Classes of Physiological Function- MiraCosta Biology


Uploaded by MeerdesIrrtums on 02.03.2012

Transcript:
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 adaptations—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 fox—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 fitness—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 function—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 scale—for example you can look
specifically at the combination of the lens and retina—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 homeostasis—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 36.4-37.1°C. Unless
you are running a fever or hibernating or dead, your internal temperature stays in this
range irrespective of the outside temperature—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 overheated—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 function—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 body—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 involved—we’ll get to this later, but let’s start with the
brain—a part called the hypothalamus (which is in a totally different area than the parts
of the brain relating to vision)—the hypothalamus has the central role in thermoregulation as
well as other kinds of homeostasis—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 in—skeletal
muscle is in there as the muscle that you use to move your tow around—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 cold—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 skin—which if you think about it
is another good example of a pretty useless vestige—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 warmth—something as simple as putting
on a sweater is a response that ultimately has the effect of conserving body heat. So
look—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 behavior—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 third—this one we’ll call “meeting a body’s need for X”—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 similar: digestive 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 thing—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 here: gathering 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 calories—but later on in this module,
we’ll be talking about a couple of aspects of digestion that are primarily homeostatic
in essence—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.