Some feedback loops make the change bigger
- The idea
- Predict
- See it
- Why
- Check
The idea
“A feedback loop always brings a value back to its set point.”
The first loops you learn, body temperature and blood sugar, hold things steady. So it's easy to assume every loop does.
The short version
The mistake: A feedback loop always brings a value back to its set point.
What's actually true: Negative feedback pulls a value back to a set point. Positive feedback grows a change until something outside the loop ends it.
- Feedback is named for what the response does to the change that triggered it. Shrinking the change is negative feedback. Growing it is positive.
- Negative feedback holds a variable near a set point, the value the body keeps it close to.
- Positive feedback has no set point. Each round makes the next one stronger, as with oxytocin in labor.
- A positive loop stops only when something outside the loop breaks the cycle. After the birth, the cervix is no longer stretched, so the oxytocin signal ends.
On the exam: Ask what the response does to the change. If it shrinks it, negative. If it grows it, positive.