How Negative Feedback Loops Work in the Body

Negative feedback loops play an important role in regulating health in the human body. 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. The body reduces the amount of certain proteins or hormones it creates when their levels get too high.

Negative feedback systems work to maintain relatively constant levels of output. For example, the body maintains its temperature, calorie consumption, blood pressure, pulse, and respiratory rate based on negative feedback loops.

Negative Feedback Loops Manage Production

Imagine that the body is a factory making Product X, and imagine that making too much of Product X is expensive, wasteful, and harmful. This means that the body needs a way to slow 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.

It might help to think of the factory as a great, big assembly line that stocks shelves at the end. When the shelves get full, the assembly line has to slow down since there's nowhere to put the product. Too much product building up can cause damage.

However, if the shelves are empty, there's plenty of space on which to stock the product. The assembly line can speed up until the shelves are full again, with the goal of keeping 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 is, the faster the plant will make more.

Examples

Several well-understood negative feedback loops control a variety of different functions in the body.

The female menstrual cycle operates through a negative feedback loop that involves structures in the brain, as well as reproductive organs, as follows.

• The hypothalamus is a gland in the brain that produces gonadotropin-releasing hormone (GnRH).
• The GnRH signals the pituitary gland in the brain to produce follicle-stimulating hormone (FSH).
• FSH triggers the ovaries to produce estrogen.
• High levels of estrogen (as well as progesterone and testosterone, which are regulated through similar loops) inhibit the production of GnRH. This causes the pituitary to make less FSH, which causes ovaries to make less estrogen.

The male reproductive axis is set up in a similar manner as the female axis, with luteinizing hormone (LH), FSH, and testosterone in a negative feedback loop associated with fertility.

Another negative feedback loop regulates vaginal acidity. The pH of the vagina varies depending on the specific bacteria that are present, as so:

• The normal vaginal pH is approximately 4—mildly acidic. This helps prevent the growth of problematic bacteria, including those that cause sexually transmitted infections (STIs).
• 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.
• One of the hallmarks of bacterial vaginosis is a pH of above 5—which will trigger the normal flora to produce acid that prevents the infectious bacteria from thriving.
• When the pH gets close to 4, the lactobacilli can slow down the production of lactic acid.

Seeking Homeostasis

One key word that is important in understanding negative feedback loops is "homeostasis." Homeostasis is defined as a system's tendency toward 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 (insulin-dependent diabetes, in which the pancreas produces little, if any, insulin), this is because there are fewer cells available to make insulin. A person's immune system has damaged the insulin-producing cells.

Similarly, breathing regulates the body's oxygen and carbon dioxide levels—which are tightly controlled by the mechanisms in the brain that mediate respiration.