Surface Area to Volume Ratio

by Jason Amores Sumpter
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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