Lesson 1 of 10 / Homeostasis and response
Negative feedback
How does a correction know when to ease off?
I can build a negative-feedback explanation from a change in the internal environment.
I can define maintenance of a constant internal environment.Syllabus
Syllabus K325, Topic 8(a). Define maintenance of a constant internal environment.
I can explain stimulus, correction and negative feedback.Syllabus
Syllabus K325, Topic 8(b). Explain stimulus, correction and negative feedback.
About 5 min
Make a guess. You are not marked.
Your body temperature is controlled by homeostasis. Does it stay at exactly 37.0 degrees C all day?
Show the answer
No. It rises and falls a little around a set point.
Homeostasis keeps conditions within a narrow range. Small changes act as the stimulus for correction.
The key idea
Homeostasis maintains a relatively constant internal environment. Negative feedback produces a response that opposes a change.
An internal condition, such as blood glucose concentration, is a variable. It normally fluctuates within a narrow range rather than staying at one exact value.
Brown dot: internal condition. Green band: usual range. Arrow: effect of the corrective response.
- Receptors detect the change.
- A coordinating centre directs effectors.
- The response reduces the deviation.
A rise triggers a response that lowers the condition. As it approaches the normal range, the response decreases.
Explanation
Cells work best within suitable internal conditions. Homeostasis maintains these conditions despite changes inside or outside the body. Examples include body temperature, blood glucose concentration and the water potential of the blood.
A change in a condition acts as a stimulus. Receptors detect it, a coordinating centre processes the information, and effectors carry out a corrective response. An effector may be a muscle or a gland. Name each part when the question supplies it.
In negative feedback, the response opposes the original change. If a condition rises above its usual range, the response lowers it. If it falls below the range, a different response may raise it. Negative means opposing the change, not harmful.
As the condition returns towards its normal range, the stimulus becomes smaller and the corrective response decreases. This prevents continuous overcorrection. A response that makes the original change larger is not negative feedback.
Step by step
- 1
State the change
Say which internal condition rose or fell relative to its normal range.
- 2
Trace the correction
Name the detector, coordinating centre and effector if they are known.
- 3
Close the loop
Explain how the response opposes the change and then decreases as the condition is restored.
Worked example
Recognising feedback
Body temperature rises. Sweating increases, and temperature then falls towards normal. Why is this negative feedback?
One way to explain it
The cooling response opposes the initial rise in temperature. As temperature returns towards normal, the stimulus for increased sweating decreases.
Why this answer works
- Identify the initial rise.
- Link sweating to cooling, which acts in the opposite direction.
- Include the reduction in the corrective response.
Watch out for this
A student says: "Homeostasis keeps every internal condition completely unchanged." What is wrong with this?
Show the answer
Conditions fluctuate within a narrow range. Homeostatic responses limit the deviations; they do not stop all change.
Check your understanding
Which sequence shows negative feedback?
Show the answer
A rise causes a response that lowers the condition towards its usual range.
The response opposes the rise, and should decrease as the deviation becomes smaller.
Quick recall
How does a correction know when to ease off?