The lymph nodes have a lot of fluid in them
so they are a reservoir for plasma. If you
start to get dehydrated, the lymph nodes are
capable of releasing that fluid to help hold
the vascular volume up.
And there’s also a constant transport that
goes on here involving the lymphatic vessels
and the lymph nodes in order to prevent oedema,
or swelling. And we’re going to talk about
oedema and swelling at the end of this lecture.
It happens when the system becomes overwhelmed
and tissue fluid accumulates. And we’ll
talk about it in more detail.
Just to give you a quick idea about the structure
of the larger lymph vessels.
They have an endothelial lining just like
the arteries, just like the veins, just like
the arterioles, just like the venules. Their
basement membrane beneath the endothelial
layer is not as tight so there is even a little
bit of leakage from the lymphatics. But basically
since they’re constantly draining, any leakage
from the lymphatic gets taken up by little
small lymph vessels and, eventually, returned
into the system. The larger lymph vessels
have smooth muscle as I said to help move
the lymph fluid up into the chest. And, of
course, on the outer layer there’s adventitia
– fibrous tissue and fatty tissue – that
holds the lymph vessels in place. And, as
I’ve also said before, the further from
the periphery you get, the lymph vessels become
more complex – more smooth muscle, more
adventitia. And the very small lymphatics
down in the tissues often have no muscular
layer and no adventitia. So the system gets
progressively complex until you arrive in
the ducts in the chest where they return to
the vein. These are quite substantial vessels.
And I’ll just say a few more words about
the lymph ducts. They are the largest vessels
in the lymphatic system. No surprise because
all the fluid arrives at them. They drain
the lymph fluid into one of the subclavian
veins and return it to the general circulation.
The large lymphatic ducts are one on the right
that drains the right side of the body and
the thorax and the arm. And the thoracic duct
is the largest lymph vessel. It drains the
lymph from the rest of the body.
Interestingly enough, the intestines also
have a number of lymph vessels. Little tiny
lymph vessels in the wall and these are used
for absorption following digestion of food.
And in fact, if you feed an animal or even
at surgery a human being a fatty meal, you
will see fatty fluid being drained by these
lacteals back into the circulation where many
of these fatty components are metabolized
– that is chewed up – in the liver and
converted into nutritional substances that
can be used by the cells.
Again to emphasize that the lymphatic system
is a major part of the immune system. It removes
cellular debris and invading bacteria, as
we talked about, in the lymph nodes. It produces
lymphocytes which are a critical part of the
immune system. We know that because, for example
when a retrovirus such as HIV destroys the
lymphocytes, terrible infections develop.
So the lymphocytes are critical to our well-being.
And, as you know, there are lymph nodes throughout
the body. They’re in the groin, they’re
in the cervical area, they’re in the axilla
– in the armpit. And also, of course, the
tonsils are part of this as well.
Interestingly enough, both the spleen and
the thymus gland are part of the lymph system
because the spleen is a blood filter, another
filter like the lymph nodes. The spleen’s
job is more destroying defective red blood
cells and acting as a reservoir for platelets
and red blood cells but also it has a lot
of lymphocytes and helps in destroying any
bacteria that get into the bloodstream.
The thymus gland up in the neck is also a
lymphatic organ. And it’s more commonly
involved in early life – in childhood and
early adolescence – in developing and maintaining
the immune system. And it partially involutes
in adults although a very nasty form of cancer
can develop in adult life if it doesn’t
I mentioned the tonsils before. You know that
they’re localized in the back of the throat
and they also help by trapping and removing
throat bacteria and they’re a kind of lymph
In the intestine, there are something called
Peyer's patches, named after an anatomist,
Peyer. And they’re also like small collections
of lymphocytes or small lymph node type areas.
And they also prevent bacteria from penetrating
the intestinal wall and getting into the system.
And there are small collections of lymphoid
tissue in the skin and connective tissue throughout
So you see that the lymphatic system is very
complex. It isn’t just a bunch of little
tubes for draining fluid. It also has a very
important defence component. And it lives
in intimate contact with the lymphatic system,
the lymph nodes and the lymphocyte production
which, as I’ve said, are critical to defence
against invading bacteria.