Explain 2
Types of Feedback Loops
As you saw in the video on the last
page, we generally think about two kinds of feedback loops: positive
feedback and negative feedback. We can tell the difference between the
two by comparing the deviation in a bodily condition to the body's response. If the response is in the opposite direction as the deviation, that is negative
feedback. The graph to the right shows this visually. The red line
represents measurements of some bodily condition (e.g., temperature,
skin thickness, blood acidity, etc.). Whenever the measurement changes
up or down, the body pushes the measurement back where it came from, in
the opposite direction. In other words, the measurements always wobble
around and move toward a central, desired value, or "set point." The
diagram below shows an example of negative feedback from our book.
If the response in a feedback loop is in the same direction as the deviation, that is positive
feedback. The graph to the left shows a couple examples of this. In
this case, a measurement of a bodily condition deviates and then the
body's response amplifies that deviation. The measurement
keeps getting higher and higher (or lower and lower), presumably farther
away from the set point. The diagram below shows an example of
positive feedback from our book.
A few important notes to avoid confusion!
Negative feedback does not mean "bad" and positive feedback does not mean "good." In daily language, we often use "negative" to
refer to something bad and "positive" to something good. This is not
the case in feedback loops. Negative feedback loops can be both helpful
(temperature regulation) and harmful (some forms of cancer) in
different situations. Same goes for positive feedback!
Also, negative feedback does not mean "lower" and positive feedback
does not mean "higher." Students often think that if something is
"negative" it must be getting lower and lower. But if you look at the
negative feedback graph above, sometimes the response pushes the values
up! Similarly, in the graph for positive feedback, one of the lines is
actually getting lower, not higher.
This emphasizes that the way we tell between the two is strictly by comparing the direction of the deviation to the direction of the response.
If the response counteracts the deviation and brings the value back
toward the set point, that's negative feedback. If the response
amplifies the deviation and brings the value farther away from the set
point, that's positive feedback.
One More Twist!
One
danger in all this talk of deviations from a "normal" set point is that
we can assume the body only ever has one preferred set point for each
of its conditions. This is not the case! Our bodies and the
environments around them are always changing. This means our set points
need to adjust to change bodily conditions one way or another to meet
our body's needs. This is a concept called rheostasis.
The graph to the right shows an example of this. The body adjusted the
set point and in this case the negative feedback loop responded by
returning the value to the set point (now higher) and keeping it there.
While this graph shows a raise in the set point, the set point could
also potentially adjust downward and that too would be rheostasis.
Click "Next" for a brief chance to pause and think about this information.
