Welcome.
In this lecture,
we will be discussing the blood vessels.
So first,
blood vessels are the delivery system
of dynamic structures
that begins and ends at the heart.
Our blood vessels work with our lymphatic system
in order to circulate fluids.
The blood vessels include the veins,
which are going to carry blood toward the heart,
and are mostly deoxygenated with the exception
of our pulmonary circulation
and the umbilical vessels of the fetus.
The second vessel are our arteries.
This carries blood away from the heart,
and it's usually oxygenated except for in pulmonary circulation,
and the umbilical vessels of the fetus.
Finally, we have our capillaries.
These are going to be vessels that come in direct contact
with our tissue cells
and are going to directly serve our cellular needs.
So if we look at the structure of our blood vessels,
we find that all vessels consist of a lumen,
which is the central blood containing space
surrounded by a wall.
The walls of all of our vessels are going to have
three layers or tunics.
Starting on the inside, we have the tunica intima,
followed by the tunica media,
and the outermost layer being the tunica externa.
Our capillaries are a little different,
and that they do not have these three layers.
Instead,
our capillaries are just made up of an endothelial layer
and a sparse basal lamina surrounding it.
So if we take a closer look at each of the layers,
we find that the tunica intima is going to be
our innermost layer
and it's going to make intimate contact with our blood.
It is composed of an endothelium
which are simple squamous epithelial cells
that line the lumen of the vessel.
It is going to be continuous with the endocardium of the heart
and has a very slick surface that reduces friction
as blood flows through the vessel.
There is also a subendothelial layer
in this tunica,
which is going to be made up of connected tissue
basement membrane.
But this is actually only found in some of the larger vessels.
The next layer or the middle layer is our tunica media.
This is going to be composed of mostly smooth muscle tissue
and sheets of elastin.
This is going to have sympathetic vasomotor nerve cells
also within this layer,
which are going to control vasoconstriction,
which leads to a decrease in our lumen diameter
or vasodilation,
which was to an increase in the diameter of the lumen.
This is our bulkiest layer
and it's going to be responsible for maintaining blood flow
as well as blood pressure throughout the body.
The third layer and outermost layer is our tunica externa.
This is sometimes also called the tunica adventitia.
It's going to be composed mostly of loose collagen fibers
that are going to protect and reinforce the wall
as well as anchor the vessels to its surrounding structures.
This is also infiltrated with nerve fibers
as well as lymphatic vessels.
Also, large veins contain elastic fibers in this layer.
Also, within this layer, we have the vasa vasorum
or a system of tiny blood vessels
that are going to be found in our larger vessels.
This is going to function to nourish
this outermost layer of our blood vessels.
So if we look at the structure of the blood vessels, to recap,
the blood vessels in the body have three layers.
They're going to be the tunica intima,
which is the innermost layer.
the tunica media, which is the middle layer.
and the tunica externa, which is the outer layer.
Also our blood capillaries,
which are going to be the smallest of our vessels
are going to just consist of an endothelial layer
surrounded by a loose basal lamina.