explained 2

 

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.