we all change in many ways as we go through life i'm a much different person than i was 10 15 20 years ago but when it comes to our bodies it's pretty important that everything stays about the same moment to moment day to day no matter what wild things we go through in life if your blood pressure gets too low you might pass out if your blood pressure gets too high it can damage your blood vessels over time and increase your risk of heart attack and stroke if the calcium levels in your blood get too low it can make your bones weaker over time if your sugar levels get too high it can damage your blood vessels or cause you to go into hyperglycemic shock but luckily our bodies have a process for keeping all those things right where they need to be and that process is called homeostasis [Music] favorite metaphor for homeostasis [Music] i'm going to use balancing the ruler as a metaphor for homeostasis and take you through the four steps of homeostasis the first idea is the idea of set point the set point is going to be the ideal position of the ruler or the ideal internal condition of the body such as 120 over 80 for our blood pressure for the ruler the set point is the ruler being actually straight up like this that's kind of the ideal position of it now if you notice when i'm balancing it very rarely is it perfectly straight up it's almost always tilted slightly to the side or slightly like this and that's a big idea in this is that even though the set point's the ideal condition it's hardly ever right at the set point it's always slightly off but we're always correcting it back to the set point okay to keep this ruler balanced then there's three steps that i have to take the first step is that i have to sense it in this case i'm viewing it with my eyes and i'm also feeling it with my fingers and so i can detect hey this is the position of it i can see that it's slanted over i can feel that the weight's more on one side than the other so step one is to detect it we can't make a change until we detect what's wrong the second step is my brain has to make a decision it's got to compare its position to the set point process of making that comparison is called integration so right now my brain has to say okay it's tilted to the side that's too far that way compared to the set point of this and so it's not where it needs to be that happens subconsciously i don't have to think hey the ruler is not right i need to correct it happens faster than that it happens faster than that with our blood pressure and all these other things as well after my brain which is the integrator or control center makes a comparison between the current position and the set point we're going to use an effector to correct it in the case of the rule of the effector is me moving my hand to correct and make that position back to the set point so let's lean over to the side like this and then i make a correction and it's back up to the set point where it needs to be any homeostasis process in the body is going to follow this set of steps that you see right here we call this whole process a negative feedback loop and the reason it's negative is because we're always making an opposite change of where the condition is so for example if this is leaning too far that way i'm going to make a change to make it the opposite or if it's leaning too far this way i'll make a change in the opposite direction to correct it so that's why we call it a negative feedback loop now let's go to the whiteboard and look at two examples of how the body actually uses homeostasis to keep internal conditions the same and to keep us alive which is pretty important our first example is blood pressure as we go through this don't worry about getting every single little detail of these negative feedback loops but focus on this whole process of sensor integrator and effector to get a variable back to its set point now the set point for blood pressure we're going to use is about 120 over 80 mmhg that's just a measure of the pressure when your heart beats and then when it relaxes now let's say something happens and that blood pressure gets a little bit too high if you get stressed out about something the first thing we have to do remember is to sense whatever that blood pressure is in our bodies we have two places that detect that blood pressure one's in the aorta which is a blood vessel that comes out of the heart and the second is in the carotid artery which is a blood vessel in our neck so those are gonna sense what that blood pressure is using a stretch receptor basically if the pressure's high the blood vessels expand a little bit and they detect that so then the aortic carotid artery will send a signal up to the brain through a nerve and the medulla in the brain is going to compare that blood pressure reading to the set point and say okay this is too high is it too low is it just right well in this case it's gonna say okay that's too high and so the medulla oblongata in the brain is gonna then make a decision and it's gonna send signals to other parts of the body defectors to tell them what to do two main effectors here in blood pressure regulation the first one is it's going to tell your heart to be slower and not to be this hard in other words your heart rate and contraction strength are going to decrease and that's going to lower your blood pressure back down it's also going to tell your blood vessels to dilate in other words your blood vessels will expand a little bit which is going to reduce the pressure that that fluid is putting on the walls of the blood vessel both those changes are going to work to lower your blood pressure back down to what that set point is 120 over 80. now let's look at the bottom loop a misconception that a lot of students have is that the top loop here is negative feedback because it got too high we're going to bring it back down so they think negative and the bottom loop is positive feedback it got too low we'll bring it back up that's positive that is a lie from the double itself in reality both of these are negative feedback because if it gets too high it does the opposite brings it down if it gets too low it still is the opposite brings back up and so those are both negative feedback we'll look at positive feedback in another video but positive feedback basically is when something changes the body makes that change even greater which is not at all what's happening here in these negative feedback loops now let's look at the bottom loop the first few steps here are going to be the same first we have to sense it that'll be the aorta and carotid artery then we have to compare it to the set point that's still going to be the medulla in the brain but the effectors will be different because the medulla is going to make a comparison say oh the blood pressure is too low and so that's going to cause the heart rate and strength of contraction to increase your heart will be faster and harder and it's going to cause your blood vessels to constrict if your blood vessels constrict there's a higher pressure between the fluid and the walls of the blood vessel both of those changes are going to raise your blood pressure back up to the set point so your blood pressure is constantly fluctuating get a little bit too high your body brings back down it gets a little bit too low your body brings it back up and it's always kind of hovering around that set point but rarely exactly 120 over 80. if you take your blood pressure right now it's probably not going to be the exact same as it is if you take your blood pressure a few minutes later but it should be around the same point no matter when you take it throughout the day our second example is body temperature i'm gonna work this one a little bit quicker our set point for our body is about 37 degrees celsius or 98.6 degrees fahrenheit so let's say your body temperature gets a little bit too high so the nerve cells in your brain and skin are going to detect that temperature or sense that temperature sensor then those are going to send signals up to the hypothalamus that's part of your brain and it's going to compare that temperature to the set point what it wants to keep your body temperature at since your body temperature is too high it's going to make a decision to send signals to some effectors in the body to bring that temperature back down the main two effectors here will be to tell the sweat glands in your skin to release sweat and when that sweat evaporates that'll bring your body temperature back down and also to cause your blood vessels to dilate that's going to cause more blood to get to the outer parts the extremities of your body so it can release some of that extra heat to the atmosphere now if your body temperature gets too low we'll go through the same first two steps first nerve cells in your brain and skin are going to detect or sense what that temperature is those will send signals to the hypothalamus and the brain to compare the temperature to the set point it's going to see if that temperature is lower than it needs to be and then the brain is going to send signals to your muscles to cause them to shiver which will warm you up and the brain will send signals to the blood vessels in your body to tell them to constrict to keep that blood closer to the internal parts of your body so it doesn't lose energy to the surroundings those changes will raise your body temperature back up toward the set point and then you're good to go again okay quick recap remember that every internal condition that you need to keep the same in the body follows this homeostatic negative feedback loop there's a set point an ideal condition whenever that condition gets too high or too low first your body will sense that condition then the body will integrate it or compare it to the set point the integrator or control center usually is the brain but it can be other organs depending on which feedback loop we're looking at and then the body will initiate effectors in order to correct that condition back to the set point [Music] hey thanks for watching there's a link in the description for cards that you can either print out or you can just manipulate on a slide to organize those into the homeostasis feedback loops which is really good practice for mastering this idea of the negative feedback loop don't forget to keep all of your internal conditions inside your body the same that's some good she's got better footwork than me