Feedback brings the variable back to the set point
- The idea
- Predict
- See it
- Why
- Check
The idea
“After a spike, the body resets its target to the new level and holds the variable there.”
After a big meal, glucose stays high for an hour or so, and it looks like the body accepted a new level. The whole time, insulin is pulling it back down.
The short version
The mistake: After a spike, the body resets its target to the new level and holds the variable there.
What's actually true: The set point stays put. Negative feedback brings the regulated variable back to it, and the response eases off as it gets there.
- The regulated variable is what gets measured, like blood glucose. The set point is the value the body defends, near 90 mg/dL for glucose between meals.
- In negative feedback, a change away from the set point triggers a response that pushes the variable back.
- Rising glucose triggers insulin release, and insulin helps cells take up glucose. As glucose falls back, the trigger fades and insulin drops. The set point never moved.
- A set point can be reset by a separate signal, as in a fever. A correction itself never moves the target.
On the exam: The set point is the target and the variable is what moves. Negative feedback moves the variable back.