Feedback Mechanisms
• Feedback Mechanisms are ways in which
“things” (cells, organisms, populations, etc.)
affect other “things” in a system
• Typically, feedback includes a:
– Stimulus - change in homeostatic
(stable/balanced) system
– Response - Effect of change, where system moves
towards reestablishing homeostasis
Positive Feedback
• Positive feedback loop – amplifies or increases
change
• Has nothing to do with good or bad outcome.
Examples:
– Birth and Population - More Births leads to higher
population, which leads to even more births, which
leads to even greater population, etc…
Positive Feedback
More Examples:
- Anxiety and Panic – the more anxious you get, the
more physical symptoms you have, the more
panicked you get, the more anxious you get…
- OXYTOCIN and uterine muscles during birth - Initial
push causes oxytocin release, which causes more
pushing, etc…
Positive Feedback
More Examples:
- Lower incidences of AIDS (-) means you need less
medical treatment (-). Higher incidences of AIDS (+)
means you need more medical treatment (+).
- Note this is a positive feedback loop despite the negative
impacts!
- Think about math – 2 negative integers multiplied together
make a positive
Positive Feedback
More Examples:
- Torn blood vessel stimulates release of clotting factors
(more tears in vessels leads to more clotting factors)
platelets
blood vessel
fibrin
clot
red blood cell
white blood cell
- Growth hormones stimulate cell division (more
growth hormones leads to more cell division)
Negative Feedback
• Negative Feedback Loop – when one “thing” is
increased, another “thing” is decreased (or vice versa)
• Has nothing to do with BAD outcome
• Most common control mechanism
• Examples:
– As duck deaths decrease, duck population increases
– As wolf population increases, deer population
decreases
Negative Feedback
• More Examples:
– Body temperature and sweating/ shivering. Body’s return
to homeostasis shuts off loop
Feedback Loops (in general)
• You can determine if the feedback is positive or
negative by looking at whether the changes in the
system are the same (see below)
• Think about MATH and multiplying integers:
Positive Feedback Loop
•SAME SIGNS
•(+,+) OR (-,-)
Negative Feedback Loop
•DIFFERENT SIGNS
•(+,-) OR (-,+)