Lymphatic MiraCosta Biology


Uploaded by MeerdesIrrtums on 27.04.2012

Transcript:
There are a couple of things I want to elaborate on before launching into this module on the
immune system. First, in the previous module we’ve gone through the composition of blood
and how it’s made up of red blood cells and plasma as well as platelets and white
blood cells, and we’ve also addressed the anatomy of the circulatory system, which for
us humans usually means the circulation of blood through the blood vessels, arteries,
arterioles, capillaries and veins. But there’s actually another circulatory system that we
can talk about in the context of human anatomy and this is the lymphatic system, which mostly
operates outside and separately from the cardiovascular system, but the fluid that it carries originates
from the blood, from the stuff that seeps out of the capillaries, and ultimately it
does feed its fluids back into the venous blood just before it returns to the right
atrium of the heart.
The spaces in between the cells of your body are called the interstices and the fluid occupying
these spaces is called interstitial fluid. In a biology class we usually talk about this
as the “in between” area or space that nutrients, cellular waste molecules, and respiratory
gases must traverse in going between the cells and the blood. Remember that blood is carrying
nutrients and oxygen “to the cells of your body” but actually your cells are getting
the nutrients and oxygen by diffusion from the interstitial fluids, and these interstitial
fluids are getting their nutrients and oxygen by diffusion away from the blood capillaries.
The other thing that sometimes “happens” in the interstitial spaces is the battle between
your defensive system—your immune system—and invading things that would be more than happy
to convert your body into a soupy compost. You can think of the interstices as well as
the whole lymphatic system that they connect to as the battlefield in your ongoing war
against potential invaders. We’ll come back to the interstices when we talk about the
inflammatory response, but now I just want to outline the anatomy of the lymphatic system
because of the importance of lymph nodes and lymphatic organs in the processes that we’ll
be considering in the remainder of this module.
The fluid in the interstices is not stagnant. It doesn’t flow swiftly like blood does,
but it is constantly being fed by the liquids that seep out of the blood capillaries. When
the capillaries increase in size (or vasodilate) their permeability to fluids increases, so
we see an increased flow of fluid into the interstices of the area where this vasodilation
is occurring—and this is the cause of swelling. These fluids come out of the blood capillaries,
but it’s a one-way flow—none goes back into the capillaries—and this liquid is
constantly seeping out even when there is no vasodilation—so where does it all go?
Well, there is another kind of capillary—the lymphatic capillaries—that penetrate into
all of the interstices of the body and these allow the tissues to drain away this fluid,
and once it enters the lymphatic capillaries, it is no longer called “interstitial fluid”
but now it’s called “lymph.” Lymph is one of the places where you’ll find the
lymphocytes that we’ve talked about earlier, but in this module they will become extremely
important. Anyways, the lymphatic capillaries flow together into larger and larger lymphatic
vessels, which collect in the lymph nodes of which there are dozens located throughout
the body. There are also other organs associated with the lymphatic system, including the vermiform
appendix, which is a small, worm-shaped blind sac the comes off of the intestine right where
the small intestine and the large intestine meet. This is exactly where some of our closest
relatives have a cecum—that large chamber where bacteria do their hindgut fermentation
that provides herbivores with their protein boost. Humans, in adapting to a more digestible
omnivorous diet, have lost the cecum but they have retained the vermiform appendix. The
appendix’s small role in the lymphatic system and immunity may contribute to our understanding
of why it wasn’t lost entirely. We can still consider it a vestige of a cecum structure
that we used to have but lost in evolution, but the vermiform appendix is not totally
functionless.
Now remember that in situations where your outer layer of protection—your skin and
mucus membranes—have been compromised, this interstitial space represents the “battlefield”
where the cells of your nonspecific immune defenses are duking it out with invading pathogens
and cleaning up the debris. All of this material is going to be carried away by the lymph where
it will have to flow through lymph nodes and lymphatic organs, and it’s in those sites
where lymphocytes have ample opportunity to examine the bits and pieces of the “enemy”
and prepare for a larger-scale attack. We might even prepare some special weaponry that
is specific to that particular enemy. And this is exactly what happens in our immune
system.
The second thing I wanted to share with you here is that in an earlier video I pointed
out that there are three sub-classes of physiological function: gathering information about the
external world, homeostasis, and meeting a body’s need for “X.” I left off one
very important sub-class from that discussion, which is defending a body against the various
nasty things that are out there in the world—and the body’s nonspecific and specific immune
defenses are definitely in this category.
Without turning this into a full-on topic on medical microbiology, let’s just point
out that there are plenty of living and nonliving things that can make us very ill if they were
to actually get past our body’s defenses. On the nonliving side there are toxins, which
usually have a fairly specific effect on your body’s cells—for example if you get bitten
by a rattlesnake, the hemotoxins cause damage to blood or cause extreme blood coagulation.
Or if you’re really unlucky you will have been bitten by a snake that has neurotoxins,
in which case the toxins affect nervous tissue. Some toxins are rather small molecules while
others are fairly large proteins. Viruses are also nonliving, but they use their host
cells to self-replicate and they end up multiplying inside your body (if you’re infected), so
a lot of people think of them as being kind of like living things. They’re also made
out of proteins and nucleic acids—the same kinds of molecules that you find in living
cells.
Then there are living things that can cause disease—bacteria are prokaryotic cells that
are mostly things that don’t pose any real risk to you, but there are some bacteria that
are specialized on penetrating and infecting living things, multiplying and then having
some of the billions and billions of cells that are produced escape and penetrate and
infect other hosts. These are the pathogenic bacteria that cause transmissible disease,
like Streptococcus. Besides bacteria, there are pathogenic fungi, which are eukaryotes,
and then there are single-celled eukaryotic parasites, which are neither fungi nor bacteria,
and also there are also animal parasites to worry about as well, like worms.
Add to this list the precancerous and cancerous cells that occasionally get started from some
of your own cells and you have quite a list of things you don’t necessarily want inside
your body, and hence there’s this whole category of “defensive features” that
we’re discussing as a fourth major component of physiological function.
Pan over previously drawn schematic showing relations bw the named items