Negative Feedback Loops - More Science on the Learning Videos Channel

Negative feedback loops are used to maintain homeostasis and achieve the set point within a system. Negative feedback loops are characterized by their ability to either increase or decrease a stimulus, inhibiting the ability of the stimulus to continue as it did prior to sensing of the receptor.

What's the temperature in the room where you're sitting right now? My guess would be that it's not exactly 98.6.

Yet, your body temperature is usually very close to this value. In fact, if your core body temperature doesn't stay within relatively narrow limits.

The tendency to maintain a stable, relatively constant internal environment is called homeostasis. The body maintains homeostasis for many factors in addition to temperature. For instance, the concentration of various ions in your blood must be kept steady, along with pH and the concentration of glucose. If these values get too high or low, you can end up getting very sick.
Homeostasis is maintained at many levels, not just the level of the whole body as it is for temperature. For instance, the stomach maintains a pH that's different from that of surrounding organs, and each individual cell maintains ion concentrations different from those of the surrounding fluid. Maintaining homeostasis at each level is key to maintaining the body's overall function.
So, how is homeostasis maintained? Let's answer this question by looking at some examples.
Maintaining homeostasis
Biological systems like those of your body are constantly being pushed away from their balance points. For instance, when you exercise, your muscles increase heat production, nudging your body temperature upward. Similarly, when you drink a glass of fruit juice, your blood glucose goes up. Homeostasis depends on the ability of your body to detect and oppose these changes.
Maintenance of homeostasis usually involves negative feedback loops. These loops act to oppose the stimulus, or cue, that triggers them. For example, if your body temperature is too high, a negative feedback loop will act to bring it back down towards the set point, or target value, of 98.6.
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How does this work? First, high temperature will be detected by sensors—primarily nerve cells with endings in your skin and brain—and relayed to a temperature-regulatory control center in your brain. The control center will process the information and activate effectors—such as the sweat glands—whose job is to oppose the stimulus by bringing body temperature down.

Of course, body temperature doesn't just swing above its target value—it can also drop below this value. In general, homeostatic circuits usually involve at least two negative feedback loops:

One is activated when a parameter—like body temperature—is above the set point and is designed to bring it back down.

One is activated when the parameter is below the set point and is designed to bring it back up.

To make this idea more concrete, let's take a closer look at the opposing feedback loops that control body temperature.

If you get either too hot or too cold, sensors in the periphery and the brain tell the temperature regulation center of your brain—in a region called the hypothalamus—that your temperature has strayed from its set point.

For instance, if you’ve been exercising hard, your body temperature can rise above its set point, and you’ll need to activate mechanisms that cool you down. Blood flow to your skin increases to speed up heat loss into your surroundings, and you might also start sweating so the evaporation of sweat from your skin can help you cool off. Heavy breathing can also increase heat loss.

On the other hand, if you’re sitting in a cold room and aren’t dressed warmly, the temperature center in the brain will need to trigger responses that help warm you up. The blood flow to your skin decreases, and you might start shivering so that your muscles generate more heat. You may also get goose bumps—so that the hair on your body stands on end and traps a layer of air near your skin—and increase the release of hormones that act to increase heat production.