So let’s look at the structure
of a capillary. There are actually three types
of capillaries, and I will describe them briefly
here, but then I’ll emphasize them more when
we look at their functional role in other
organ systems. On the left-hand side, you
can see a picture or an image taken through
a connective tissue. And there are blood capillaries
running through them, very, very thin.
You can make out just the very thin walls, you
can see through them, and just make out the
elongated structures that represent the nuclei
of the endothelial cells. 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.
A continuous capillary is one that I’ve
referred to you before. This is where the
endothelial cells join together to make up
the capillary lining. And they join together
by very strongly occluding junctions. The
strongly designed occluding junctions that
include tight junctions. So the junctions
between endothelial cells is very very solid,
very strong. Nothing passes through those junctions.
And if you look back on your knowledge
of epithelia, these occluding junctions are
very important in many epithelial tissues
because they restrict movement of fluid and
other substances, pathogens, between cells.
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, and
then release on the other side into the interstitium.
It’s called transcytosis, the movement of
fluid and other substances by endocytosing that
material at the lumen, and by those vesicles
moving across and releasing the products on
the other side. That enables these capillaries
to restrict the sorts of substances that they
later pass across them. The basal lamina wrapped
around the endothelium is always continuous.
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,
all be it restricted and all be it very finely controlled.
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. In that situation,
there is a very thin diaphragm between the
fenestrations because they need to be further
controlled of what passes from the blood in the
kidney. 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. Well, the final type of
a capillary, we call a “discontinuous capillary”
that has a discontinuous basal lamina. Sometimes
we refer to this as being sinusoidal. 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. They are
very leaky. They allow substances to pass
out of them, and also into them. And in some
organs, such as the spleen, cells actually
pass through these large gaps. The liver is
another example of these sorts of capillaries.
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.