What Is a Negative Feedback Loop?

A negative feedback loop is a normal biological response in which the effects of a reaction slow or stop that reaction. A negative feedback loop helps regulate health by ensuring that a reaction is appropriate and that the systems of the body are in a constant state of equilibrium, also known as homeostasis.

When a system falls out of homeostasis, your body will react with hormones, enzymes, and other substances to correct this imbalance. A negative feedback loop effectively "puts the brakes" on the reaction once homeostasis is achieved and systems are stabilized.

This article illustrates how a negative feedback loop works. It offers examples to help you understand what's involved in achieving and maintaining biological homeostasis.

How Negative Feedback Loops Work

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 of Negative Feedback Loops

To understand negative feedback loops, it's important to understand homeostasis, or the body's tendency toward stability. Homeostasis is very important in the human body. Many systems have to self regulate for the body to stay in optimal ranges for health.

Negative feedback loops regulate and maintain different functions in the body to keep the systems in balance. Examples include:

Menstrual Cycle

The female menstrual cycle operates through a negative feedback loop that involves various structures in the brain:

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

Blood Glucose

Glucose (sugar) is the body's main source of energy. In humans, a blood glucose level of 4 grams, or roughly one teaspoon, is essential for normal function.

The body's positive and negative feedback loops keep glucose levels within this narrow range. Whenever blood sugar rises or falls, the body will respond by releasing hormones that either "turn on" or "turn off" the release of glucose from the liver to keep levels stable.

In the simplest terms, two different hormones produced by the same organ are involved in maintaining tight control of blood sugar:

• Glucagon is a hormone produced from so-called alpha cells in the pancreas. When blood sugar levels are low, glucagon is released as part of the positive feedback loop.
• Insulin is a hormone produced by beta cells of the pancreas. When blood sugar levels are sufficient, insulin is released as part of the negative feedback loop.
  

Vaginal Acidity

Another negative feedback loop regulates vaginal acidity. The body needs to keep the acidity within a narrow range to prevent disease-causing microorganisms from invading vaginal tissues and to prevent the overgrowth of bacteria and yeast naturally found in the vaginal flora.

Vaginal pH is regulated by a "good" bacteria found naturally in the vagina called lactobacilli. Lactobacilli are tasked with producing lactic acid to maintain an optional vaginal pH of between 3.8 and 5.0.

Conditions like bacterial vaginosis, in which "good" and "harmful" bacteria are imbalanced, can disrupt the vaginal pH and trigger positive and negative feedback responses:

• When the vaginal pH is high—meaning the vagina is more alkaline than acidic—lactobacilli will produce more lactic acid as part of the positive feedback loop.
• When the vaginal pH is normalized, lactobacilli will slow the production of lactic acid as part of the negative feedback loop.