Surface Area, MiraCosta Biology

One of the key themes that plays into your development of a good understanding of organs
and organ systems is how they provide the body with vast amounts of surface area across
which the body exchanges chemicals with the external world.
“External” here is in a manner of speaking, since these vast surface areas are usually
packed into organs that sit inside of your body. From the earlier videos you’re probably
tired of hearing about your lungs—we’ll get back to them soon enough—what I just
want you to remind you of here is that there is a vast surface area in both the alveoli
and the pulmonary capillaries, and all of this surface area is folded and packed into
around five pounds of lung-meat sitting inside of your chest cavity.
Another example—and one that is directly relevant to this module—has to do with the
lining of your small intestine. This is where the nutrients get absorbed into your bloodstream
from the digested food that is passing through the intestinal tube of your digestive tract.
Okay so here’s a diagram showing the basic anatomy that we’re talking about. Looking
at a cross section of the small intestine taken right here—it’s basically a tube
with a space in the center—this space is called the lumen and it is where the food
must pass through in going from mouth to anus. Now the first thing you need to understand
is that this lumen— the space inside of your intestines (and your stomach and your
throat) is all part of the external world. It’s easier to see this if you consider
that the lumen space is contiguous with the space inside your mouth. Obviously the space
in your mouth is contiguous with the outside world, and so is all of the space inside of
your digestive tract.
Now if we zoom in a bit—not to a microscopic level yet—the epithelial lining of the intestine
where your cells are in contact with the food passing through—is called the tunica mucosa,
and it is highly folded (or involuted) with millimeter-long finger-like projections called
villi that make the actual surface area of the intestine far greater than what you would
have in a tube with a smooth interior surface. This is an electron micrograph showing these
projections.
On top of this, there is even more infolding occurring at the cellular level that creates
additional surface area. If we zoom in on a single epithelial cell of the lining we
would see that each mucosal cell lining the lumen has microscopic fingerlike projections
called microvilli where the cell is in contact with the intestinal space—this gives the
cell a roughly Bart Simpson-like appearance the way I’m drawing it here, though in real
life there are thousands of microvilli on top, so the appearance is really more like
a brush. The effect of all of these finger-projections and infoldings at both the microscopic level
and the visible-with-the-naked-eye level is a 30- to 60-fold increase in the surface area
of the small intestinal mucosal lining. There’s actually thirty to sixty times as much surface
area inside of your small intestine than what you would have if its interior lining were
smooth. Okay, so here is the factual detail of an anatomical feature of the small intestine,
and so how do we want to understand this structure in terms of its physiological functional significance—why
have so much surface area?
Here is where we return to the general functionality of meeting the body’s need for X, via exchange
with the outside world. In this case “X” is the nutrients that are gotten from the
warm, mostly digested “soup” that passes through the intestine—remember that we’re
considering the lumen as part of the outside world. The nutrients of food aren’t truly
in your body until they pass from that soup into the blood, where they can then be processed
by the liver and delivered to all the parts of the body where the nutrients will be used.
The uptake of nutrients across the mucosal lining is the exchange, and having a large,
highly folded surface area allows the exchange to occur more efficiently—basically if your
small intestine had a smooth interior surface it would need to be 30 to 60 times longer
in order for the same amount of absorption to take place. Being large animals with relatively
high metabolic rates, we have a greater need for nutrients and calories, as compared with
smaller animals with lower metabolic rates, and having this large surface area as part
of the structure is crucial for the physiological function of adequate uptake of food calories
and other nutrients.
Animals that are much smaller than humans have simpler digestive systems, and yet we
can still see the recurrent theme of surface area maximization as part of these animals’
adaptations relating to nutrient uptake. This little flatworm has a gastrovascular cavity
that has only one opening—the pharynx (labeled P in this photo) is the short tube through
which food passes as it goes both into and out of the gastrovascular cavity (labeled
G)—basically it’s both the mouth and the anus. But you can see how the cavity itself
is highly convoluted in structure—with lots of little outpockets which makes for a much
greater epithelial surface area, which is where the worm absorbs nutrients from the
digested food that is in its gastrovascular cavity.
Even more bizarre is another flatworm—the tapeworm, which has no digestive space at
all—no mouth, no stomach, no intestines. You probably know that the tapeworm lives
in its host’s intestine, so basically this lucky worm is continually bathed with nutrient
soup every time the host eats. It absorbs all of its nutrients directly through its
outside epithelium—basically it soaks up all of its nutrition through its skin. But
even this very simple animal living in such a nutrient-rich environment has surface area
issues—this mechanism of using absorption through the skin is possible only because
the shape of this flatworm is, well, flat, which means that its surface area is much
greater than what you would have on a worm with a rounder body profile. You might say
that the tapeworm’s flat shape is part of its physiological mechanism for nutrient uptake,
and that natural selection has maintained this very flat body profile in tapeworms because
all of the tapeworm’s ancestors were successful because they were very flat-bodied, and this
success was directly related to their ability to absorb nutrients.