http://www.ck12.org Chapter 1. Human Biology
1.2 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 calledhome-
ostasis. 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 feedbackis a response to a stimulus that keeps a variable close to a set value (Figure
1.3).
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
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.