Introduction to Homeostasis
MODEL 1: This model shows a heating system for maintaining home temperature in cold weather. Most people
consider a value around 23 degrees Celsius to be comfortable.
QUESTIONS:
1. What is the temperature range within the house over the time period shown?
22 to 24 degrees Celsius
2. What two values are used by the thermostat in its functioning?
Target value and measured temperature
3. At what temperature does the furnace turn on? At what temperature does it turn off?
Turns on at 22 degrees Celcius and off at 24 degrees Celsius
4. In this scenario, what is the most likely value of the target temperature? Explain your reasoning.
23 degrees Celcius. This value is considered most comfortable by the homeowner and in order for the system
to not be “on” all the time, the temperature falls below the target temp by a degree and then the furnace
turns on until the house is heated 2 degrees above where it turns on, and 1 degree above the target
temperature.
5. The overall system used to control the temperature in the house is an example of a negative feedback
system. Have each member of the team write their definition of a negative feedback system.
See next question for an example answer.
6. Using a grammatically correct sentence, write your best team definition for a negative feedback system.
A negative feedback system is a system used to control a variable by instituting a process that counteracts a
change from the target setting, bringing the value of the variable back toward the target.
7. The homeowner has Northern Australian orchids growing in the house, which only grow in temperatures
above 22 degrees Celsius. If the thermostat or the wire from the thermostat to the furnace breaks, what will
happen to the temperature in the house? What will happen to the orchids?
Because this system is meant to keep the house “warm”, the house will cool. This will either kill the Orchids or
keep them from growing.
8. In the summer, how could this loop be modified (settings or components) to keep the house around the
same target temperature?
The furnace could be replaced with an air conditioner (or an air conditioner could just be added in) that the
thermostat communicates with.
MODEL 2: Negative Feedback Loop
Homeostasis is an important ability associated with living organisms. It is typically carried out through a process
using a negative feedback loop, similar to the loop that controls the temperature in a house. Figure 2A shows the
generic components of the loop, and Figure 2B shows an example loop important in maintaining blood pressure.
Figure 2A: General Homeostatic Feedback Loop
Figure 2B: Homeostatic Feedback Loop for the Regulation of Blood Pressure
QUESTIONS (Refer to Models 1 and 2):
9. List the 3 components that you would expect to find in any feedback loop. Refer back to Model 1 and indicate
which item from Model 1 represents each component. Then refer back to Model 2, Figure 2B, and indicate
which item from Figure 2B represents each component.
Components of a
feedback loop
Items from Model 1 that are
representative of each component
Items from Model 2, Figure 2B that
are representative of each
component
Sensor
Thermostat
Pressure receptors
(baroreceptors)
Integrator
Thermostat
Brainstem nuclei
Effector(s)
Furnace
Heart rate (heart)
Force of contraction of heart
Constriction of blood vessels
10. Besides the main components of the loop (listed in question 9), what other feature is important in the
functioning of a feedback loop?
The ability of the components to communicate with each other.
11. Is the goal of a negative feedback loop to maintain a regulated variable at a constant value? Explain.
Not necessarily constant, but within a suitable range around a set point or target value.
12. Is a negative feedback loop limited to one effector? Explain.
No, in the control of blood pressure there are multiple effectors, such as responses by both heart and blood
vessels.
13. Do you think the integrator always a separate anatomical structure from the sensor? Explain.
No, it is often the same structure, similar to a thermostat being both the sensor an integrator.
Teachers note: An example of this would be in the regulation of plasma glucose levels. Cells of the pancreas act
a both the sensor and integrator.
14. The ability to maintain homeostasis is considered critical to the normal functioning of life and is normally
carried out through the use of a negative feedback loop. Individually write your best definition of
homeostasis.
See answer to next question
15. As a team, write your best definition of homeostasis.
Answers will vary, but a good definition is “the body’s ability to maintain certain variables in a range suitable
for life”. In your discussion, please point out that we may not all have exactly the same set point and that it is
hard to keep us at a “constant” value for just about anything. But the system works well at using a setpoint as
a target to keep the variables in a suitable range.
Extension Questions
16. Is heart rate a homeostatically-controlled variable? Explain.
No, although most individuals have a HR that falls within a specific range (such as 50 to 150), it is actually an
effector for regulating blood pressure. There is no receptor for measuring HR.
17. What happens when a homeostatically-controlled feedback loop is damaged or interrupted? If possible,
provide a specific example.
Answers may vary, but it is likely that damage or disease will ccur because the regulated variable is likely to
end up outside of the appropriate range.
a. Body temperatures. If we are too hot or too cold, we tend to feel ill or die.
b. Blood pressure. Hypotension (too low) makes us feel dizzy. Too high and it damages blood vessels, kidneys,
etc.
c. Plasma glucose. Too low and we can feel dizzy and pass out. Too high and it can make us lose water in our
urine and cause longer-term damage to blood vessels and nerves.
18. Based on your current knowledge, sketch a feedback loop for the control of temperature in the body. Be
as specific as possible with the structures involved.
An example loop might be:
19. Predict what would change in this feedback loop when someone is experiencing a fever.
The set point (target value) in the hypothalamus changes to a higher temperature value. The rest of the
loop stays the same.