In this video we're going to think about some of the general themes that we can associate with physiological function. Remember that earlier we came to understand that it's natural selection that shapes an organism's specific adaptationsโ€”including all of the anatomical structures that we might be seeing or studying on an animal. The details of a particular structure on an animal have come about because of the way they have positively contributed to the animal's long and unbroken history of success in surviving and reproducing. Let's say you're studying a wild foxโ€”the desert kit fox in southeastern California. The reason that fox is here at all is because each of its ancestors was successful in surviving and reproducing, and their specific phenotypic traits contributed to this success. We refer to this type of success generally as Darwinian fitness, and there are several major components of fitnessโ€”things that are directly and undeniably related to survival and reproduction. Now in the context of this lesson in thinking very generally about physiology, you can identify three major elements of fitness and call them general themes in physiology. One of these major components is sensing the external world. An animal's success depends on its ability to gather information about its world because having such information is useful and enhances the animal's likelihood of survival and reproductive success. In the previous video we talked a lot about the eye and how vision represented a pretty general and over-riding physiological functionโ€”a classic example of a function in the class of gathering information about the external world. It's because vision is important for fitness that the eye as a whole is maintained and kept from degenerating due to the effects of mutations. It's also possible to think about vision at a smaller scaleโ€”for example you can look specifically at the combination of the lens and retinaโ€”this represents an anatomical structure that suits the specific function of image projection, which is part of the overall task of vision. You could go to smaller and smaller scales and think about smaller and smaller subsets of physiological function, yet all these could be classified under the general umbrella of relating to vision and the animal's success in gathering visual-based information about its environment. A second of these general classes of physiological functionality is that of homeostasisโ€”the maintenance of a relatively stable internal environment despite fluctuations in external conditions. The most obvious example of this is thermoregulation in a warm-blooded animal like yourself. Your body temperature remains within a narrow range from 36.4-37.1ยฐC. Unless you are running a fever or hibernating or dead, your internal temperature stays in this range irrespective of the outside temperatureโ€”you have warming mechanisms to heat up your tissues and minimize heat loss when the outside temperature is cooler than your body (which for most of us is most of the time), and you have cooling mechanisms to reduce your temperature if you get overheatedโ€”either because it's too hot or your muscles have generated an overage of heat through exertion. You shouldn't have to think too hard to understand why this is a pretty important physiological functionโ€”you know about enzymes, and so you can see the advantage of maintaining a steady body temperature that suits all the enzymes and other proteins of your cells, even when conditions on the outside are too hot or too cold. And as we now know, anything that is this important for your general success in surviving and reproducing is very likely going to be associated with specific anatomical structures that make this function happen. . . So where do the organs involved in thermoregulation reside in the body? Well thermoregulation is different from something like vision in that it's not anatomically restricted to a localized part of the bodyโ€”vision is taken care of by the eyes and certain portions of the brain that process information from the optic nerves. Your ability to see doesn't involve your stomach or your right big toe. In the case of thermoregulation, however, your right big toe is involvedโ€”we'll get to this later, but let's start with the brainโ€”a part called the hypothalamus (which is in a totally different area than the parts of the brain relating to vision)โ€”the hypothalamus has the central role in thermoregulation as well as other kinds of homeostasisโ€”when it comes to body temperature it's your internal thermostat. It senses slight changes in blood temperature and causes appropriate responses in different parts of the body. If the body gets too cold, the brain activates skeletal muscle which generates heat. Here's one place where your big toe comes inโ€”skeletal muscle is in there as the muscle that you use to move your tow aroundโ€”it's this same muscle tissue that starts to work a little harder without causing movement that releases energy in the form of heat. At the same time, arterioles supplying blood to skin capillaries will constrict, and this has the effect of dramatically reducing the blood flow to the skin surface, and as a result your skin gets coldโ€”which is a good thing because you lose a lot less heat this way than when your skin is warm and flush with blood. The same thing happens on a larger scale in all of your extremities, so when your toes get cold it's almost always going to be because your body is activating this mechanism that conserves your body's heat. If you're a furry animal there's mini-musculature in your skin will make the body hair stand up tall, increasing the thickness of insulation. Humans, by the way, actually still have this musculature despite our lack of body hair, and goosebumps are the result of activating the erector pili musculature in our skinโ€”which if you think about it is another good example of a pretty useless vestigeโ€”what is the function of these erector pili muscles if there is no body hair to make stand up? On top of all this, there are also behavioral responses that go along with conserving body warmthโ€”something as simple as putting on a sweater is a response that ultimately has the effect of conserving body heat. So lookโ€”this homeostatic function of thermoregulation is a multi-faceted affair, involving many different parts of the body the brain, the skeletal muscles, the blood vessels, the skin, and your overall behaviorโ€”it's really a team effort. So we've now talked about two general classes of physiological function and we're ready to move on to a thirdโ€”this one we'll call meeting a body's need for Xโ€”something that is gotten through exchange with the outside world. Earlier we talked about meeting the body's need for oxygen and the exchange of gases with the air that occurs at lung surfaces. The remainder of this module will focus on something that is fundamentally simila igestive anatomy and physiology and this is similar to gas exchange in that it's how animals meet their daily need for calories and other nutrients. One more thingโ€”I'd like you to think about each of the specific physiological functions that we talk about in this unit as being part of one or another of these classes of functionalities that we've talked about her athering information about the external world, homeostasis, or meeting the body's need for X. Digestion is mostly going to associate with the meeting the body's need for X, where X is nutrients and caloriesโ€”but later on in this module, we'll be talking about a couple of aspects of digestion that are primarily homeostatic in essenceโ€”the first of these is the maintenance of a relatively steady level of blood sugar despite the fluctuating availability of nutrients from the food we eat. The second is the maintenance of a relatively steady body weight.