Hello students. In this lecture we're going to be talking about homeostasis and feedback loops. Homeostasis is where we maintain a relatively constant internal environment despite external changes. Note that this internal environment is not like an exact set point. It is not one particular level. It is going to be a range, so it's really considered a state of dynamic consistency. This is very important. We're going to stay within a livable range for whatever variable that we're talking about, whether it's blood glucose, or oxygen levels, or body temperature, our body stays within a within a a range for a specific variable that is livable and acceptable for our body, despite external changes in the environment. We're going to determine how our body does that and what are the ways that we maintain that homeostasis. We're going to study that and we're going to look at what happens when we get out of a homeostasis. That's what physiology. It is basically the study of homeostasis and what happens when we get out of homeostasis. Homeostasis is maintaining this internal environment and it is a dynamic constancy. It's very important that we understand it's not one exact point, it's a range. Most of the time, this is maintained by negative feedback loops. We'll talk about positive feedback loops and we'll talk about negative feedback loops. You've probably had some experience with these if you've taken BIO 5 and definitely with anatomy. You should have had some some introduction into this. We will look at each of these and how they help to maintain homeostasis. If we are out of homeostasis, if we get out of this livable range, whatever this range may be for for the variable that we're we're looking at, this is disease. This is disorder and this can be detrimental and lead to death if it's uncorrected. When we have disease states, basically we're out of homeostasis and we have to figure out what's causing that disruption in homeostasis and get us back, if possible, to the homeostatic state. Understanding the internal environment and how we maintain that, That's going to be most of my questions to you in this class. How are we maintaining our internal environment? My second question will be: "If something went wrong, what went wrong? How do we fix it? If you think like that, it will help you in understanding as we go along in each of the systems that we study. You know what's normal. You know if something's out of whack, what's not normal, what went wrong. Then how do we fix it? Focus on that as we go through each of these. When we talk about preventing imbalance, how do we do this? How do we maintain this regulation? How do we control and stay within this range? First of all, you have to be able to test for this range. You have to have receptors that pick up and check for this range. If you don't have receptors, if you can't test a variable, you can't maintain homeostasis. There are no controls for it. So number one, you have to have receptors that are going to measure your variable. An example here would be blood glucose. We have to have receptors that detect glucose levels. These are typically going to be in the pancreas. They're going to monitor blood glucose. There are receptors elsewhere, but when we're talking about adjusting blood glucose a big part of that is going to be at the pancreas. Our normal homeostatic range is generally between 70 and 110 milligrams per deciliter. This is where we want to be. This is a good number to remember. It does vary a little bit between labs. Note that the units can vary. So the numbers will look different, with different units. Here is a list of some variables, some homeostatic variables, that you need to know, (definitely pH, this one you need to know) for this class. For the first test, write this down. It's on the first test. Right here, know this, your partial pressure of oxygen. This is a good one to know. You will not have to write this down until probably the third test, but you do need to know it. This is pretty standard. It's very important and you will need it to know it for the third and fourth test. Blood glucose is good to know. Again, you notice that this range is a little bit different, so ranges will vary a little bit. When ranges vary I'm not going to have you memorize them, because it's going to depend on what lab you're at. They're going to vary a little bit. Look at your lab. Look at what the normal for them. Bicarb- this is one of those that varies. Sometimes it's 22 to 26. Sometimes it's 24 to 28. Know about where it is but you don't have to memorize the exact numbers. Sodium- most of the time it's 135 to 145. This one's easy to remember just because of pH 7.35 to 7.45. so 35 to 45 I think that's really easy to remember. This one's easy for me because they're very similar. Calcium- this one varies. It does vary depending on the units. You need to look at the units, but this is the normal range for calcium. Again I'm not going to make you memorize that one. Just look that one up. These are some good variable ranges that we're going to talk about throughout this class. We'll look at other ones as well, but these are ones that we'll be we'll be focusing on multiple times during this class. Definitely, these top two, are two parameters that you need to know. The top one is on the first test. Do you see me winking there. Okay? Make sure you know that, all right? When we talk about homeostasis, it's maintaining that internal environment with changes in the outside. You can see here in this picture, these individuals are in an extreme heat situation. How are they acclimating to this extreme heat environment? How is their body adapting? What changes can you think of that they are making, other than behavioral changes, like wearing loose fitting, light colored, reflective clothing, other than those things? What physiologic changes are they doing to help them adapt and acclimate to this high temperature environment? You can ask yourself in another way, how do you acclimate to this type of condition, where it is very cold and at high altitude and low oxygen? How do we acclimate to these different environments or adjust to these different environments and still maintain homeostasis? We're going to look at: "What is a negative feedback loop? A negative feedback loop is where you determine you're out of range for a set point, you initiate a response, and your response takes you back to your set point. You get out of range, you initiate a response, and it brings you back. So negative feedback is you're coming back towards your set point. You're going in the opposite direction that your stimulus was. Your stimulus is out of the set point and we're going away. Going back towards the set point is the goal. We have a set point, a range we can detect. If we are out of the range, we have to have receptors. Our action whatever that is, our response is going to return us to that set point. This is very important. That's negative feedback. The stimulus is detected by a receptor. The control center is the processor, who determines what is going to be your actions, what what is your response. It could be one action. It can be multiple actions that are going to be carried out to make the changes to get you back to home to homeostasis, to that set point and range. Control centers will determine the response. They're going to signal effectors. Effectors are the organs that are going to carry out the response. They're the the "doers", they're the ones that actually carry out the actions that will bring you back to your range that you want to want to be in. Once you're in that range, the receptors are no longer picking up that you're out of the range, so this should turn off the response. The control center will then no longer be signaled to make the changes. Then that should turn off. So we have a stimulus, the sensor picks up that we're out of out of range, the control center determines a response, the effector carries out the response. The response gets us back to the normal range. Now our stimulus is no longer there. This will actually shut off the response. An example here is body temperature increases. Remember body temperature is around 37 degrees C (36.4-37.6). If temperature goes above that range, it is detected. The control center, the hypothalamus, will then initiate mechanisms. The heat releasing center, will be initiated. We will plan on signaling effectors to release heat. How might we do that? Can you think of ways that we would reduce heat in the body? Good! Sweating allows for evaporative cooling good. Vasodilation. If we dilate blood vessels and we increase blood flow to the surface, we lose heat through radiant loss. Good. These are two mechanisms that heat is lost by. Now our body temperature goes back down. The stimulus is no longer going to be there. The temperature is no longer signaling the hypothalamus. We no longer are going to sweat and vasodilate. Then these get shut off. Can you provide another negative feedback example? Take a minute write this down and describe how it works. Good. Practice. If you are having trouble with this, reach out to me. shoot me an email, shoot me a text. I'll be happy to walk through this with you. If you aren't sure, you can also email me, text me, give me a call, and we can walk through this, and see how it goes. Positive feedback loops, these are less common. In this scenario, we have a situation in which the stimulus, is we're out of range, but in this case, being out of range actually increases the stimulus. We're going to initiate mechanisms that reinforce the stimulus, rather than shut it off. Positive feedback increases the stimulus rather than shutting it off. You can imagine though, that this would just go on, and on, and on, and on, unless there's some sort of end point to the game or end point to the process. That's exactly what happens. There's always an endpoint that shuts this off. We'll take a look at that. Ultimately these do result in homeostasis, but initially it kind of looks like they're going the opposite way, because they're reinforcing the stimulus early on. An example of this would be blood clotting. You get damage to a blood vessel, (we're going to look at this in unit 3) and when you get damage to the blood vessel, collagen is exposed. Von willebrand factor and collagen bind together and this causes platelets to stick to the collagen and the von willebrand factor. When the platelets get sticky, when they stick together, they get activated. When they get activated they release chemicals. The chemicals they release, actually cause more platelets to stick to the platelets that are already there. So they become more sticky. More platelets stick. They become activated and they release their chemicals, and more platelets stick. It's almost like a snowball rolling down the hill, it gets bigger, and bigger, and bigger. This clot gets bigger, and bigger, and bigger. This allows for a clot to happen very quickly, so we can plug off that blood vessel and prevent blood loss. Ultimately, that helps homeostasis because we prevent blood loss and so the end point is, we stop blood loss. We have a clot. The chemicals that are released from the platelets will help with the coagulation process. That goes further- we'll talk about that later in unit 3. This is a positive feedback mechanism. We reinforce the stimulus. The stimulus was the platelets being attracted to the damage. We're going to get more platelets attracted and more release of chemicals, and more platelets attracted and more release of chemicals, and so on. Here the attraction of the platelets and the activation is amplified, until we have a full clot and the blood loss stops. Very important. Another positive feedback loop, is birth. During birth, oxytocin is released, and this stimulates uterine contractions. Before birth, the muscle increases the receptors for oxytocin. Oxytocin is binding to the muscle and now we're contracting and pushing the baby on the cervix. The stretch of the cervix and the pushing of the baby on the cervix and the stretching of it, actually there are neurons that pick up that stretch, and signal the hypothalamus to cause the release of more oxytocin, from the posterior pituitary so more oxytocin gets released, more contractions of the uterus, more pushing on the cervix, more signaling to the hypothalamus. You can imagine this is going to keep going until baby comes out. Once baby's out, there's no more pushing on the cervix no more stimulus to the hypothalamus, no more oxytocin release, until something else happens which is really important. Anybody know? Suckling- very good! Baby needs to suckle. Then we will continue some contractions. This will help with the third stage of labor, which is the release of the placenta. Can you come up with any other positive feedback mechanisms and describe how they work? Again, if you have any questions or you just want to describe the last one that we talked about a little bit, better that is fine. There's not a lot of these actually in the body. Give me a call, or email, or text, if you have problems with this. I'm here to help. This is what we're going to talk about throughout our class. It's important you understand how these work and how homeostasis is maintained. Our next lecture is going to be on medical imaging. I hope to see you there.