00:01
Okay, kind of beating
a dead horse here,
but we're just kind of now
repeating, so it is a useful review.
00:09
So arteries,
are your resistance vessels.
00:13
They are the
small arteries and arterioles.
00:17
They regulate arterial
blood pressure and blood flow
within the organs.
00:21
They are highly responsive
to hormonal and nervous stimuli.
00:24
So, I mentioned the nervous input.
There is also hormonal input.
00:28
And we'll talk
more about things like
the renin-angiotensin
system shortly.
00:34
Veins, on the other hand,
are capacitance vessels.
00:37
Those are the veins in the venules.
They're gonna store blood.
00:40
They can dilate up more,
and they don't squeeze as well.
00:45
But that's where
two thirds of the blood
is stored at any given time.
00:49
Okay, bridging between
the arterial circulation
and the venous circulation
is the capillary circulation.
00:56
That's the next level down.
00:58
Capillaries are simple tubes
lined by a single endothelial cell
sitting on a basement membrane,
and then there are cells
around the outside
kind of look like fried eggs
sitting on the surface there
that are called pericytes,
P-E-R-I-cytes.
01:15
And these are
smooth muscle like cells
that give some structure
to the endothelium
but they're not wrapped
all the way around.
01:22
They're kind of loosely arranged
over the surface of the
endothelium within capillaries.
01:28
So again, the capillary is an
epithelium on a basement membrane.
01:33
And it's going to be
very thin wall
specifically so that we can get
gas exchange.
01:38
It's at the capillary level
that we want to bring oxygen
and release it into the tissues,
and take carbon dioxide
and get it back out.
01:45
So we need a very thin wall.
01:47
Hence just an endothelium
on a basement membrane.
01:50
We also have to
exchange metabolites.
01:52
We have to move glucose,
and all the ions
everything else
to make the system run
that also has to be
provided by diffusion
out of a very thin walled capillary.
02:05
This is now where I've identified
that fried-egg look that pericytes
sitting on the surface
kind of scattered around
on the surface of endothelium
within the capillaries.
02:15
And now we're going to
come out of the capillaries
into the postcapillary venule.
02:20
Alright, so we've coalesced,
we've gone from an artery on the
left hand side to an arterial,
to a capillary bed
where all that
nice exchange occurred,
that's all coalesced back down
to a postcapillary venule.
02:33
So what's so special about that?
Well, this is actually going to be
the site of all the action
with regard to inflammation.
02:40
Ah, this is where inflammatory cells
are recruited
into sites where we have
infection or injury.
02:46
It's also going to be where
most vascular edema occurs.
02:49
So when we have
increased vascular pressure,
when we have
increased permeability of vessels,
the fluid is generally leaking
out of the postcapillary venules.
03:00
So you need to
pay attention to that.
03:03
One other part of the vasculature
that we need to talk about
and often doesn't
get much attention,
are the lymphatics.
03:10
Lymphatics are lined
by endothelial cells too.
03:12
These are different than
the endothelial cells
that line the blood vessels.
03:17
Yes, they're endothelial cells,
but they have some
different features.
03:21
They line the lymphatics
and are thin-walled structures
that then drain into
various lymph nodes,
eventually, we'll take
edema fluid from the tissue
and drain it back
into the circulation.
03:37
So if you have edema, it gets out.
03:40
Lymphatics pull it back in.
03:42
It gets sampled along the way
in the various lymph nodes
and eventually will return
through the thoracic duct-
main structure in the middle-.
03:52
And that returns
the contents that fluid
that would otherwise be lost
that returns it to
the left subclavian vein.
03:59
So that's how we get fluid
in a circulation.
04:02
Once it leaves the vasculature,
it comes back via the lymphatics.
04:07
The lymph nodes along the way,
are not just kind of way stations
they're actually
going to be very important
for antigen presentation
and immune surveillance.
04:14
What's going on in that tissue
that's draining that fluid?
Is there infection?
Is there a tumor?
Is there something else going on?
So the lymph nodes are
going to be important elements
that will allow us to kind of assess
what's happening in the tissues.
04:29
Alright, so, we're gonna
be talking more again about
resistance and capacitance vessels.
04:34
Again, a little bit of a review,
but it's an important
set of concepts
that you really need to kind of
understand a visceral level.
04:42
Alright. So initially, we have,
I'm going to just start
with the blood leaving the rigt heart
and coming out
through the pulmonary artery.
04:52
Note that it's blue.
This is deoxygenated blood
even though it's an artery.
04:56
It's going to go
into the arterioles in the lung,
and then eventually
into the capillary bed of the lung,
where we'll get gas exchange,
and we will release carbon dioxide,
and we will take up oxygen.
05:08
Now, the blood becomes red.
05:10
So a venule here actually
has oxygenated blood,
That post-capillary venule.
05:16
It returns via the pulmonary veins,
through the left atrium,
and then
through the left ventricle,
and eventually out
through the aorta.
05:26
High pressure system.
05:28
That high pressure system branch is
into arteries and arterioles.
05:32
And then finally
into a capillary bed
where gas exchange and
metabolite exchange occurs.
05:38
That coalesces back to a venule
and back into the systemic veins,
which is indicated here
as a big dilated lump,
mainly because that's to indicate
there is high capacitance.
05:49
That returns to the right atrium,
to the right ventricle,
the circuit is complete.
05:55
On the right hand side here,
in the high pressure,
oxygenated vessels of the aorta,
that's conductance.
06:04
Basically, the ascending aorta,
and the descending aorta,
the thoracic aorta,
all those are just conductance.
We're just trying to get blood out.
06:13
And then we're going
to distribute it
at the level of the
arteries and the arterioles.
06:17
It turns out that the
major resistance vessels
in this entire circuit are at
the level of the arterioles.
06:25
So whatever
the arterioles are doing
in terms of squeezing or relaxing,
will determine for the most part,
the pressure within the
systemic arterial circulation.
06:36
At the capillary level,
we're now getting diffusion.
06:39
Again, oxygen into the tissues,
carbon dioxide out of the tissues,
metabolites into the tissue,
waste out of a tissue.
06:48
We then return blood
to the venous circulation,
that's our capacitance, that's
where the blood is basically stored.
06:54
And we're off and running.
06:56
Okay. Blood pressure
varies systematically
across the vasculature.
07:01
So let's start
with the left ventricle.
07:03
And basically, we're looking
at the left ventricle pressure.
07:06
It goes almost from zero to
120 millimeters of mercury.
Okay,
that's the trough to the peak.
07:14
That's in the left heart.
07:15
But we don't see zero to 120,
out in the vessels. Why?
Well, that's because
what's happening in the aorta
is that we get that peak pressure
of 120 millimeters of mercury.
07:28
But soon as we get below the level
of pressure in the ventricle,
the valve closes.
07:35
So we get a diastolic pressure
of 80 millimeters of mercury,
roughly, 90 shown here.
07:40
roughly, 90 shown here.
07:40
So thats your peak and your trough
within the aorta.
07:43
As we go more distally,
then we're looking at the arteries.
07:48
And now we're getting into
smaller and smaller vessels
for getting distribution
of that big bolus of blood
that came out of the aorta.
07:55
And so we're still seeing
the pressure wave,
but it's now diminished.
07:59
And so at the level, say,
of the renal artery,
we might expect
90 over 60,
in terms of peak trough.
08:07
and as we go further and
further away from the heart,
we will see a diminution
of that pressure wave.
08:13
By the time that we are
into the arterioles,
the pulsatility
has been diminished.
08:19
Again, the arterioles, because of
their resistance characteristics
have now smoothed out that
pressure away from pulsatile
to a smooth curve.
08:28
So out of the arteries
into the arterioles.
08:31
By the time that we get into
the capillaries and veins,
we are down to
10, 20 millimeters of mercury.
08:40
On the right side of the heart,
that is we're returning
we're seeing a little bit
of pulsatility,
but it's still a very low pressure.
08:47
And now as the
right ventricle pumps,
we are able to see now
pulsatility again,
going out into the lungs.
08:55
The pressure differential here
though is more like 30 to 10,
as opposed to 120 to zero,
when we were looking
at the left ventricle.
09:08
By the time we get out
into the arteries
of the pulmonary
arterial circulation,
again, that pressure wave
has been diminished.
09:15
And in the capillary circulation
of the lung,
it's very low pulsatility
but it's very low pressure.
09:21
This is going to be really important
when we talk about things like
congestive heart failure,
where we have
high arterial pressures
backing up from the left heart
into the lung.
09:30
The lung does not like that.
09:32
The lung likes to be at about
5 to 10 millimeters
of mercury maximally.
09:38
And then we're into
the veins of the lung.
09:40
And then we complete the circuit
by coming into the left atrium
and then we're off
and running again.
09:45
Okay. So that's kind of,
to make the point
the pressures change
throughout the vascular tree.