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.
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 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.
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.
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.
- "Lehninger Principles of Biochemistry"; David L. Nelson and Michael M. Cox; 2008
- "Biology"; Neil A. Campbell, et al.; 2008