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Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

In order for biological processes to proceed, organisms need to maintain standard internal balances. They do so through systems of negative feedback called homeostasis which ensure that all things happen in moderation. The processes and controls involved in homeostasis vary from organism to organism, but often involve hormonal signals.

Homeostasis is a basic idea in Biology and what it is, is an organism's ability to keep a constant internal balance no matter what's going on in the environment. Now this sometimes is hard to understand in the beginning but once you get it, it makes a lot of Biology very simple and it makes things like why do we get addicted to foods or why do people have problems losing weight. And it makes it make a lot more sense, so yeah I can give you this definition but what really helps you get it is to give you a few examples. One of the easiest examples of homeostasis to understand is our response to changes of the environment in terms of temperature.

Now you know what happens if you get cold, well as the environment changes our bodies as mammals are always trying to keep our internal temperature around 98.6 degrees Fahrenheit which is roughly 37 to 38 degrees Celsius. So as I get cold what do I do, what does my body automatically do? Well my fingers tend to lose heat a lot easier and my ears tend to lose heat than say my chest. So it's very important that I keep my chest and my torso area warm because that's where my heart and lungs are and I need to keep my brain warm. So what happens is that I close off the blood supply to my fingers and to my ears and they get colder because I'm no longer sending blood there. That allows me to keep my warm blood in the mid section in my torso and in my head to help keep my body working or I'll start to shiver.

That shivering generates heat because my muscles are sitting there doing work really fast and I've read an estimate an somewhere where shivering can ramp your body's heat production by upwards of 500 percent. So this is how we respond to changes, well what if it gets too hot? Well then I open up the blood vessels to my extremities and my fingers will start to get red and my ears will redden up. And I start to sweat, that sweat on my surface helps absorb some of the heat and evaporate and take it out to the environment. So that helps cool me down, so my body temperature should be here if the external environment gets too hot I start doing changes to lower my body temperature down to keep it down around here. If it gets too cold I start doing changes to bring it back up to its normal range. So that's a normal standard example of homeostasis.

Another one that you may not have thought about is blood sugar. There's this sugar called glucose that's in our blood that is our basic fuel for doing all sorts of things whether it's thinking, running, watching TV so what, how do I control that? Well my blood glucose level needs to be at a certain level to assure every part of my body has adequate fuel, if my blood sugar gets low there'll be all sorts of changes going on, first I'll start to feel hungry and that will drive me start eating food to help bring the amount of glucose that's going into my system up, I'll also start to release extra stored up glucose from things like my liver. It'll start breaking down a complex molecule called glycogen to start throwing out glucoses into my blood stream bringing that blood glucose back up.

If my glucose gets too high that can cause all sorts of problems like high blood pressure etcetera. So I need to start bringing that into my cells and that's when my pancreas will release insulin a hormone that signals all my cells to open up channels to allow the blood glucose to enter into the cells thus going out of your blood. So if my blood glucose gets too high my insulin level starts to raise bringing that blood glucose back down to its normal environment. Which will signal the pancreas to stop releasing so much insulin so that will bring it back down. And this is now that you understand homeostasis this explains why for example you can easily become addicted to things like caffeine or cocaine or tobacco. Because they become part of your standard homeostasis your body adjust to them, so if you don't have them all of a sudden your body has problems figuring out how to accomplish, how to bring itself back up to this standard internal state and you start craving the caffeine or the cocaine or whatever it is that you've been adding to your environment.

Okay so that's the base concept of homeostasis now to add a little bit of complexity to it I'm going to clue you in on 2 basic ideas that are used or terms that are used. One of them is called negative feedback, now negative feedback is one of the most standard or basic ways that homeostasis works. Now negative means that, it gets rid of something. What is it getting rid of? Well it's getting rid of the change, your reaction to normal environmental temperature changes is a great example of negative feedback. So what happens if you get cold? Cold stimulates shivering, what do shivering do? It creates heat, what does that do to the starting off cold it makes it go away. So negative feedback means that as I get more and more warmth in response to the cold I start stopping felling so cold. So I feel better and I stop shivering, so I start responding and then I return back to normal and negative feedback you'll tend to see oscillates up and down around your set point to stay at that stable environment.

That's very different from what's called positive feedback, positive feedback is rare in Biology because instead of keeping you at some stable operating temperature or some stable blood glucose level positive feedback tends to increase the response so you get more and more and more. And that tends to lead to rather than a nice oscillating up and down it leads to an exponential growth. This is very rare in Biology because it tends to lead to things being over used, there are a few examples of positive feedback, your immune system for example tends to respond in what's called a positive feedback manner, but that's because it's for emergency use only.

You don't want to have a constant level cholera in your body or constant level of pneumonia virus. Instead if a little bit of flu virus comes into your body you ought to kill it right now and so you want to have a very quick fast response where you go faster and faster and faster ultimately though you want to have that come back down to normal and that's why in your immune for example you have specialized cells called suppressor T-cells that turn this off. One last thing I'll mention heat your body temperature your parents may have when were a little kid gotten worried a couple of times when you had a fever, that's because if your body temperature gets too hot you can actually wind up in a positive feedback loop because if you get above temperatures around 106 degrees Fahrenheit that heat speeds up chemical reactions in your body and chemical reactions often give off heat so if you heat them up they heat some more which heats them up some more, which heats them up some more and you can wind up having major problems with a positive feedback loop if you get too hot it can cook your brain. So that's homeostasis.