Structure and Function Miracosta Biology


Uploaded by MeerdesIrrtums on 11.02.2012

Transcript:
When it comes to the kind of biology we’ll be talking about from here to the end of the
semester—basically a survey of animal organ systems—there are really three pieces that
have to fit together in order for you to have the complete picture of whatever it is we’re
talking about… it might be a lung or a kidney or the tiny units within lungs—let’s zoom
in on the alveoli--these are tiny spaces in close contact with tiny blood vessels called
capillaries that you’d need a microscope in order to see—or in the case of kidneys,
we might be talking about the whole-kidney level of things—think in terms of a wad
of roughly ? pound of organ meat that fits nicely in the palm of your hand that is a
critically important organ for your physiology. It’s composed of millions of various microscopic
functional units which include bizarre little blood vessels known as glomeruli, which is
where blood gets pressure-filtered in a manner very similar to espresso coffee. In thinking
about organs and their functional units, you will need to zoom in and out from macro to
micro quite often, but this isn’t the big challenge for most students.
I mentioned before that there were three pieces--the first piece I want you to have is pretty straightforward
and obvious and I call this the “structure” part—largely a descriptive understanding
of the actual physical or anatomical entity itself. You’ll need to develop a good picture
on both the macro and micro scales, and it’s entirely possible for a student to get bogged
down in just learning the details of anatomical structure –maybe even reaching the point
where you are so focused on the microscopic details of what the parts are and how they
fit together that you actually lose sight of the significance of that structure—and
this would be no good. You can study a lung to death and this would be useless unless
you take into account what the lung does for you as an organism, and here is where the
next piece fits in.
So really, the only reason why we would even care about detailed descriptions of anatomical
structures is their significance in physiological function. Function is the second piece—it’s
whatever has to happen—by virtue of the structure—physiologically in order for the
body to be successful in living. The lung, for example, has an enormous interior surface
area—an expanse equivalent to about one side of a singles tennis court—where the
air space comes into close proximity with the blood running through tiny pulmonary capillaries.
Knowing this fact would be meaningless if it were not that this surface area were a
feature of lung anatomy that’s totally crucial for providing the gas exchange needed for
the aerobic metabolism responsible for the bazillion ATP molecules that we produce every
second.
You see, I want you to understand the connection between the physiological function of meeting
the body’s need for gas exchange and the anatomical structural feature of the lung’s
surface area. The anatomy is the means by which the physiological function is achieved.
Structure is anatomy. Function is physiology. The two are interwoven and interdependent.
Some people will hear this and ask if there is one that is more important than the other—does
structure drive function, or is it the other way around? Which is the dog and which is
the tail? On first glance you might conclude that structure is in the driver’s seat.
Let’s take the example of one particular aspect of the human lung: there is about 70
m2 of surface area—about the same area as one side of a singles tennis court—this
would be an important aspect of your lungs’ structure, and you could argue that this is
what is responsible for the function of meeting your body’s need for gas exchange. So maybe
it’s structure that drives function. But there’s another way of looking at this.
Ask yourself: “how does the structure come about in the first place?” In the case of
the interior surface area of the lungs, why have a singles tennis court’s worth of surface
area in the lungs and not a doubles court? or a soccer pitch? Or why have an interior
surface area (i.e., a lung) at all? The human lung and its surface area—like all important
anatomical structures in all animals—have been shaped by natural selection over eons
of our species’ history, stretching back to well before the origin of humans, and even
before the origin of mammals. Our lungs came about because our amphibian-like fish ancestors
that were becoming more adapted to life on dry land needed an interior surface area for
gas exchange with the air to supplement and eventually supplant the use of gills, which
had been used for gas exchange with water. In the time since, the surface area in our
lungs has been driven upwards due to greater need for gas exchange as our metabolic rates
and body size increased. For a typical-sized human, anything less than a singles tennis
court would cost us in terms of fitness, whereas whatever benefit we might gain by having a
doubles court is presumably not substantial enough to outweigh the costs of making such
a lung. Here’s where the third piece comes in: natural selection. Ultimately it is the
need for physiological function that drives the evolution of the structure. If you and
your singles tennis court-sized lung are sitting between two people with different sized lungs--maybe
the guy on your right has a mini-lung with a ping pong table of surface area and the
guy to the left of you has an oversized lung that’s too big for what he needs—you can
see that your fitness is greater than either of your neighbors. Your lung is the right
size to meet your need for gas exchange and your rate of survival and reproduction will
be predictably greater than either of your neighbors’.
Is it necessarily true that all anatomical structures have a physiological function?
Why actually no. Remember that life changes through evolution, and a structure that is
important for a species in one particular time might be totally useless for the descendants
of that species living at a later time. Take for example the eyes on a blind cave-fish
that lives in total darkness. You may be aware that blind forms of life often evolve in caves—fish
are just one of many—there are also crickets, shrimp, salamanders—all of these show varying
degrees of reduction in eye structures, and many appear to be completely eyeless.
Why is this important? Well for one, it shows us that it is the function of sight—which
is required for those of us living in the light—that causes the natural selection
that maintains the continued existence of the anatomical structure of eyes. Function
is required for structure. When you remove the need for function (in this case sight)
the structure degenerates. How does this degeneration occur? Well think of mutations that destroy
eyesight but leave other bodily functions intact. These mutations would be harmful in
a non-cave population and would be eliminated by natural selection. You could say that in
a cave population living in perpetual darkness these mutations could have no adverse effect
at all and they might even be a little beneficial—if you have ever had an eye infection, you know
that in spite of all the wonderful things that eyes do for us, they are also a potential
site of infection and for a wild animal living with no access to medical treatment any infection
is potentially lethal. Thinking about it this way, having no eyes could actually give you
a fitness advantage for no other reason than that it makes you immune to eye infections.
Another thing about a blind cave fish—one that has no eyes at all—is that it may very
likely still have an ocular orbit—the eye socket in the skull. This would be an example
of an anatomical structure with no physiological function whatsoever. There’s no eye to go
into the socket, so why should the socket exist? The answer here should be pretty obvious
to you by now—this fish evolved quite recently from ancestors that did have eyes to go into
those eye sockets. The mutations that resulted in the loss of external eyes would not have
eliminated all the parts of the eye structure. So yeah, you might find ocular orbits in blind
fish as well as any number of other diddly bits of eye structures that may still be there
but have no physiological significance—these are just what remains of that complex anatomical
structure of the eye, which did have physiological significance in an earlier chapter of this
fish’s evolutionary history.
Professor Leigh Van Valen had a special name for these structures—he called them “ghosts
of selection past.” Most biologists call them vestiges or vestigial structures, and
while these structures are largely useless (i.e., without function), there may in some
cases be a physiological function remaining. For instance, if the eye were responsible
for something other than vision—say it also was the site for the production of an important
chemical for the body—it might be true that a tiny vestige of an eye in a cave fish could
be totally useless for vision but still adequate to produce that necessary chemical. Knowing
that many organs have multiple physiological functions like this, it should not be surprising
that some vestigial structures may retain some physiological function while others may
be perfectly useless.