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Using Feedback Systems to Regulate Growth, Reproduction & Dynamic Homeostasis

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  • 0:03 Feedback Mechanisms
  • 1:17 Negative Feedback
  • 3:12 Positive Feedback
  • 4:35 Alterations
  • 6:59 Lesson Summary
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Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In order to maintain specific internal conditions, organisms use feedback mechanisms. Feedback is essential to the regulation of internal environments, which allows for proper biological functioning.

Feedback Mechanisms

Whether you realize it or not, our bodies are constantly working hard to maintain specific internal conditions. This stable internal environment, known as homeostasis, is important because it allows our bodies to function normally.

In order to maintain this internal environment, we use feedback mechanisms, which are responses that trigger other activities or processes. There are two types of feedback mechanisms: negative and positive. Negative feedback mechanisms are the most common because they attempt to maintain a target level. In contrast, positive feedback mechanisms are amplifications away from a target level.

Negative feedback is like seeing a speed limit sign and either speeding up or slowing down to reach that speed. Positive feedback would be seeing the speed limit sign and then flooring it to see how much faster than the speed limit you can go.

So, if negative feedback is more common, why is it called 'negative?' It's not because it's a bad process - it's actually quite important! It's called negative feedback because it reduces output, essentially a 'negative increase.' This is the opposite of positive feedback, which is when there is a 'positive increase' in output.

Negative Feedback

Let's look at an example of why negative feedback is so important and how it works in your body. You know that iguana sunning itself on a rock? It's doing this to help increase its body temperature. Iguanas are ectotherms, meaning that they rely on outside sources to regulate their body temperature. The iguana is warming itself up by sitting in a warm environment. This is great for the iguana because it doesn't take very much energy, so it doesn't need to spend as much time looking for food.

You, on the other hand, are an endotherm because you regulate your own internal body temperature through metabolism. Mammals, many birds, and even some fish are endotherms. Being an endotherm takes much more energy than simply warming up in the sunshine because metabolism is a constant and active process and needs a constant supply of energy.

Human bodies use negative feedback for this internal temperature regulation. Here's how it works. The hypothalamus, which has a sensor that sits at the roof of your mouth, tells your brain when you get too hot or cold. If you get too hot, your body tries to cool you down by sweating. The water that comes out as sweat takes a lot of heat with it, and as it leaves your skin, you cool down. At the same time, your body is also doing other things to increase heat loss, like moving blood up to your skin.

But what happens if you cool down too much? Well, then your body reverses these processes - goosebumps appear and your blood gets moved back from the skin to more internal locations. Both of these processes help minimize heat loss, increasing your internal temperature again.

This temperature regulation is a constant process. Though you may not notice it, your body is always working hard to maintain this constant, internal temperature through negative feedback.

Positive Feedback

Now let's look at how positive feedback works and why it is a less common process. Positive feedback occurs in fruit-bearing plants so that their fruits can ripen. When fruits ripen, they become more enticing to animals that will eat the fruit and then spread the seeds around. As a fruit ripens, it produces a gas called ethylene. As ethylene is given off by a ripening fruit, the gas is sensed by other nearby fruit, which triggers them to start ripening and producing ethylene as well. This then triggers other nearby fruits to ripen and produce ethylene, and in no time, the whole plant is full of ripe fruit.

This is the same reason that if you want unripe fruit at home to be ready to eat sooner, you can put them in a paper bag with a ripe piece of fruit. The already ripe fruit will produce a large amount of ethylene, triggering the ripening process in the other fruits in the bag. But don't leave them in there too long - otherwise, you'll just end up with a bag of rotten fruit!

It's in this way that positive feedback is an amplification process - all the fruit ripen quickly at the same time because it is beneficial for this to happen on the plant. Positive feedback is used when we want something to happen quickly, not for maintenance or long periods of time.

Alterations

Sometimes, the feedback mechanism has an error so we get an alteration in the response. For example, diabetes is a disease that is the result of a faulty feedback mechanism for blood glucose levels.

In our bodies, negative feedback regulates our blood glucose levels (glucose is a sugar). Blood glucose levels are regulated by your pancreas, which has two types of cells for this job: beta cells and alpha cells.

When you take in large amounts of glucose, say in a carbohydrate-heavy meal, this signals the beta cells to produce a hormone called insulin. The insulin that is excreted from the pancreas tells your cells to take up more glucose and also instructs your liver to store more of this glucose as glycogen. Both of these processes help remove glucose from your blood, which returns your blood glucose levels to normal.

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