pulse in 4 steps. __________________________________________
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5. How do you expect the SHAPE of your graph for Exercise A to differ from that of Exercise B? Explain!____________
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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, and blood pH 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 you will measure heart rate which is determined by two antagonistic effectors. Antagonistic effectors work
in tandem to maintain homeostasis (like both the AC and the heater working to maintain temperature). Sympathetic nerves
stimulate an increase in heart rate while parasympathetic nerves produce an inhibitory effect that slows the heart rate.
Procedure
1. Work in pairs. Take turns serving as the subject in this procedure. You will be determining heart rate (HR) by 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 against the radial artery in the subject’s wrist or the carotid artery in the neck until you feel a pulse.
2. Count the number of pulses in a 15 second interval. Record this number in the Data Table below.
3. 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 recorded below.
4. Since HR is expressed as beats per minute, the values you recorded will need to be corrected by multiplying each pulse
count by 4. Record the corrected HR (expressed as beats per minute, or bpm) in the data table below.
Results
Data Table________________________________________________________________________
a. Graph the corrected heart rates (expressed as bpm) by placing a dot at the point corresponding to the HR for each
measurement interval on the graph below.
b. Connect the dots by drawing straight lines (use a ruler/straight edge!).
180
160
140
Beats
per
Minute
120
100
80
60
40
20
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Measurements
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Exercise B: Homeostatic Recovery after a Disturbance
In this procedure you will determine the HR changes during the recovery phase following exercise. The exercise will consist
of running in place for two minutes.
Procedure
1. Run in place for 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 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 will be obtained. Record your results in the
Data Table below.
2. Since HR is expressed as beats per minute the values you recorded will need to be corrected by multiplying each pulse
count by 4. Record the corrected heart rates in the data table provided. You now have a record of HR once every 30
seconds for 5 minutes.
Results
Data Table___________________________________________________________ ____________
a. Graph the corrected heart rates (expressed as bpm) by placing a dot at the point corresponding to the heart rate for
each measurement interval on the graph below.
b. Connect the dots like you did in Exercise A.
180
160
140
Beats
per
Minute
120
100
80
60
40
20
1
2
Turn over to complete the Analysis section.
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Measurements
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Analysis
1. Compare and contrast your graphs from Exercises A and B. __________________________________________
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2. Use your knowledge of homeostasis and negative feedback systems to explain the shape of your graph in Exercise A
(why was it not a perfectly horizontal line?).___________________________________________________________
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3. Use your knowledge of homeostasis and negative feedback systems to explain the shape of your graph in Exercise B.
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4. How do the sympathetic and parasympathetic nerves respond to an increase in heart rate? How do they respond to a
decrease in heart rate? (The Procedure DOES mention the role of both in maintaining resting HR).
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