Metabolism and Homeostasis

hi. In this video, we'll be talking about two very important aspect of animal physiology. Those are metabolism and homo Stasis. Now, before we get there, we need thio. Understand some of the constraining factors on the animal form. Now, body size and functions are always going to be constrained by physics. For example, larger animals will weigh mawr and therefore require thicker skeletons to support that weight and also bigger muscles to move that weight around. Now, one of my favorite examples of how physics can constrain the body size of animals has to do with terrifying giant insects. Believe it or not, millions and millions of years ago, there used to be some really scary big bugs out there. And fortunately for us, there are not today insects, in fact basically having much smaller maximum size than they did way back in prehistoric times when there were, you know, just terrifyingly large insects. The reason for this, it's believed, has to do with the amount of oxygen in the atmosphere. There's, ah limit to how much oxygen condition fuse into a new organism. We'll get into the details of all that when we talk about respiration, but you know. Just know that there's a physical limit on, uh, the availability of those gasses to defuse into tissues. Now, back in these prehistoric times, there was a lot more oxygen in the atmosphere, which allowed for organisms like insects to grow larger than they can today because there's less oxygen in the atmosphere. Today, there's a smaller upper limit on bug size, so never gonna have to worry about that scenario. Fortunately, now what this kind of gets into is this very important idea of surface area to volume ratio, which essentially determines the physiology of an animal. And it sells now. The reason for this is as organisms get bigger, this ratio of surface area to volume actually decreases. And we can see a nice example of that in this graph here that looks at area on the Y axis and volume on the X axis. Now, as you can see, a czar shapes get larger. If you know you look at the line from one shape, so to simulate a cell, let's just look at the ball for argument's sake. Eso. As this ball gets bigger, you can see that as it gets bigger. The line of its area versus volume curves and it actually increases in volume at a faster rate, then it's area. So what does this mean? This means that as animals get bigger, they get are they have less surface area compared to their volume. And this comes into play with ideas like molecular diffusion, right. The more surface area you have, the more efficient your diffusion will be. It also relates to nutrient use and heat loss It. Organisms that are smaller basically use relatively mawr energy compared Thio. Organisms that are larger will look at that in just a moment. Another way to think of this is that smaller organisms will actually lose more heat to the environment relative to their larger counterparts. And this is going to have a number of implications in terms of metabolism. Now, one way that animals have our one strategy, animals have found to increase surface area is by flattening, folding and branching structures. Thio essentially give them more surface area. A lovely example is the human brain right here on day, and the brains of many organisms will show similar features, though some organisms don't have all of these folds that you can see in the brain. So here in this image, we're looking at a side view of the brain that's been cut in half. So we're seeing basically the center of the brain, and you can see all these spaghetti folds in the brain tissue that actually are increasing its surface area. And here we've cut the brain in half. According to this image here, we basically took a slice through the middle and are now looking at it head on, and you can see that there's tons of folds, which are called Sulka and gyre I. If you're curious, there's lots of folds in the brain that increased surface area. This folding can also be seen in the intestine. This is super important for digestion. Let me actually hop out of the way here now the surface of the intestine, As you can see here, it has the tissue folded around. It's around itself to create additional surface area. In addition to that, the surface of the tissue is lined in these structures, called villi, that air little projections that come out of the folds, further increasing the surface area. And if that weren't enough, there's also Micro Valli at the surface of each of these cells that make up the Vialli. These, um, in terror sites, these cells have what's sometimes referred to as a brush border, basically little hair like projections that cover them. So, in a sense, this is like triple compounded surface area increase right. We fold the tissue in the intestine that folded tissue is covered in these ville I projections And those ville high projections Aaron turn covered in these little hairs called Micro Valli. They're not actually hairs their hair like objects that don't wanna don't confuse you there. Now branching is another strategy that we conceive very nicely in our vascular system. So by branching are vasculature, we can create tons of additional surface area, which is going to be really important for exchange with tissues. And here you can see how the branching of vasculature looks in a hand. We have some, you know, thick arteries, and they branch into much smaller what we'll talk about later. Capital Aries that allow for, uh, you know, much more efficient diffusion with the tissues. So with that, let's turn the page

energy is an essential ingredient for life. In fact, it's so important. There's a whole field of study dedicated to examining how energy moves through living systems and especially looking at how energy flow is related to an animal size and its metabolism. Now metabolism is through the some of the chemical processes of an organism that sustain its life. A great example of one of these processes is cellular respiration, which is, of course, the process. Oops process by which, uh, cells will break down carbohydrates from proteins, sugars and fats and convert it into energy in the form of ATP. Through that amazing process known as oxidative phosphor relation, go check out the videos on cellular respiration. If you want a refresher, want to Nome or about this process. Now metabolism is often conceived of in terms of a metabolic rate were a rate of energy consumption, and there's different ways to measure metabolic rates. But I think this chart is super neat example of, uh, metabolic rate, a specific type that we Noah's basal metabolic rate, which is basically the minimum rate of energy consumption of an endo therm at rest. Now, basically, at rest means you know, not exerting themselves physically not stressed out. So just like, you know, chilling out, kind of like what it sounds like. They're just relax and chilling out maxing relaxing, all cool, uh, and a therm, of course, that Z organisms like us that generate our body heat from internal processes. And what's so nifty about this chart is you can see how different types of food we eat actually sustain our energy in different ways. Carbohydrates, as you can see, give us a nice initial boost of energy. But they kind of don't last right. They plummet after a while. Proteins, on the other hand, don't give us as quick on initial boost of energy, but you can see that they sustain us much longer. Right? That curve goes way above the carbohydrate curve. They provide us with more long term energy. Now, basal metabolic rate is looking at endo therms. There's another measure we know a standard metabolic rate that is the minimum rate of energy consumption. Oven ecto therm. A rest and an echo therm, you may recall, is an organism that absorbs most of their body heat from an external source. It doesn't mean they can't generate any heat internally. It's just that, uh, their main source of it is coming from outside. Now, when you start comparing metabolisms of different animals, some really cool patterns come out. One of the more obvious ones is that warm blooded organisms are going to have higher metabolic rates, fan cold blooded organisms. And hopefully that comes as no surprise, warm blooded organisms. And you know, these terms warm blooded, cold blooded, very imprecise, kind of like common terms we throw around, we'll talk about technicalities and of all of that in a different lesson. So don't stress now. It's just, you know, warm blooded, cold blooded. You know, uh, you know, we can think of this in layman's terms for the sake of the explanation. So anyways, warmblooded organisms have to consume energy. Thio warm their bodies, whereas cold blooded organisms are going to be again absorbing. Most, uh, are going to be, um, well absorbing heat for their bodies, but also not expending a ton of energy to warm their bodies. There's others, uh, sort of strategies in there, too. But the main point is that these guys are consuming energy and therefore adding to their metabolic rate in order to heat themselves, and these guys aren't so much. And hopefully it comes as no surprise that uni cellular organisms which, of course they're very simplistic, have smaller energy requirements than these multi cellular organisms and therefore will have even lower metabolic rates. That's what these lines are showing us the metabolic rate increase of these particular types of organisms. Now, a really interesting pattern to note comes out when you compare larger animals and smaller animals. So looking at an elephant and a mouse, obviously obviously the elephants is far larger than a mouse. And in terms of, you know, total like tonnage of energy, sheer quantity of energy, they need more, obviously, right that they're much bigger organisms. They have much larger muscles that need to be powered. So of course, they're going to require more energy, then a little mouse. Here's the thing when you look at their metabolic rates compared to their body size, though, so essentially you find the relative metabolic rate of metabolic rate. Um, you know, look sorry when you when you look at the metabolic rate relative thio the body size of the organism. What you see is that smaller animals like the mouse will actually have a larger relative metabolic rate than a larger animal like an elefant, essentially like pound for pound. The elephants metabolic rate is lower than the mouse's, and this again has to do with those patterns of surface area to volume. Right. Larger organisms are going to be less prone to heat loss, for one thing than smaller organisms. So smaller organisms we're gonna have to dedicate a greater percentage of their metabolic rate to warming themselves. Just lots of interesting patterns and things to look at. When you start delving into the comparisons between energy use of different animals. With that, let's flip the page.

circadian rhythms are those daily cycles that result in regular physiological and metabolic fluctuations. One of the most famous and highly studied is the fluctuation between cortisol and melatonin over the course of the day and the night. Cortisol is the main stress hormone, and as you can see, it's concentration in the body. That's what this axis is supposed to be, you know? Think of it as like concentration of thes hormones. You know, I'm sorry for using a graph that doesn't have labeled access. I know that's a real no no, but it's a pretty picture. So the point is that cortisol levels shoot up right? As you're about to wake up and they peak, uh, you know, early in the day and then steadily drop over the course of the day and into the night where sort of in the middle of sleeping, they start to increase again in preparation for the next morning. Now, court is all, as I said, is the main stress hormone, and it would make sense that you would want high levels of stress or alertness. You could think of it as early in the day when you're getting up when you have Thio, you know, look for food. Blah, blah, blah. You know, of course, we just go to the fridge these days. But, you know, we used tohave toe actually struggle to get our food. Anyhow, melatonin is almost like a counter to that in a really interesting way. See, melatonin promotes sleep and sleepiness. And as you can see, while cortisol is shooting up in the morning right here melatonin is actually chilling out, right? Melatonin levels drop precipitously, right? As you are about to wake up. And that's, you know, in part so that you don't feel super groggy in the morning. So don't be, You know, don't be thinking that melatonin is the main thing that makes you groggy in the morning. There's actually a lot of other stuff going on there. Uh, it's it's just related to sleepiness and sleep. Now it levels of melatonin stay low throughout the day, right? You don't wanna be sleeping during the day, But as night time sets in, as you should be getting to bed right as a, you know, an organism that's not living in the age of electricity and works based off a night day night cycle those levels shoot up to make you sleepy, right? So you go to bed, you have a nice good night's sleep, and right when you're about to wake up, all that melatonin dries up so that you're not all sleepy and morning and, you know at the same time your cortisol is popping up in order to make you nice and alert when you wake up. Now these air just daily cycles. But organisms can show interesting fluctuations in their metabolism and physiology over longer terms. You've probably heard of hibernation, but a lot of animals also, uh, do what's called tor poor or experienced what's called tor poor, I should say, which is a short term state of decreased physiological activity and metabolic rate. It's not as long as hibernation, but you can think of it working to the same effect, essentially hibernation. Of course, I'm sure you're familiar with animals fattening up before winter, where they'll go to sleep for a really long time and wake up when it's spring again, You know, so that they can kind of wait out the winter when there's not a lot of food and conserve energy. Well, that is not actually a sleep. You know its hibernation. It's it's not a long nap. It's an actual state of depressed metabolic activity. And it's something that's specific to end of therms. Right? We need lots of energy on on the daily in order to sustain ourselves. And when. Energy in the environment food is really scarce like in the winter. This is a nice way Thio get us so that we can live until the spring and then wake up and start to eat again. Wake up right now it z you know hibernation is going to be again like tor poor but on a longer term basis. Now organisms can't just let their metabolic, their metabolic physiologic processes run wild. They have to be very tightly controlled and maintained. Regulation of physiological processes is super important in order. Thio stay alive because you know things are changing around us. Things were changing within us, and our bodies need to be able to cope and to maintain ideal conditions for our survival. So this regulation of physiological properties eyes called homo Stasis and a great example of why this is so important is, for example, enzymes, right, Those proteins that basically do everything in ourselves that they make the magic of life happen. In a large part, they function best in very specific physiological conditions. Um, and in fact, proteins are very sensitive to temperature changes and changes in pH. And if you can't maintain these specific conditions for enzymes, they can actually cease functioning, which could obviously be very dangerous and potentially lead to death. That's, of course, just one example. There's other reasons why we need to maintain homeostasis now. There's kind of like two strategies that animals will take when they are trying to maintain, um, you know, their internal environments basically. And those two strategies are confirmation, like being a conform ER and conformers don't actively regulate what's going on. Instead, they'll kind of conform to their environmental conditions. They it's more like making do with what's around them instead of trying to fight against it. Like regulators, which actively control their internal environment, Uh, regardless of what fluctuations are occurring in the external environment. So one way you could think of this is in terms of body temperature. You know you'll have fluctuations in environmental temperature, and conformers will just kind of go along with that. They'll let their body temperature fluctuate mawr with the environment. Whereas regulators will, uh, you know, if it gets colder, for example, burn more energy to maintain that desired body temperature. So they're kind of fighting against what's happening in the environment instead of being like Zen with it. Now, homie, a static systems, uh, are often conceived of as having, um, certain properties. And we're gonna talk about those properties in a very generic way right now. And depending on who your professor is or you know what book you look at, they might use different terms here. So if you see different terms, come up in your course or something, don't worry. It's It's the same idea, really thes air, just generic terms. So, you know, don't worry about necessarily memorizing these names just kind of understand the ideas. That's that's what's really important. So a homo static system will be based off of a set point. This is kind of like the temperature that you set your thermostat to write in your home. If you have a thermostat, you say, like Okay, I want my house to be 70 degrees now. Obviously your house isn't going to be exactly 70 degrees all the time. That's the set point that your heating system is trying. Thio Get to write. It might go a little above sometimes and then compensate and go a little below and Mac forth back forth. It's It's the ideal point in the system. Now a sensor is going to detect stimuli related Thio the property of the home yo static system s O, for example, if we're talking about temperature, there will be, uh, sensors that will pick up on body temperature cues. Now, it's not always as direct as that. For example, the sensors in your brain that look at oxygen concentrations in your blood actually detect pH right there looking for something, and they detect a property that's related to that. So it's it's not always, you know, so directly connected. But the point is, they're looking at some particular property and using some sort of stimulus to keep track of that property. Now the integrator is going to evaluate the sensory information that comes in and determine the appropriate response. This is going to be like the, you know, little, uh, wiring system in the thermostat that goes, Oh, I'm detecting that it's two degrees colder than the optimal temperature. And so here's what I need to dio now. Lastly, you have the effect, er, which is the thing that actually generates a response to restore the Homo static system to ideal conditions. And if I jump out of the way here, as you can see, we have a nice example of body temperature behind me. You know, body temperature usually want to keep it around 37 degrees Celsius. You have cells in your skin and your brain that can detect temperature, and you have a A. You know, you could think of it a za ah regulatory center in the brain for temperature, and that's going to decide what to do based on the information coming in from these sensors and the response, Let's say that it z getting a little too hot. Body temperature exceeds 37 degrees, so it's a little too hot. Well, you're gonna want to sweat, right? So it's going to stimulate those sweat glands throughout the body to secrete sweat, which will evaporate and cool you down. So that's just a nice generic example of a homo static system, and we will be looking into some more specific examples as we examine different physiological systems in the animal body. With that, let's turn the page.

regulation is super important. Thio all metabolic and physiological processes. One of the best strategies for regulating is known as negative feedback. And this is a type of regulation where the output of a system will actually reduce the systems output. Now, that's kind of ah, confusing general way toe put it. So let me give you an example. That kind of makes this a lot more clear. Here we are looking at like Hollis ISS, which is the first step of cellular respiration and a super important process. No matter what type of biology you're doing now, you don't need to worry about memorizing any of these names or technicalities. You'll have plenty of time for that. If and when you take biochemistry. I just want you to get a sense of how negative feedback works. So, like Hollis is begins with glucose and you wind up with pyro of it. In the process, you produce Sameh tp right now. Like Collis, this is the first step of cellular respiration, which ultimately results in the production of a lot of a teepee through oxidative phosphor elation. So a teepee is not only a direct product of like Hollis iss It's also the major downstream like endgame product that this whole process is gunning for. So, like Hollis ISS is under, uh, negative feedback control. The way this works is one of the enzymes that catalyze is a reaction very early in the process. It's a very important step of the reaction for reasons that you don't need to worry about these enzymes called phosphoric geekiness, and it is negatively regulated by ATP. So ATP will feedback and shut off fossil for tackiness, shutting down this chemical pathway. So essentially, if there's too much ATP being produced either directly from like Hollis ISS. But more likely, you know, through the downstream cellular respiration. Oxidative sorry, the downstream oxidative phosphor elation process. Um, if there's too much a teepee being produced, it's going to negatively feedback and shut off the very beginning of this whole process to conserve Resource is, you know, not waste energy and just maintain the balance of generating Justus much 80 p as is needed so you can see how powerful and eloquent a system negative feedback is where ah systems output will actually reduce this output of the system in order to control the levels in a nice passive way. Now. Positive feedback is very much so. A different beast, and it's also a lot more rare to see. And the reason for that is because with positive feedback, the output of a system actually increases the systems output, right? So the most common example of positive feedback is in birth, where the infants head, uh, pushes and sets off some receptors that send a signal which induce greater labor contractions, which in turn are going to cause the infants head to push harder against those receptors, which, of course, means more labor contractions. And so these two things just upped the ante and feed back positively on each other, creating a bigger and bigger effect. So you could see why something like that you wouldn't want to use in a lot of systems. It could very easily get out of control, which is why negative feedback is everywhere, and positive feedback is a lot less common now to look at a example of negative feedback that involves actual, uh, systems in the body. Want to take a look at something known as the HP a axis? Now this is going to involve the nervous system and the endocrine system. The nervous system is going to be a A system responsible for transmitting information throughout the body, as well as receiving information from the body and the environment. It's going to transmit the signals via nerves through those electric signals Act called action potentials. Um, if you wanna know more about this, check out the nervous system videos. The endocrine system is also a signaling system, but it functions differently than the nervous system. The endocrine system is a hormone signaling system, so it's going to involve glands that secrete hormones into the bloodstream. And those hormones are going to target and set off reactions at various organs that have their appropriate receptors. So both of these air signaling systems and they're actually connected by this really cool brain structure called the hypothalamus, which basically just means underneath the thalamus, which is where it's located. So very creative naming here. Ah, this structure coordinates the autonomic nervous system, which is going to be the part of the nervous system that we don't have direct control over right things like breathing heart rate, that sort of stuff we don't have direct conscious control over is what I mean, you obviously are. Our hypothalamus is controlling that. So we have We have control. We don't have conscious control. It's not like the part of the nervous system where I can say all right, Finger wants you to poke and move or whatever. So the hypothalamus links the nervous and endocrine system by coordinating the autonomic nervous system. And also by coordinating the pituitary gland, which is a very important gland in the endocrine system and in it has a variety of functions. I don't want to get ahead. I don't wanna get ahead of myself because I could go off on tangents and all of this forever. So here's the important thing to note you have the hypothalamus that is a brain structure, right? And in the h p A axis, which stands for hype of phylum IQ, pituitary adrenal access, right, H p. A. Um, essentially, what you have is a stress hormone, uh, you know, signaling system so the hypothalamus can release something called Kordic Atropine releasing hormone. Don't worry about these names just now. This will stimulate the pituitary, the P in the h p A to release, uh, a C T. H or a Drina Adrenal cortical Tropic hormone. Again like don't worry about these names. Thes air. Just stress hormones. That's all you need to know that will eventually lead the adrenal cortex to secrete and let me jump out of the way here. Cortisol, which is that main stress hormone. Now the thing about cortisol is it actually feeds back negatively to the pituitary and the hypothalamus. As you can see here court, this is standing for cortisol. We'll actually have a negative feedback effect on the hypothalamus and the pituitary to cause them to stop releasing quarter trope in releasing hormone and adrenal cortical tropic releasing hormone. Or, as it's much easier to say, CRH and th essentially the downstream output of that system. Cortisol will go back to earlier points in the system and cause them to shut down that pathway again. This is known as the HP access and is a really nice example of negative feedback regulation. That's all I have for this video. I'll see you guys next time