A hormone falls when the job is done
- The idea
- Predict
- See it
- Why
- Check
The idea
“A hormone level drops because the gland has used up its supply.”
A gland putting out hormone for hours sounds like it should tire, like your legs on a long run. Glands keep making more, and many hold a reserve.
The short version
The mistake: A hormone level drops because the gland has used up its supply.
What's actually true: A response ends when the variable is back at its set point, or when an outside event breaks a positive loop.
- In negative feedback, the response pushes the variable back toward its set point. Once it gets there, the stimulus is gone and the response winds down.
- A gland that had run out would give a weak response to the next stimulus. A full-size second response means it was never empty.
- Positive feedback keeps building until an event outside the loop ends it. Birth ends the oxytocin surge of labor.
On the exam: A falling hormone means its stimulus is gone. The variable was fixed, or an outside event broke the loop.