this is mandra 1 Section 1.5 and we're going to be talking about homeostasis we've mentioned homeostasis earlier um and all it really is is what your body's constantly trying to do we're trying to maintain this relatively constant internal environment that allows all of our physiological processes to occur the best that they can at the fastest rate that they can we are going to have different variables um we also refer to these as conditions that are going to fluctuate around a set point the set point is your normal value the most ideal value for whatever um normal range or homeostatic environment we're looking at so if you look to the image below you can see that we're looking at body temperature here so we have time of day on the x-axis um body temperature on the y axis and you can see that we have a normal range so what this is showing is that the normal range is the range that our body will not have a response to that change temperature so you can see from 98.8 to 98.4 as long as we are within this range our body's not going to have um a reaction to try to change it back we're not going to go through a feedback loop so the set point is the center of this range so in this example it's 98.6 this is the ideal temperature that does not mean that's where we necessarily need our temperature at all the time as you can see by the graph here temperature can fluctuate throughout the day um going up and down but as long as we're within this normal range we're not going to have a response our body is not going to have a response to try to bring it back within now if our temperature is too high and it exceeds and it goes out of this normal range what we're going to try to do is try to bring it back down within the normal range if we're too cold and it goes below the normal range what we're going to do is we're going to um have a response that's going to try to warm us up to bring that temperature back into the normal range again so when looking at feedback loops um homeostasis is regulated by feedback loops whether it's a positive or A negative feedback loop what these are going to do is that they're going to allow us to either deviate as far as possible from our normal or normal range or set point in terms of positive feedback or it's going to bring us back within the normal range in terms of negative feedback there's different components to a feedback loop so every feedback loop is going to have a receptor that receptor is going to monitor whatever that variable or value is um by detecting some form of stimulus that has caused change in that variable so in the last example for example if we walked outside and we got really hot you would have a receptor that would pick up that stimulus which would then be excessive heat that will then send it to what's known as a control center the control center is what's going to establish the set point and is going to receive the input from the receptor so once that control center receives the signal that oh we're too hot it will then send that signal back out to something that we're going to refer to as the affector and what the effector does is the factor is going to generate a response it's going to counteract in terms of a negative feedback loop in our example to bring that temperature back to normal range and change the value of that variable or that condition so looking at a negative feedback loop specifically um this is how we're going to regulate most of our body systems so when we're looking here you can see we have this little bar here and we have stimulus so homeostasis has been disturbed by some form of stimulus the receptor is going to pick up that stimulus realize that there's been a um change send it off the control center is then going to integrate that stimulus what is going on what do we need to do to fix it the control Senter will then send a signal out to the uh affector that affector will then cause some form of response to occur which will then bring our set or our that condition or that variable back to the set point within the normal range now once that variable returns to the set point we restored homeostasis we're back within our normal range and we have a negative feedback loop that will inhibit that means stop the control center from receiving anything coming through because the receptor shouldn't be receiving anything because we've brought it back within our normal range it's important to note that negative feedback loops um act as a counteract system if you will um they're going to do the complete opposite of what's happening so if we have an increase in temperature the way that we're going to try to counteract that increase in temperature is to bring our temperature temp back down and that's going to allow us to maintain homeostasis so here's an example that we're going to go through of a woman playing tennis so please note um this is going to be a negative feedback loop you can see the progression over time so what happens is that as she's playing tennis receptors in the skin are going to detect this increase in temperature she's running around it could be hot outside both could um stimulate a response from a receptor so then what happens is that the receptor are going to send that signal to the control center which is going to be the hypothalamus here in the brain in this example the control center is then going to um integrate that information if you will figure out what to do with that information it will then send a signal out to the affectors which in this case is going to be the sweat glands the sweat glands will then produce sweat the act of sweating will cool her down it'll bring her temperature back within normal range and then we're good homeostasis has been restored so now over here when she's no longer playing tennis and she's sitting and she's resting the value of the variable that we've been looking at which is temperature has returned back to its set point we no longer need the negative feedback loop of sweating so as she's continuously playing tennis she will continue to sweat in um by the negative feedback loop trying to bring homeostasis and the temperature back within a normal range when she's no longer exerting herself so much her body temperature is not Rising so therefore we are now going to cancel that signal from the receptor saying that oh it's hot we need to cool ourselves down so that will inhibit the signal going to the control center that control center is not receiving a signal so it has no signal lend to the affector so you will eventually stop sweating once you're in a restful state so this is kind of what we just talked about um in terms of body temperature and this is a little hard to see on this slide um but basically what we just went through is that when we have an increase in temperature and we exceed our normal range what we're going to do is we're going to start sweating our homeostasis has been Disturbed receptors on the skin are going to pick up that we're hot that's going to send a signal to the hypothalamus that's then going to send a signal out to the effectors that are going to generate a response so if we're extremely hot there's a couple things that can happen one we're going to enable or enact those sweat glands that we talked about in the previous example that's going to help cool us off the other thing that we're going to do is we're going to go through a process called vasodilation and what vasodilation is is that the diameter of your blood vessels that are close to the surface are going to expand and what that allows us to do is that the blood that is warm circulating throughout your body will be closer to the surface of the skin and that will allow you to cool off uh naturally um hopefully bringing you back to a homeostatic level within our normal range so that we are have restored home ostasis now in the example where you have went out of normal range but in terms of your temperature is too low what's going to happen is that receptors on the skin are going to recognize that your temperature is too low that's going to send a signal to the brain the hypothalamus hypothalamus is then going to send that signal out to the different effectors now the effectors are different depending on if you're hot or cold so in terms of being cold what's going to happen is that the effectors are going to be your skeletal muscle it's going to cause what we normally refer to as shivering and that's going to generate body heat what else is going to happen is that your vasculature you're going to have Vaso constriction so instead of um the dilation of the blood vessel like we talked about when heating up you're going to have constriction of the blood vessels and what that does is that takes b or the um blood further away from the surface if the external environment is extremely cold you don't want to expose the warm blood to that you know chilliness so you want it to be more toward your core that's why when you are extremely cold you'll start to lose sensation in your fingertips and you'll notice that your core is more warm along with your head what your body is trying to do is it's trying to vasoconstrict it can also shunt blood which means stop sending blood to the periphery and try to maintain the survival of those internal organs um something that you'll also notice when you're really cold is that you can get um Goosebumps little erector pilly muscles of your skin will pull and your hair will stand up and that's to also help you Main maintain heat um within your body hopefully by doing those three things what you've now done is you've increased your body temperature is now back within the normal range and homeostasis is restored so it's important to note that the normal range is um able to move depending on the state that our body is in so we're going to look at this example of changes in blood pressure during exercise so obviously when you're sitting at rest sitting in a chair like I am right now not much expenditure of energy I'm not going through tons of oxygen my muscles aren't working really hard but if I were to go exercise I have an increased demand for oxygen energy expenditure is going to go up I'm G to have more Demand on my muscles because I'm be working them so depending on the situation that I'm in my normal range is going to be able to change so looking at our example here in this graph so notice we have time on the x-axis we have on the y-axis blood pressure so this is our normal range here during rest you can see our set point during rest and what's going to happen is that our normal blood pressure at rest will be within this normal range and we're fine so this is when I was sitting down now let's say I go and I start exercising because of the new environment that I'm putting myself in which is now let's say you know exercise my normal range of blood pressure is going to increase while I'm exercising just naturally but that increase does not mean that I'm outside of normal range per se all that means is that my new normal range during an exercise period is going to be different than my normal range during rest so you can see that my set point will adjust to a higher blood pressure my normal range also increases because there's going to be a higher demand for oxygen um energy expenditure ATP in um energy and the amount of glucose and things I need to keep my muscles moving and just the general Demand on my muscles themselves so during that period of exercise my set point has increased my normal range has increased and as long as I'm staying within that normal range I'm good now so I finished my workout um I'm going to go sit down now obviously and rest so when I leave that exercise environment and I now go back to a normal resting environment my normal range will come back down to what it was originally what my normal range is at rest my set point will come down to what it is during rest and that will be my new um normal range for blood pressure so it is um environment dependent if you will or activity dependent is probably the better word to put or associate it with if I'm just sitting and resting I have a normal range if I'm exercising I have a normal range for my blood pressure and that's to facilitate the different demands of the different parts of my body during those two different time periods so positive feedback kind of talked about positive feedback earlier um positive feedback is not nearly as common as negative feedback is I would say 99.9% of all feedback loops are negative feedback loops positive feedback loops are pretty far and few between but they play some really important roles uh in physiy ology in reproduction um so that's why we talk about them pretty thoroughly so what happens with a positive feedback loop is that we're going to have some form of deviation um and we're just going to continue to deviate further and further and further away from our set point until that stimulus no longer exists so we typically won't find these in healthy adults young adults this is more of what we'll find um people that are older if you will that have a higher risk of physiological issues that could lead to death um in young adults positive feedback loops can lead to death they can be dangerous not always I don't want you to think that positive feedback loops are associated with dangerous things but they can be just as negative feedback loops can so some examples of a normal positive feedback loop are going to be childbirth lactation in blood clotting those are really the big three examples that you need to know um there are harmful positive feedback loops so um if we're looking after a hemorrhage let's say that there's some form of bleeding your blood pressure is going to drop so what happens when your blood pressure drops is that the heart's ability to pump blood is going to decrease what your body is going to tell your heart to do is then to pick up the speed I need more blood because I need more oxygen I need more nutrients so your heart rate increases but that's going to be harmful for you because all that's doing is that that is now allowing you to Hemorrhage faster you're losing more blood than you would have because you're pushing out more blood um so that positive feedback loop obviously would is one that would lead to a more catastrophic end result but that is not always the case and we'll go over many examples of positive feedback loops in the course and uh here shortly so what happens is that the effectors involved in this response are going to continue to push beyond the set point until that original stimulus is revoked so looking at the positive feedback loop of child birth what we have here and I know this image is a little small and hard to see but so we have the baby within the uterus here and what's going to happen is at the stretch of the uterus um we have mechano receptors in there which are uh receptors that are going to notice stretch and what's going to happen is that those are going to send signals to a control center which we're going to see up in um pituitary gland up here here hypothalamus pituitary gland that sensation of stretch is going to cause the release of oxytocin and what oxytocin is going to do is oxytocin is going to increase the smooth muscle contraction within the uterus so what happens is that the more the baby's head is pushing on the uterus more signals are being sent up to the hypothalamus and the pituitary gland to secrete more oxytocin more oxytocin is then going to cause more uterine contractions put more pressure on those walls so as you can see here this positive feedback loop just keeps going and going and going and going until eventually the contractions increase so much that the baby's going to be pushed out of the uterus through the cervix through the vaginal canal and then they are you know have completed the birthing process once the baby is no longer there and its head and its body are no longer putting pressure on The mechan receptors of the uterus then there's no signal to be sent to the brain to produce oxytocin so this is one of those situations where our set point is um you know minimal oxytocin as the baby's head is pushing we're getting further and further and further and further away from our set point which you can see in this image here we're just increasing the amount of oxytocin exponentially until the baby is birthed and then once the stimulus is gone which is that baby's head pushing on the uterine wall that stimulus is gone the positive feedback is over this is an example where positive feedback plays a role that doesn't have a catastrophic ending um it's a very normal process as you know for people to give birth