Hi everybody, Dr. Mike here. I think it's time I let you in on a little secret. Every single day the environment is trying to kill you. Whether that be extreme changes in temperature or the availability of nutrients or little microbes infecting you via cuts or scrapes in your skin, that environment is trying to kill you.
Luckily for us, we have a body that can respond to these changes in the environment to make sure that we maintain a stable internal. environment. And it's this response that the body has to these drastic changes in the environment that we term homeostasis. Now homeostasis means similar balance. Now what does that mean?
Well I want you to think about this. Pick any function of the body. It could be blood pressure, it could be blood glucose management, pH balance, it could be carbon dioxide levels, it could be temperature regulation, doesn't matter.
Pick any physiological function and you'll find that they all have this happy healthy range that they like to work within. So let me just quickly draw that up. There's always a range in which there's upper bounds and lower bounds, upper boundaries, lower boundaries, and it needs to stay within this in order for you to be happy and healthy. If it goes a little bit too high above the upper bounds then you don't just get unhappy but you get sick.
And likewise, if it goes too low, you don't just get unhappy, you get sick as well. Homeostasis is the body trying to maintain this happy, healthy balance of its physiological range. Now, some ranges can be quite wide, like blood pressure.
It has higher boundaries and lower boundaries that are quite wide, but some are quite narrow, like blood pH and temperature regulation. But regardless... The homeo in homeostasis is referring to the fact that there is a range you can sit within.
If there was a single value, like let's just say for core temperature, regulating your core temperature, and you know that that's at around about 37 degrees Celsius, right? It's not just one value that you need to maintain. It's not just 37 degrees and you can't go either side of that. If that was the case, it would be called homostasis, but it's homeostasis and you can go up by around about 0.5 degrees and you can go down by around about 0.5 degrees celsius before you start to notice ill effects or again you start to get sick.
So let's talk about homeostasis because it is the most important concept in anatomy physiology and medicine really is and you need to understand it. So when you open up a textbook and you look at homeostasis you'll find that regardless of the physiological function you're referring to there's always the same components that keep coming up, right? And some textbooks say there's three components of homeostasis. Some say there's five, some say there's six. We're going to go through all of them because they're all important.
Let's take the example of trying to regulate our internal body temperature. And like I said to you before, it's around about 37 degrees, the internal core body temperature, and you can go up by around about 0.5 degrees Celsius and down by around about 0.5 degrees Celsius. And that is your happy, healthy. range.
But let's just say you decide to spend the day out in the sun. Now we know that the sun is hot and the sun is going to increase or try to increase your body temperature. This is what we call the stimulus.
So there is always a stimulus when we talk about homeostasis. That is the first part, the first component of homeostasis. What is stimulus? Stimulus is the change in the environment. That's homeostasis.
That's the stimulus. That's the change in the environment. All right.
Here, the temperature is going up. Increased temperature. Now, your body can't respond to these changes in the environment, these stimuli for plural, unless you have something that can pick it up and measure it. So this is a receptor.
So the second aspect is that the stimulus must get picked up by a receptor. And this is a super important concept because without a receptor, you cannot pick up the change in the environment. Now, there are some things like UV light that we don't have receptors for.
We have receptors for pretty much everything, right? Temperature, mechanical pressure, even just other types of pressure in the environment. chemicals, all these different types of receptors, right?
There's so many different types, but we don't have receptors for UV light. And therefore it's really hard to detect when our skin is getting burnt by UV light and damage. So our body can't respond. So we go from playing in the sun to getting a skin burn, right? But here we're talking about temperature and the temperature goes up.
That's the stimulus. The receptor that picks this up is going to be a thermoreceptor. Thermo meaning temperature. So the receptor picks up the change.
It picks up the change. which is the stimulus. Now I said there's different types of receptors, temperature receptors, pressure receptors, mechanical receptors, chemical receptors and so forth. What needs to happen is the receptor needs to what we call transduce.
It needs to turn that stimulus whatever it may be into an electrical chemical, a signal. So the receptor picks up the change, transduces it and then needs to send it somewhere and it sends it either via nerves or through Generally, the endocrine system. So either through the nervous system or endocrine system.
And where's it sending it to? Well, it's sending it to the third part of homeostasis, which is the control center. So there's always going to be a part of the body that takes the information sent by the receptor and decides what we need to do with it. All right. Now.
Remember the stimulus here is an increase in temperature. That's the stimulus The receptor picked it up, sent it to the control center, and the control center decides, what's going on? Well, the temperature's gone up.
It's going above that 37 degrees. That's not where I want to go. So I need to bring it back down. What can I do to bring the temperature back down?
I know I can send a signal to the sweat glands in my skin to sweat. And if sweat then accumulates on the surface of my skin, when a breeze goes past through the process of convection it can take that sweat and heat away from the body cooling us down. That's what the control center decided to do so it goes alright you know what to do that I need to send the signal to sweat glands and that's what the control center does.
It's decided what it needs to do and it sent it off to the place or the area that makes the change and this is what we call the effector. The effector has the effect it makes the effect. So the control center decides what needs to be done. It's the area that decides.
Sends it to the effector. And in this case, the effector are sweat glands. But depending on the situation or the physiological function we're referring to, the effector, it can be cells, glands, muscles, right?
Can be a whole range of things. But in this case, it's sweat glands. Now, what was the effect?
The effect was to sweat. And what did this end up doing, right? What did this end up doing?
Let's have a look. The effector allowing us to sweat resulted in a drop in temperature, which is the opposite of the stimulus. But that's what we wanted, right?
When it went too high, we brought it back down. It brought it back down. So it did the opposite of the stimulus. It negated the stimulus.
It's negative feedback. And this is what we call negative feedback. This is one type of homeostasis, which is negative feedback.
Really, really important. The most common type of homeostasis to try and bring things back. So what does the effector do? It negates the stimulus. And so you can see here we've got four important parts of homeostasis, but I said there can be six.
So what are the other two that we need to add in here? Well, when the receptor sends the signal to the control center, it sends it like I said through either the nervous system or the endocrine system generally. This is what we call an afferent signal.
Afferent. When the control center has decided what it wants to do and needs to send it out to the effector, this is called an efferent signal. Now the way you remember...
F, efferent, goes to the effector, right? It's moving away from the control center. Efferent's going towards the control center.
So these are two additional aspects of homeostasis, the efferent and the efferent signals going to the control center and away from the control center. Now, we didn't say what the control center was here in temperature regulation. It's the brain. Specifically, it's a part of the brain called the hypothalamus that can deal with temperature control and sends that signal out.
And again, negative feedback. Now, you can look at this and think about what happens in the opposing scenario. What if it's not a sunny day, right? What if it is a cold day? Now, this is me trying to draw a snowflake, which is a Not the easiest thing for me to do.
And I suppose a snowflake, how many sides does a snowflake have? It's a good question, and I'm not sure. Anyway, it's cold. So, the temperature drops.
So you're now going below that 37 degrees. down here to where you're starting to get unhappy, unwell. That's the stimulus drop in temperature. Receptor, still a thermoreceptor, picks up the change, sends it via an afferent signal to the control center and it says what do I need to do?
Well it's cold I need to bring the temperature up. How can I do this? I can shiver.
I can contract and relax, contract and relax my muscles. That's what shivering does. Why?
Because when you contract, relax, contract, relax muscles, it generates heat. Wow, that's perfect. The effector here are the muscles now. And... What it did was increase temperature.
That's still negative feedback because have a look, the effector negated or did the opposite of the stimulus. So it's still negative feedback. So that's perfect.
Now the last example. talk about is positive feedback. So positive feedback is a tricky one. There's not a huge amount of examples, but there are some that we can refer to. And probably the most important is that of the process of labor, giving birth.
Now all these things are the same. There's still these components. So if we start with the stimulus, think about this.
It's time for mum to deliver the baby. Now that baby is sitting in the uterus and needs to move past the cervix. Now as this is happening, right, as the baby starts to move through, that big old noggin that the baby has starts to stretch the cervix.
Okay, the stimulus is stretch. It's stretching the cervix. This is picked up by receptors in the cervix itself.
It sends an afferent signal to the control center, again, the hypothalamus. And the hypothalamus... goes, oh, what's going on here? The cervix is stretching because bub wants to come out. What do I do here?
I know, I need to help bub come out. So I'm going to release a chemical that tells the uterus that the bub's sitting in to contract. And if it contracts, it helps to push bub out.
So the hypothalamus says, all right, I'm going to release oxytocin. Oxytocin is a hormone. This hormone is in the posterior pituitary gland. It gets released, moves through the bloodstream via the, so that's the efferent signal, and it goes to the muscles of the uterus.
And what does the uterus do? The uterus contracts. Now, when the uterus contracts, it pushes Bub. What do you think that means? Bub's head starts to push even harder on the cervix.
and continues to stretch. So the stimulus here is stretch. What do you think the outcome? The outcome increases the stretch.
So in negative feedback, it would have done the opposite. But here in positive feedback, the effector, the outcome exacerbates, it amplifies, it reinforces the stimulus. And this is positive feedback.
Now you might be thinking, well, what's the outcome here? Well, this whole thing continues. Because the uterus contracted to push bubs head against the cervix and it further stretched, further stimulated, further released oxytocin, more uterine contractions, more stretching, this whole thing happens until bubs out. Then when bubs out, there's no more stimulus and the whole thing stops.
And that's a perfect example of positive feedback. So what have we learned today? That homeostasis is the body trying to maintain some internal balance, internal stability. Doesn't want to go outside the upper and lower limits, but there's a range that it sits within which is different for all functions.
But if it does go too high or too low, the body wants to respond to try and bring it back into its appropriate range. And to do that, it can use negative feedback, for example. And all the same components happen, work. within homeostasis regardless of negative or positive feedback. They are the stimulus, the receptor.
the afferent signal, the control center, the efferent signal and the effector. In negative feedback the outcome of the effector is to negate or do the opposite of the stimulus. In positive feedback the outcome is to exacerbate or amplify the stimulus. Hi everyone Dr Mike here if you like our videos please hit subscribe and leave a comment.