tissue in the aorta. Now, here is a large
vein, the vena cava. You know it’s a vein
because look at the image on the left-hand
side, it has got a large lumen, but the walls
collapsed. Look at the thickness of the wall
relative to the lumen. Totally different
to what you see in a large artery. Now, look
very carefully on the right hand side, because
this is a section through the wall of the
vena cava. The tunica media shown here is
very thin. The tunica adventitia has smooth
muscle in it running longitudinal along the
length of the vena cava. That’s an unusual
situation. You don’t have smooth muscle
in blood vessels in the tunica adventitia.
But here, you’ve got it in the vena cava.
And the reason for that is because the vena cava
are so important returning blood to the heart,
to the right side of the heart. And that smooth
muscle in the tunica adventitia helps the
blood vessel, helps the vena cava elongate
or shorten depending on the postural position
of the individual because the vena cava lie
up against the posterior wall of the body.
So that’s a very important feature of the
vena cava, to have the ability to elongate
and shorten when we bend or change our posture.
It’s a characteristic feature of the vena cava.
Let’s have a look at our capillary bed.
Capillaries are very, very hard to see