– Hello Bio 242.
In this video, we're going to
introduce the lymphatic system
and talk about how the
lymphatic system circulates
fluid from the tissues
back to the blood.
This is the first video
on the lymphatic system.
In the second video, we'll
talk about the lymphoid organs.
Together, these videos cover
approximately pages 765 to 769
in your textbook.
We'll skip over the discussion
of the lymphocytes for now
and pick the lymphatic system
back up in pages 772 to 776.
Before getting into
lymphatic drainage,
I want to take a moment to
talk about the functions
of the lymphatic system.
The functions of
the lymphatic system
can be divided into
two categories.
First, it acts as a
second circulatory system.
The lymphatic system picks
up interstitial fluid
from the tissues of the body and
drains it back into the blood.
The lymphatic structures that
participate in this function
are the lymphatic vessels
and the lymph nodes.
The lymphoid organs serve as the
anatomy of the immune system.
The immune system is
a functional system.
It consists of immune
cells that protect
your body from infection.
Immune cells gather in the
lymph nodes, the spleen,
and the lymphoid nodules.
These are the structures
where the immune response
to infection gets started.
The lymphoid nodules
include the Peyer's patches
of the small intestine,
the tonsils that
surround your pharynx, your
throat, and the appendix
off your large intestine.
The thymus is a
lymphoid organ too,
but its function is
different from the others.
The thymus is
where T lymphocytes
go to mature after they
leave the bone marrow.
You saw this image before
when we were discussing
the cardiovascular system.
It shows the relationship of
the lymphatic system in green
to the cardiovascular system.
Remember that fluid
leaks out of the vascular
capillaries into
surrounding tissues.
More fluid actually
leaks out than can return
into the vascular capillaries.
This excess tissue fluid drains
into the lymphatic capillaries.
Once the tissue fluid has
entered the lymphatics,
it's called lymph.
Adults generate about three
liters of lymph every day.
The lymph contains cells
and proteins, as well as
water and small solutes.
It passes through lymph
nodes and is returned
to the blood near the heart.
This lymphatic drainage is
critical to normal functioning
of the body.
If lymphatic drainage
gets blocked,
fluid will accumulate
in the tissues,
causing swelling and discomfort,
and possibly limiting mobility.
Lymphedema is the term for
this accumulation of fluid
due to blocked lymphatics.
Lymphedema is more common
in obese individuals.
And you also tend to see
it in cancer patients
after treatment.
You see, when cancer begins
to spread through the body–
a process called metastasis–
cancer cells get swept
into the lymphatics
along with tissue fluid.
So if a cancer has
metastasized, you'll
find cancer cells in the
lymph nodes that drain
the tissue around the tumor.
Often, the draining
lymph nodes will
be removed as part of
the treatment for cancer.
This is done to get
rid of any cancer
cells that may have
spread to the lymph node.
However, removal
of the lymph nodes
impairs lymphatic drainage.
One severe form of lymphedema
is called elephantiasis.
Elephantiasis is also known
as lymphatic filariasis.
It's caused by a parasitic
worm found in tropical regions.
These worms get trapped
in the lymphatic system,
blocking drainage.
The immune response
to these worms
can cause scarring
further blocking drainage.
You get massive fluid
accumulation and inflammation
that can lead to thickening
and hardening of the skin.
This diagram shows
the distribution
of the lymphatics in the body.
Note that lymphatic
vessels are found
throughout the body in the head
and neck, the arms, the legs,
the chest, and in the abdomen.
Lymph nodes are found
throughout the body
as well, but they are
particularly concentrated
in the inguinal region–
the groin– the
axillary region–
the armpit– and the
cervical region–
the neck.
Lymph nodes trap the
lymphs, slowing it down,
enabling immune cells in
the lymph nodes to sample
the fluid for
infectious organisms.
When bacteria or
viruses are present,
the immune cells
in the lymph node
initiate an immune response.
If you've ever had a
cold or sore throat
and felt swelling in your neck,
it's your cervical lymph nodes.
They swell with immune
cells, proliferating
to fight off infection.
These images show you
what the lymph nodes look
like in a cadaver.
They are small
pearlescent lumps that
blend in with the surrounding
connective tissue.
I want to talk now about
the lymphatic vessels.
We'll go over the
structure of the lymph
nodes in the second
video when we
talk about the other
lymphoid organs as well.
The smallest lymphatic vessels
are the lymphatic capillaries.
Lymphatic capillaries
are found throughout
the connective
tissue of the body.
There are specialized
lymphatic capillaries
in the wall of the digestive
tract called lacteals.
Lacteals take up fats
from the intestines
that are too large to be
absorbed into the vascular
capillaries.
The fatty lymph that
flows through the lacteals
is called chyle–
C-H-Y-L-E. Tissue fluid drains
directly into the lymphatic
capillaries– including
the lacteals–
passes into the larger
collecting vessels,
then into the still
larger lymphatic trunks.
The lymphatic trunks drain into
one of two lymphatic ducts–
the right lymphatic duct
or the thoracic duct.
These two ducts then return
the lymph to the blood.
The lymphatic capillaries
differ in structure
from the capillaries of
the cardiovascular system.
First, they are a
bit wider so they
can take in cells and large
proteins as well as fluid.
Lymphatic capillaries are
also different in that they
are blind-ended tubes.
This basically means that the
capillaries come to a dead end.
Compare this to the
vascular capillary beds.
Remember that in
vascular capillaries,
blood flows into the
capillary bed from an arteriole
and out of the capillary
bed into a venule.
For the lymphatic
capillaries, they just
terminate in the tissues.
The end of the
lymphatic capillaries
is particularly well
suited to their function.
The end of each
lymphatic capillary
consists of loosely
overlapping endothelial cells
that form miniature one way
valves called minivalves.
These minivalves open when the
interstitial fluid pressure
goes up, allowing
fluid in cells to drain
into the lymphatic capillary.
If interstitial
fluid pressure drops,
the minivalves fall closed
again and keep the lymph
inside the lymphatic capillary.
The lymphatic endothelial cells
are anchored to the surrounding
tissue by collagen filaments.
These collagen filaments hold
the walls of the capillary
apart so that it
doesn't collapse
when interstitial
fluid pressure rises.
Instead, you just get
opening of the minivalves,
making the lymphatic
capillaries extremely permeable.
When you have infection
or some kind of injury,
you get inflammation
at the site of damage.
One effect of this
inflammatory response
is an increase in the
amount of interstitial fluid
at the site of inflammation.
With a greater volume
of interstitial fluid,
the interstitial
fluid pressure goes up
and the lymphatic
minivalves open.
This allows inflammatory cells,
cell debris, and any bacteria
or viruses to flow into
the lymphatic system
and drain straight
to the lymph nodes.
And in those lymph nodes,
you have immune cells
waiting ready to attack
any harmful organisms that
come their way.
Lymphatic capillaries drain
into larger collecting vessels.
Collecting vessels travel
alongside arteries and veins.
The collecting vessels
are similar to the veins
in structure, but they
have even thinner walls,
more internal valves
to prevent back flow,
and they're very
highly networked.
The collecting vessels are
very low pressure and receive
no push from the heart
to circulate the lymph.
Instead, lymph is pushed
through the vessels
by skeletal muscles
and respiration.
When skeletal muscles
contract, they
squeeze the lymphatic vessels
running through the muscle.
This squeezing
pushes lymph along.
The lymph moves in
only one direction
because the valves
prevent back flow.
When you breathe, the pressure
in your thoracic cavity
decreases and the pressure
in your abdominal cavity
increases.
This combination
helps lymph move up
towards the thoracic cavity–
the area of lower pressure.
The lymphatic trunks
in the thoracic duct
are surrounded by
smooth muscle that
also helps push lymph along.
The lymphatic collecting
vessels join together
to form the lymphatic trunks.
You have right and left
lumbar trunks, right and left
broncho-mediastinal
trunks, right and left
subclavian trunks, and right
and left jugular trunks.
The jugular, subclavian, and
broncho-mediastinal trunks
on the right side of
the body join together
to form the right
lymphatic duct.
You also have a single
intestinal trunk
that collects chyle–
fatty lymph– from
the intestinal tract.
The lumbar trunks and
the intestinal trunk
join together to form
the cisterna chyli.
About 100 milliliters
of lymph per hour
pass through this
enlarged lymphatic sac.
The cisterna chyli marks the
beginning of the thoracic duct.
You can see the cisterna
chyli and the thoracic duct
in these images.
The lymphatic ducts
return lymph to the blood,
roughly at the junction
between the internal jugular
vein and the subclavian veins.
Here it is on the right,
with the right lymphatic duct
emptying in.
And here on the left, you have
the thoracic duct entering back
into the blood.
Remember, the right
lymphatic duct
drains the upper right
half of the body,
including the right arm.
The thoracic duct
drains everything else.
After reviewing this video and
studying the lymphatic vessels,
you should be able to
answer these questions.
If you'd like to read
up more on lymphedema,
check out this website
run by the Mayo Clinic.
If you'd like to find out
more on elephantiasis caused
by microscopic worms,
check out the page
on lymphatic filariasis at the
Centers for Disease Control.
I do not recommend that
you Google elephantiasis–
at least not an image search.

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