How Negative Feedback Loops Work in the Body

Negative feedback loops play an important role in how many of the systems of the human body stay in control. A negative feedback loop, also known as an inhibitory loop, is a type of self-regulating system. In a negative feedback loop, increased output from the system inhibits future production by the system. In other words, the system controls how much product it makes by shutting down manufacturing when levels of output or the amount of accumulated product gets too high.

Negative feedback systems are responsible for many types of hormone regulation in the human body. They are good at maintaining relatively constant levels of output. 

Imagine that the body is a factory making Product X. Furthermore, imagine that making too much Product X is expensive and wasteful. Therefore, the body needs a way to shut down the factory when enough Product X has been made. It does this through a negative feedback loop. What that means is that the speed of production is sensitive to the amount of Product X. When it starts to build up, production slows and then stops.

It might help to think of the factory as a great big assembly line that feeds shelves at the end. When the shelves get full, the line has to slow down. There's nowhere to put the product. However, if the shelves are empty, there's plenty of space. The assembly line can speed up until the shelves are full again. The more responsive the assembly line, the more the factory can keep the shelves filled at the right level all the time.

The opposite of this would be a positive feedback loop. In that case, the more Product X there was, the faster the plant would make more. This type of system easily gets out of control. In contrast, a negative feedback loop is self-regulating.

One very well understood negative feedback loop is the female menstrual cycle. The hypothalamus produces gonadotropin-releasing hormone (GnRH). The GnRH signals the pituitary to produce follicle-stimulating hormone (FSH). FSH tells the ovaries to produce estrogen. High levels of estrogen (as well as progesterone and testosterone, which are regulated through similar loops) then inhibit the production of GnRH. This causes the pituitary to make less FSH, which in turn causes ovaries to make less estrogen.

Another negative feedback loop regulates vaginal acidity. The normal vaginal pH is approximately 4—mildly acidic. This helps to prevent the growth of problematic bacteria and STDs. In fact, one of the hallmarks of bacterial vaginosis is a pH of above 5. The lactic acid that maintains this pH is made by lactobacilli—part of the normal vaginal flora. These bacteria grow faster and produce more acid at higher pH. Then, when the pH gets close to 4, they slow down and stop. This is how the pH is regulated in the vagina. It also explains some of the differences in different women's vaginal pH. The pH varies depending on the specific bacteria that are present.

One keyword that is important in understanding negative feedback loops is homeostasis. Homeostasis is defined as a system's tendency towards stability. Homeostasis is very important in the human body. Many systems have to self regulate in order for the body to stay in optimal ranges for health.

For example, in diabetes, the pancreas does not respond properly to high blood sugar by producing more insulin. In type 1 diabetes, this is because there are no cells available to make insulin. A person's immune system has damaged or destroyed the "factory."

In type 2 diabetes, this is because the pancreas is not as sensitive to blood sugar signals from the body. Therefore, it doesn't produce enough insulin in response to blood sugar rises. In either case, the person is no longer able to maintain homeostasis in their blood sugar system without the help of medical or behavioral intervention.