Hey y'all, Dr. Brown here. Let's learn about capillary shall we. So capillaries
are the smallest of the blood vessels in the human body. They can be usually
between 7 and 9 micrometers in diameter, but they can be smaller than 6 microns
in diameter. Keep in mind that red blood cells average about seven-and-a-half
micrometers in diameter, so these these can get very, very, very, small.
Structurally they are the simplest of the blood vessels, containing only an
endothelium and a basement membrane. They don't even have a complete
sub-endothelial layer, and these capillaries are going to come in three varieties as
I think you could have guessed by the title at the top of the screen that says
three types of capillaries. So what I want to do is go through each of those
three types of capillaries, and kind of point out their their major structural
characteristics that give them the degree of contact between the blood
space inside of the capillary, and the tissue space outside of the capillary
that is characteristic of each different type of capillary. We're gonna begin with
continuous capillaries, and they get their name because the cells, the
endothelial cells, form a single continuous sheet. So continuous
capillaries tend to be relatively tubular. They can contain a single and up
to three layers of endothelium surrounded by a continuous basement
membrane. Now the cells are actually held together. They're connected to one
another or in the case of the smallest ones you can have a single cell that's
wrapped around and connected to itself like you see in a transmission EM on the
right. And the cells are gonna be held closed by a structure called a macula
occludens. Let's write that for you so you know how to spell it. If I can get my
pen to work. There we go. Very good. Mac-you-la O-clue-dense. And here's one
right here, and if we zoom in and look at that structure, at very high
magnification, we can see that the macula occlusions is actually a series of
discontinuous tight junctions. This is going to form for better...for all
intents and purposes, an impenetrable barrier. Stuff can't diffuse unless its hydrophobic.
Can't diffuse from the blood space in here. So this is where the blood would be,
and then the tissue space would be out here. The only way substances can get
from one side of the capillary endothelial membrane to the other, or
the endo...yeah the endothelial cells is through trans cellular transport or
vesicles. So what you're actually seeing inside the cell are a bunch of little
endocytic and exocytosis vesicles. So these are called micropinocytic vesicle,
because they're doing pinocytosis, but they're very, very, very, small, and this is
going to give us the strictest control over contact between the blood space in
the tissue space. The only way something can get from the blood space to the
tissue space or vice versa is if this endothelial cell has taken it
up via endocytosis, transported it across a cell, and released it via exocytosis.
So this extreme vetting results in very limited contact between the blood and
the tissue, and in some...most cases this is what you want. Especially in the brain.
So we need an example of where we're going to find all of these things, and
our example location for continuous capillaries is the blood-brain barrier.
Blood brain barrier. Cool. The next kind of level of contact between the blood and the tissue
space is the result of fenestrated capillaries. Fannett...Ha ha! I'm making words.
Fenestrated capillaries get their name from the Latin word finestra
meaning window. Fenestrated capillaries have continuous layer of cells. They tend
to be tubular like continuous capillaries. They have a continuous
basement membrane like continuous capillaries. The difference is they don't
have the the tightly controlled contact or limited contact between the blood
space in the tissue space. Instead the cells are
with numerous fenestrae or fenestrations, and these fenestrations or
windows, are gonna act like little pores. Now they do have a membrane across them.
This is not a unit membrane, you can see right here, but these little pores are
going to allow more things through than could get through via trancellular
transporter, vesicle mediated transport. So fenestrated capillaries have slightly
more contact between the blood space and the tissue space. Much more so than we
saw with the continuous capillary, but not as much as we're gonna see with our
last one. So our continuous capillary, our example location was the blood-brain
barrier. These are the capillaries that you find in the central nervous system.
An example for fenestrated capillaries is actually twofold.
So one location were we can find these is endocrine gland, and this is a situation
where we want to get stuff from the tissue space made by the endocrine cells
into the blood. Right? Endocrine glands use the blood stream to transport their
signaling molecules or hormones. The other place where we find these is in
the glomeruli of the kidneys. In this case we want to get ma...we want to get
increased transport to get stuff out. We want to get waste products from the
blood space into the tissue space of the glomerulus, so that then those waste
products can be excreted by your kidneys, since that is in fact their job. So
continuous capillaries, limited contact between blood space and tissue space
only transcellular transport. Fenestrated capillaries, more contact
between the blood space and the tissue space thanks to these fenestrations
or little windows in the endothelial cell. Finally, sinusoidal capillaries are
gonna have maximal contact between the blood space and the tissue spaces. Unlike
what we saw in the first two kinds of capillaries, the endothelial cells are
not continuous in sinusoidal capillaries; they're gonna have these big gaps in
between the cells. Additionally, the basement membrane is
incomplete, meaning that substances are free to go from the blood space into the
tissue space unhindered. We're gonna get maximum contact
between the blood and the tissue. In fact if...oh, one other thing, before I go to the
other picture, if we look we also see that these are rarely, perfectly
cylindrical. So if I outline the sinusoidal capillary in this light
micrograph of the liver, you can see that it takes kind of a tortuous, weird little
pathway. So these guys are very odd shaped, and they're discontinuous
allowing maximum contact between the blood space in the tissue space. And if
we look at more closely a sinusoid, and the best example of these is in the
liver, so this is a hepatocyte or a liver cell out here. Inside the liver cell...
..sorry...not inside the liver cell, inside the capillary itself, we can see these gaps
or holes in the cell membrane through which substances can flow. So
transporting across capillaries we have in increase, or sorry, decreasing
stringency of getting things into or out of the capillary. We have trans cellular
transport with vesicles. So vesicles are going to be the most stringent. That's
going to be in continuous capillaries. Then we have fenestrae, which are much
less stringent, more things are going to be allowed to cross. And then finally, we
have sinusoids, which are going to be the least stringent allowing maximum contact
between the blood space, and the tissue space. So that's just a brief overview of
the three different kinds of capillaries we find in the human body.
I hope you enjoyed it, and had fun. Study hard. See you later.