00:01
Let's have a look at our capillary bed.
Capillaries are very, very hard to see
when you look at sections of tissue because
they're under very low pressure, if any pressure
at all may collapse them. So they're almost
impossible to see. Here's a diagram illustrating
on the left-hand side, the nature of the capillary
bed. And on the right-hand side, you can see
again the image or the section that I've used
previously. On the bottom left-hand side
of the histological section, you can see a
little small artery. It's going to get smaller
and smaller to be only one or two layers of
smooth muscle and become an arterial.
00:45
And on the bottom right-hand side of the image,
you can see half a lumen or half a section
through a venule, a small venule. Look at
the thin wall. And at top, on the top of the
section, you can see a structure with a large
lumen, a very very thin wall. That's probably
going to be a postcapillary venule or even
a small venule. It's very hard to know,
but just get an idea of the relative thickness
of the walls of these vessels. Well, blood
flows through the arterial and goes into the
capillary bed. And you can see that illustrated
in the diagram on the left-hand side. That
arterial sends blood through branches, through
very small branches, into the capillary bed where
there's a lot of networks, a lot anastomosis,
a lot of connections. So blood flows through
that capillary bed in an enormous network
or surface area, supplying all the surrounding
cells and the interstitial fluid with all
the nutrients that the cells need, and collecting
all the waste products from the interstitium,
from the cells, and then returning it back
towards the heart to be dealt with by other
organs. Sometimes histologists like to describe
metarterioles or precapillary sphincters.
02:10
They can sometimes close off the supply of
the capillary bed or blood from the arterioles,
or sometimes they can close off in one area
and open up in another area and divert blood
directly from the arteriole side of the cardiovascular
system directly back into a venule and bypass
the capillary bed all together. It's called
an arteriovenous anastomosis. And that occurs
in some parts of the body, for instance in
the dermis of skin that I've mentioned earlier.
02:50
There are sweat glands penetrating down from
the epithelium, from the epidermis of
the skin. Those sweat glands have a role in
thermoregulation. They secrete fluid water to the
surface of the epidermis where it's evaporated
and that cools the body down. Well, sometimes
when we overheat, when we play sport for
instance, and we increase our sweating, we
also like to dissipate more and more heat
from the body. So sometimes the dermis of
skin opens up. The blood supply to the skin
is opened up by these channels opening up
and sending more and more blood to the skin
surface towards the skin surface. The epidermis
is avascular, but there is a vast network
of blood vessels within the dermis right underneath
that epidermis. So in conditions, hot
conditions where we want to try and dissipate
body heat, we can open up these capillary
channels and the heat can be dissipated across
the surface. Conversely, if we're in very
cold weather, we can close off that blood
supply to the dermis, and therefore, not lose
body heat. So these are often very important
structures. But let's concentrate now on
blood flow into the true capillaries.
04:17
They go to capillaries where there's this exchange,
then they move into postcapillary venules
and then back into the venules to be
returned to the heart.
04:28
I want to concentrate now on the structure
of these capillaries, which as I've emphasized
is very hard to see when you
look at histological sections.
04:43
So let's look at the structure of a capillary.
04:45
There are actually three types of capillaries and I'll describe them briefly here,
but then I'll emphasize them more when we look at their functional role in other organ systems.
04:58
On the left hand side, you can see a picture or an image taken through connective tissue,
and there are blood capillaries running through them.
05:10
Very, very thin, you can make out just the very thin walls.
05:15
You can see through them and just might get the elongated structures
that represent the nuclei of the endothelial cells.
05:23
Well, probably the most common type of capillary is what we call a continuous capillary,
and this is illustrated on the diagram,
particularly on the bottom diagram on the right hand side.
05:38
A continuous capillary is one that I've referred to before.
05:42
This is where the endothelial cells join together to make up the capillary lining
and it's joined together by very strongly occluding junctions.
05:55
These strongly designed occluding junctions include tight junctions,
so the junctions between endothelial cells is very, very solid.
06:05
Very strong, nothing passes through those junctions.
06:10
And if you'll look back on your knowledge of epithelia,
these occluding junctions were very important in many epithelial tissues
because they restrict movement of fluid and other substances, pathogens between cells.
06:28
So, in many organs of the body, skin and particularly the brain,
you have these continuous capillaries
and the only way in which substances are transported across the capillary wall,
is by pinocytosis, by these substances being invested by the cell, taken in by the cell,
wrapped up in little membranes, and transported across the endothelial cell surface in cytoplasm.
07:04
And then it's released on the other side into the interstitium.
07:08
It's called transcytosis.
07:11
The movement of fluid and other substances by endocytosing that material, the lumen,
and by those vesicles moving across and releasing the products on the other side.
07:24
That enables these capillaries to restrict the sorts of substances that they allow to pass across them.
07:33
The basal lamina wrapped around the endothelium is always continuous.
07:38
Well, another sort of capillary is one where the actual cytoplasm of these
very, very thin endothelial cells have little fenestrations, little windows, little pores,
and that allows substances to pass out through those pores, albeit restricted
and albeit very finely controlled.
08:07
We'll see an example of these fenestrated capillaries in the kidney
because they form part of the filtration component of our blood forming a glomerular filtrate.
08:21
In that situation, there's a very thin diaphragm between the fenestrations
because there needs to be further control over what passes from the blood in the kidney.
08:33
But just to summarize these little capillaries, again, the basal lamina is continuous,
but there are little windows or little pores within the cytoplasm,
allowing substances to pass through them.
08:48
Well, the final type of a capillary, we call a discontinuous capillary.
08:57
It has a discontinuous basal lamina, sometimes we refer to this as being sinusoidal.
09:04
There are large gaps in the cytoplasm,
in the lining of the epithelium of the blood vessel, the endothelium,
and these large gaps are quite typical in some endocrine tissues.
09:18
They're very leaky. They allow substances to pass out of them and also into them.
09:25
And in some organs such as the spleen, cells actually pass through these large gaps.
09:32
The liver is another example of these sorts of capillaries.
09:37
And again, let me stress that we will look at these types of capillaries
when we look at the different organ systems where they have a functional role.