Here we're going to talk about one of the major uses of energy in our body which is to maintain something called homeostasis. uh our physiology, the way that we function optimally, that occurs in a relatively constrained set of conditions, including temperature, uh pressure, um uh also things like, you know, like levels of certain solute, levels of of stuff in our blood, in our system, levels of water, right? All of that stays pretty consistent. we don't have dramatic changes inside our body throughout the day. Uh the values that those things tend to fluctuate around are called set points. So if we use body temperature for example, our set point, our average body temperature is 37° C. Now, we might get a little warm with a fever or we might drop a couple of of degrees, but generally we're staying around that 37 degrees C as long as our body is able to maintain homeostasis. If we are in within a set of reasonable parameters or within a reasonable set of parameters around this set point, then we are in homeostasis. And our body is generally going to respond when there are any changes to these homeostatic conditions. And that's what you're seeing on the bottom here. And we'll kind of spell out each component of that on this slide. So first is the stimulus. This is any change to our uh homeostasis, any change to our internal environment. Now that might come from an external thing like for example when it's really hot outside right that'll raise my internal body temperature and then some um uh the body my body's going to respond to that in a certain way. So the stimulus is the thing that causes a change to homeostasis. The thing in our body that uh detects that stimulus is called a sensor or a uh receptor cell. Generally a sensory receptor cell. Um uh so yeah, these are going to detect those stimuli um when they cause a change in homeostasis. Then information about that stimulus is going to be carried to our control center or our central nervous system by neurons that are part of the apherant pathway. So they carry information to the control center here which is the central nervous system. So in this example we're talking about the thermostat being a control center. So it's not our an example from our body but an example of of how our body could respond to a change in homeostasis. So here we have a uh room temperature increasing as a stimulus. There's a thermometer that's going to detect that. So that acts as our sensor here. And then that thermometer is going to send information through an aphrine pathway to our control center which in this example is a thermostat. But in our body is our central nervous system, our brain or our spinal cord. The control center then processes that information and responds appropriately to it. Here the thermostat sees that the temperature has risen above our set point which here the the thermostat is set to 22 degrees C. It detects a rise in that temperature. So it's going to respond to that through efferent pathways. This carries information away from the control center in our body. This would be peripheral neurons leaving our central nervous system going out to the rest of the tissues in our body. And so I like to remember this because apherrant A comes first before epherent E in the alphabet. Apherrant and then epherent pathway. The epherine pathway is going to travel out of the central nervous system and then target something called an aector. In this example, the aector is an air conditioner. In our bodies, an aector is going to be a muscle or a gland. It's something that is going to be instructed by the central nervous system to respond to a certain stimulus. And so it's the aector that that carries out the response. So here the aector is the air conditioner. The air conditioner produces the response which is blowing cold air and as a result of blowing cold air. We've returned back to homeostatic conditions. Okay. So often these changes to um homeostasis, our body responds to them by restoring homeostatic conditions where the response is going to stop, reduce or somehow counteract the stimulus. In the example in the previous slide, we had a rise in temperature as the stimulus and so the response was to reduce the temperature. This is an example of a negative feedback loop where the response stops, reduces or dampens or counteracts the stimulus in some way. However, our body also has positive feedback loops where the response instead will activate, increase, speed up, um, encourage that stimulus, encourage more further movement away from homeostasis. And both of these are really important for our physiology. And I'll give an example of both, a biological example of both. For negative feedback, we have blood glucose levels. Anyone who has or knows someone who has diabetes has u familiar with this is familiar with this. We have a set point of blood glucose levels, right? That's the concentration of glucose in our blood at around 90 milligrams per deciliter. When blood glucose increases, for example, when we eat a big meal, that'll cause our glucose levels in our blood to spike and go higher. When our body detects that change, there are cells that are going to detect that change in glucose. They're going to communicate with the uh central nervous system and the central nervous system is going to respond by activating uh cells in our pancreas that produce the hormone insulin. Insulin tells the body to start using up some of that glucose. It tells the liver to take some of that glucose and store it away as glycogen. and it stops our body from producing more glucose. As a result of insulin being released, our blood glucose levels go back down to our set point. People with diabetes, for one reason or another, aren't able to produce high enough levels of insulin to return to um homeostatic levels, which is why they need insulin as a drug to help uh regulate this homeostatic level. When our blood glucose levels drop, then something in our body detects that. It communicates with our central nervous system and the central nervous system is going to respond this time by activating different cells in the pancreas that instead are going to produce and secrete glucagon. Glucagon is a um hormone that's going to essentially do all the opposite things of insulin. It's going to tell the body to stop using up that energy. It's going to tell the liver to start breaking down the glycogen it has stored and it's going to tell the body to start producing more glucose. As a result, the blood glucose levels are going to increase and we'll return to homeostasis. So, in both of these examples, the response returns us to homeostatic conditions. It does the opposite of what the stimulus did here. The stimulus was a rise in blood glucose. The response was to drop blood glucose. Here the stimulus was to was a drop in blood glucose. The response increased blood glucose. With positive feedback, we are pushing further away from homeostatic conditions in order to achieve a certain goal. There aren't as many examples of positive feedback, but there are a few key ones. One of them, one of the go-to ones is during childbirth. Uh, here is the a baby in a uterus. And when it gets large enough, it starts its head starts to push on this structure here called the cervix. Just to orient you, right? Here's the uterus, cervix, birth canal, vagina, right where the the baby's going to be pushed out. the uh baby's head pushes on the cervix and as a result the tissue in that cervix is going to stretch here head of baby pushes against cerv cervix and it stretches the cervix that activates sensors in the cervix to send information to our central nervous system the brain and the brain responds by activating the pituitary gland to to create oxytocin. Oxytocin then gets carried through the bloodstream back to the uterus where it's going to stimulate contraction of the smooth muscle in that uterus. When the smooth muscle in the uterus contracts, it pushes the baby's head closer to the cervix. When the baby's head pushes against the cervix, that cervix stretches and we get another round of this feedback loop triggers the brain. The brain secretes o tells the pituitary gland to secrete oxytocin. Oxytocin gets carried to the uterus and it causes the uterus to contract which causes more stretching which causes more oxytocin which causes more contraction which causes more stretching and so on and so on and so on until the baby is finally delivered and it pushes through the cervix and that stimulus stops. So there is a homeostatic level of of oxytocin in our blood and this feedback loops loop the response is to increase that right the stimulus is stretching the cervix and by releasing oxytocin we end up stretching the cervix more. So that's why this is a positive feedback loop because we continue to move further and further away from homeostasis. Whereas a negative feedback loop we want to restore homeostasis. Uh we're going to stop there and then the next recording I think it'll be the last one um we're going to talk about uh some like basic anatomical principles um that are important kind of like talk about like the language of anatomy and physiology a little bit.