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Feedback in living systems

Feedback mechanisms maintain the internal conditions of an organism within certain limits and mediate behaviors, allowing the organism to remain alive and functional even as external conditions change within some range. Feedback mechanisms can encourage (through positive feedback) or discourage (negative feedback) what is going on inside the organism. Created by Khan Academy.

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Video transcript

- [Instructor] So last weekend, my family and I went out hiking in the desert. And as you can tell from these pictures I snapped along the way, it was a gorgeous hike. We made our way to this lake around a small canyon range and up and down this mountain trail. Now, all of this was really great, but there was just one problem. It got super hot. And because we were exercising out in the hot sun, we started sweating buckets. And all I wanted to do after a while was to find some water and shade as soon as possible. So finally, after we sat down to have a nice picnic in the shade and our sweat provided some evaporative cooling, our bodies were able to cool down without overheating. So you might be wondering why did our bodies in our behavior respond that way? Why did we sweat and want to find shade? Well, the answer is our bodies were protecting us from harm. The human body isn't able to function at too high of a temperature. So our bodies helped cool us down through a combination of physiological and behavioral responses. Physiological responses being the internal, chemical, and physical changes that our bodies carry out unconsciously, and behavioral responses being the actions we carry out consciously in response to what our body needs. So in this case, the physiological response would be sweating, which our body does in order to cool itself down. And in addition to sweating, other physiological responses were also happening such as, our blood vessels were dilating and we were getting thirsty. And the behavioral responses were our attempts to find shade and get out of the sun and to drink water. This tendency of an organism to maintain internal conditions within an acceptable range despite changes in its external environment is called homeostasis. And I'll write down our definition. So it's the tendency to maintain internal conditions despite changes in external conditions. So homeostasis is incredibly important because without it, we could have overheated and been in real danger. So in other words, homeostasis is necessary in order for organisms to survive. Now, you might also be wondering how living things generally maintain this homeostatic condition of theirs. And this usually involves negative feedback loops. So let me draw this diagram for us. We have our stimulus, we have a detection and then a response. So in negative feedback, a stimulus or a detectable change in internal conditions triggers the body to carry out a response that will counteract or oppose this change. So it'll bring conditions back within an ideal range. And this is what is represented right here by this blocking symbol in the diagram. So going back to my family's hiking trip, we can say that the stimulus was the increase in our body temperatures as a result of hiking in the hot desert. Our bodies detected that our internal temperature was moving outside of the acceptable range, which typically falls between 97.7 to 99.5 degrees Fahrenheit or 36.5 to 37.5 degrees Celsius. And the cool thing is that, once our bodies detected the stimulus, they produced a response to counteract this change through negative feedback. We were hot, so we wanted to become less hot. And in this case, the negative feedback loop caused responses like sweating that helped cool our body temperatures down to the acceptable or the ideal range. It's also worth noting that our bodies can elicit negative feedback mechanisms in response to our body temperature dropping too low. So if our body temperature drops below the ideal range or our body temperature decreases, then the body counteracts this change through responses like shivering and blood vessel constriction, all with a goal of helping to keep us warm. So negative feedback mechanisms help cool us down when we get too hot or they warm us up when we get too cold. So they help to keep our body temperatures just right. And this process of maintaining body temperature, otherwise known as thermoregulation, let me write that out for us, it can be seen in all different kinds of organisms. You might've seen dogs pant when they're hot or spotted lizards sunbathing to stay warm. And these are all homeostatic responses that help keep the organism's body temperature within the acceptable range that we talked about. Another really awesome example of a negative feedback loop is osmoregulation, specifically in salmon. And here's a picture. Now, salmon spend part of their lives in freshwater streams and the other part of their lives in salt-water oceans. So in fresh water, the salt concentration of the water is lower than the salt concentration you would find in the fish's internal body fluid. While in saltwater, the salt concentration of the water is higher than this fish's internal salt concentration. So this means that in fresh water, the fish will tend to absorb water and lose salts through their skin. Well, the opposite is true in saltwater. Any large change in a fish's internal salt or water levels could be fatal. So how exactly can salmon tolerate these extremely different environmental conditions? Well, they also use negative feedback mechanisms. So salmon have a negative feedback system, which detects changes in internal salt concentrations and causes a response that involves either taking up or excreting salt through the gills or having more or less dilute urine in order to reestablish ideal internal salt concentrations. And this is otherwise known as osmoregulation. So again, we have a feedback loop that acts to oppose a stimulus, which in this case is the change in internal salt concentrations. So now we know that homeostatic mechanisms usually involve negative feedback loops, but what about positive feedback loops? Well, many organisms actually use positive feedback loops to bring processes to completion. So while negative feedback loops dampen stimuli or oppose stimuli, positive feedback loops do the opposite. They amplify stimuli. And as you can tell from this diagram, instead of having a blocking symbol here, we have an arrow to indicate the amplification of the stimulus. So in humans for instance, a positive feedback loop is used for childbirth. So as you can see from this diagram, the stimulus in childbirth comes from the baby's head, which presses against the cervix here. And this stimulates neurons in the cervix, which send a signal for the brain to release a special kind of hormone called oxytocin. Now, oxytocin is responsible for causing the uterus to contract, which as you might've guessed causes more pressure on the cervix which sends more neural signals, which releases more oxytocin. And this loop continues on and on all the way until the baby is born. So when the baby is born, because the baby's head isn't pressing up against the cervix and the pelvic floor anymore, the neuron stops sending the signal and the brain stops triggering the release of so much oxytocin. So that's how the loop will eventually come to an end. So to recap on what we've talked about. Today, we learned that organisms maintain their internal conditions through homeostasis. And this is usually accomplished through negative feedback loops, which dampen or oppose stimuli as we talked about with thermoregulation and osmoregulation. But on the other hand as we saw in childbirth, positive feedback loops work to amplify stimuli in order to bring processes to completion.