HOW DO FEEDBACK LOOPS IN
THE BODY WORK?
Sep 4, 2011 | By John Brennan
Photo Credit Hemera Technologies/AbleStock.com/Getty Images
Feedback loops are a salient feature of human biochemistry and
physiology. Whether you're looking at blood glucose, osmoregulation
or blood chemistry, you'll find feedback loops are integral to
maintaining stable conditions inside your body. Broadly speaking,
feedback loops in biology come in two different flavors: positive
feedback and negative feedback. The latter is by far the more
common of the two.
NEGATIVE FEEDBACK
A negative feedback loop is a little like a thermostat. When the
temperature in your office building falls, the thermostat kicks on the
heater. When the temperature rises, by contrast, it kicks on the air
conditioning. Either way, when the temperature changes, the
thermostat counteracts the change. Negative feedback loops in your
body behave in a similar way. A change in a variable like temperature
or blood glucose concentration triggers a process that counteracts
the original change.
POSITIVE FEEDBACK
Positive feedback loops are just the opposite. When a change occurs
in the system, a positive feedback loop acts to exacerbate and
amplify it. If your thermostat worked on a positive feedback loop, for
example, as the temperature increased, the thermostat would
respond by turning on the heater, boosting the temperature even
more and causing the thermostat to respond by turning the heater
higher still. Positive feedback loops tend to destabilize a system by
causing it to go to an extreme.
FEEDBACK INHIBITION
A common kind of negative feedback loop is feedback inhibition,
where the product of a biochemical pathway inhibits an enzyme or
enzymes higher up in the pathway, thereby inhibiting its own
production. This often works through so-called allosteric inhibition,
meaning that a small molecule binds to an enzyme and changes its
shape in such a way that it can no longer perform its usual function.
Binding by an allosteric inhibitor is reversible, so the small molecule
can also come unstuck from the enzyme. The fraction of the total
enzyme bound by the allosteric inhibitor at any given time depends
on how much inhibitor is present, so the activity of the enzyme
decreases as levels of the allosteric inhibitor increase.
EXAMPLES
Negative feedback loops are abundant in physiology. Body
temperature is perhaps the most obvious example. If your body
temperature sinks, the portion of your brain called the hypothalamus
senses the change and reacts by triggering mechanisms that
increase body temperature like shivering. If the temperature rises, by
contrast, the hypothalamus may react by triggering mechanisms like
sweating to bring the temperature down. Childbirth, by contrast, is an
example of a positive feedback loop. Contractions induce secretion of
the hormone oxytocin, which in turn induces more contractions,
creating a feedback loop that persists until the baby is forced from the
mother's body.
REFERENCES
"Lehninger Principles of Biochemistry"; David L. Nelson and Michael
M. Cox; 2008 "Biology"; Neil A. Campbell, et al.; 2008