Download Anatomy and Physiology

Anatomy and Physiology
Blood vessels
Blood vessel overview

Blood travels from the heart through arteries.
Initially these are large and very elastic. They
soon become muscular and these arteries deliver
blood to organs. Arteries become smaller until
they become arterioles and finally they enter a
network of capillaries. The blood moves through
this capillary bed and then flows into small
venules, then larger venules before becoming
veins and delivering the blood back to the heart.
Blood vessel structure

Arteries and veins have the same basic
structure. The inside layer of the artery
(left hand side) is composed of simple
squamous epithelial cells called the
endothelial layer or tunica intima. Next to
that layer is the tunica media. The outside
layer is called the tunica adventitia. Look
carefully at this slide to visualise the three
layers.
Arteries

There are two types of arteries and these are
elastic and muscular. Elastic arteries have thick
walls and contain lots of elastin. You will
remember that this is the main protein of elastic
connective tissue. This gives the elastic arteries
greater flexibility and blood pressure in these
arteries remains relatively constant. On the other
hand muscular arteries (which distribute blood to
the organs) contain smooth muscle and it is in
these arteries that blood pressure can change.
Large elastic artery

In this slide you can see the tunica media as the layer with lots of
coiled elastic tissue. The outside layer is called the tunica adventitia.
This is the green layer and it is composed of connective tissue. This
section of elastic artery has been stained with a special stain to show
the layers and magnified about 500 times.
Small Elastic Artery (such as
renal artery)

As arteries become smaller the adventitia
becomes smaller as does the tunica media. You
cannot see the endothelial layer in this highly
magnified small elastic artery but you can see the
purple tunica media and the blue tunica
adventitia.
Muscular Artery

Muscular arteries deliver blood to organs. The
elastin of the elastic arteries is mostly replaced by
smooth muscle cells and this limits the capacity
for the artery to change shape. Unfortunately,
damage to the endothelial layer through stress,
cigarette smoking and a number of other causes
can lead to cholesterol plaques building up and
restricting blood flow. This can lead to what is
called ischaemia or reduced blood flow.
Arterioles

Muscular arteries become arterioles when
the tunica media is reduced in size to one
or a few layers of smooth muscle cells. At
the same time the tunica adventitia also
reduces in size.
Arterioles

Arterioles control blood flow by dilation or
constriction. They contribute to peripheral
resistance and a significant fall in blood
pressure. You can see in this slide that the
blood vessel has become quite small.
Small arterioles

A small arteriole has one layer of tunica media smooth
muscle and the endothelial layer. You can see the single
layer of squamous endothelial cells in both blood vessels
in this picture. Can you see the nuclei bulging out into the
lumen of the blood vessel? This is because the nuclei of
squamous cells is fatter than the cell itself. The cells in
the middle of the pipes are red blood cells.
Capillaries

Arteries carry blood away from the heart and veins carry
blood back to the heart. These are connected by the
capillary system. Capillaries consist of one layer of
squamous epithelial cells. Blood flow is small and
substance exchange occurs by osmosis and diffusion.
Blood pressure drops significantly in capillaries because
the blood that was running through one pipe is now
running through many.
Capillaries of the heart

Capillaries are all through our body. This
is a section of heart to show you the dense
capillary structure in the heart.
Venules

The blood then leaves the capillaries and
enters the venules on its way back to the
heart. These have very thin walls and the
endothelium is associated with a thin
smooth muscle layer.
Venules

As the venule becomes larger it adds smooth
muscle. Here you can see a valve. When these
are pushed together by blood flow they stop back
flow. Have a look at your arm when flexed. You
might see the valves running down the arm.
These stop the blood running back down the arm
between heartbeats
Medium Sized Vein

A sympathetic nerve supply to the tunica media
causes smooth muscle contraction and this assists
blood flow back to the heart. Blood pressure in
the veins is much lower than the arteries. On this
slide you can see the smooth muscle layer.
Activity

Make a schematic diagram of the blood
vessel network in our body.