Homeostasis
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Printed: September 1, 2013
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C ONCEPT
Concept 1. Homeostasis
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Homeostasis
• Describe homeostasis and how it is maintained.
How does your body react to cold?
These people may be having fun in the icy water, but their bodies are struggling to react to the cold. For example,
they may begin to shiver. Shivering helps the body return to a stable temperature. The body is always working to
achieve stability, or homeostasis.
Homeostasis and Feedback Regulation
When you walk outside on a cool day, does your body temperature drop? No, your body temperature stays stable
at around 98.6 degrees Fahrenheit. Even when the temperature around you changes, your internal temperature stays
the same.
This ability of the body to maintain a stable internal environment despite a changing environment is called homeostasis. Homeostasis doesn’t just protect against temperature changes. Other aspects of your internal environment
also stay stable. For example, your body closely regulates your fluid balance. You may have noticed that if you are
slightly dehydrated, your urine is darker. That’s because the urine is more concentrated and less water is mixed in
with it.
Maintaining Homeostasis
So how does your body maintain homeostasis? The regulation of your internal environment is done primarily through
negative feedback. Negative feedback is a response to a stimulus that keeps a variable close to a set value (Figure
1.1).
For example, your body has an internal thermostat. During a winter day, in your house a thermostat senses the
temperature in a room and responds by turning on or off the heater. Your body acts in much the same way. When
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body temperature rises, receptors in the skin and the brain sense the temperature change. The temperature change
triggers a command from the brain. This command can cause several responses. If you are too hot, the skin makes
sweat and blood vessels near the skin surface dilate. This response helps decrease body temperature.
FIGURE 1.1
Feedback Regulation.
Positive Feedback
Some processes in the body are regulated by positive feedback. Positive feedback is when a response to an event
increases the likelihood of the event to continue. An example of positive feedback is milk production in nursing
mothers. As the baby drinks her mother’s milk, the hormone prolactin, a chemical signal, is released. The more the
baby suckles, the more prolactin is released, which causes more milk to be produced.
Vocabulary
• homeostasis: Ability to keep a stable internal environment; the ability of the body to maintain a stable internal
environment despite a changing environment.
• hormone: Chemical messenger molecule.
• negative feedback: Response to a stimulus that keeps a variable close to a set value.
• positive feedback: Response to an event increases the likelihood of the event to continue.
Summary
• Homeostasis is the ability of the body to maintain a stable internal environment despite a changing external
environment.
• Homeostasis is maintained primarily through negative feedback, when a response to a stimulus keeps a
variable close to a set value.
Practice
Use the resources below to answer the questions that follow.
• Homeostasis at http://www.phys.unsw.edu.au/biosnippets/
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Concept 1. Homeostasis
1. What are three variables affected by homeostasis? What is the normal human range for these variables?
2. What is negative feedback? What is a sensor and an effector? How does this relate to homeostasis?
3. How does temperature affect enzymes? Why is this important to organisms? How do the enzyme systems of
ectotherms differ from the enzyme systems of endotherms?
4. How is the body temperature of ectotherms related to environmental temperatures?
5. What happens if a fish is exposed to a rapid change in water temperature? What mechanism is involved in this
response?
• Homeostasis at http://www.think-bank.com/iwb/flash/homeostasis.html
1.
2.
3.
4.
5.
List four internal conditions that organisms regulate?
What happens to blood flow in your body when your internal temperature increases?
What happens to blood flow in your body when your internal temperature decreases?
How does your body react to maintain water levels in your body? What is ADH? What is its function?
What are the roles of insulin and glucagon? What do they help the body regulate?
Review
1. What is homeostasis?
2. What is the difference between negative feedback and positive feedback?
References
1. CK-12 Foundation - Megan Totah. . CC-BY-NC-SA 3.0
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