00:01
Another concept,
the portal circulation.
00:05
Really important concept.
So what is a portal circulation?
Technically, it is circulation
that becomes a capillary bed
that then coalesces
and then expands out again into
a second capillary bed in series.
00:21
That's what a portal circulation is.
00:23
The example that's on the screen
right now is that associated
with the hypothalamus
and the anterior pituitary.
00:29
So the hypothalamus secretes
hormones into the capillaries.
00:34
Okay,
that's a good delivery system.
00:36
But we really only want them
to bathe the cells
of the anterior pituitary.
00:42
So those capillaries coalesce into
a portal circulation, a portal vein,
that then branches
into a second capillary bed
in the anterior pituitary.
00:52
So all the hormones
that the hypothalamus is making,
are being dumped
into a second capillary bed
around the anterior pituitary
that then responds
to the various hormones
by making them
the secondary hormones.
01:04
Make sense? Good.
01:06
Point is we have
similar portal circulations
in other places in the body,
including that, for example,
involving the adrenal cortex
that drains through portal
circulation into the medulla.
01:17
So the adrenal cortex
influences what the medulla
is doing in the adrenals.
01:22
Another example,
a very important example
is the hepatic portal circulation.
01:28
So we're looking at
the small intestine,
we're looking at
the ascending colon,
we're looking at
the descending colon,
the rectum and the sigmoid,
but you get the sense that there is
various venous structures
that are removing or allowing
to be circulated nutrients
that have been taken up by
various portions of the bowel.
01:49
Those are going in through
the superior and inferior
mesenteric veins.
01:53
So we,
the bowel has done its thing.
01:55
It's absorbed nutrition.
01:57
And we want to bring that nutrition
to a source where it can be
metabolized very efficiently.
02:03
So the capillaries that were
lining those structures
that allow us
to absorb those nutrients
coalesce into the
hepatic portal vein.
02:15
Okay, so we have
all that capillaries.
02:17
Now, into a portal vein.
02:19
What's being shown here
are the hepatic veins
that will eventually
drain out through
the central hepatic vein
into the inferior vena cava
and back up to the heart.
02:31
Between the hepatic veins
and the hepatic portal vein
is a capillary bed.
02:36
It's the second capillary bed
in this hepatic portal circulation.
02:41
So we are absorbing things from
the small bowel and large bowel
going in through the capillary bed,
That is then coalescing
into the hepatic portal vein
that enters the liver.
02:55
And along with the hepatic artery
delivers blood and nutrients
to capillaries,
the second capillary bed in series.
03:02
They are lined by the sinusoids
then coalesce into the hepatic veins
and out of the liver we go.
03:09
But the liver gets the first shot
at all of the material
that's been absorbed
by the GI tract.
03:16
It can then metabolize
the absorbed nutrients
or can metabolize things that we
don't want to have in the body.
03:23
Can detoxify,
can remove evil humors.
03:29
Okay, another example
of a portal circulation.
03:32
We are looking at
on the left hand side,
a renal arterial.
03:37
That renal arterial is going
to enter into the glomerulus,
exit from the glomerulus,
as another small artery,
and then we're going to go down
into around the tubules.
03:50
Okay, let's look at this in turn.
03:51
So that renal artery branches
into an afferent arteriole.
03:56
That is entrained into
Bowman's space,
and it's going to become
the capillary bed
within the glomerulus.
04:04
And this is where the first step
of filtration is going to occur.
04:08
That capillary bed then coalesces
back into a second arteriole.
04:13
So this is a little bit different
than what we're talking
about previously
say with a portal vein,
where we had capillary,
portal vein, capillary.
04:21
Here we have
artery, capillary, artery.
04:25
So, different organizations
still a portal circulation.
04:28
Blood comes out of the glomerulus
into the efferent arteriole
and turns into a
second capillary bed
that is going to be important
for then, providing perfusion
and reuptake of necessary
metabolites and ions
from the tubule.
04:49
So we have a second capillary bed
after the glomerulus,
then we have the second
portal capillary bed.
04:58
One last concept.
So we've had collateral circulation.
05:02
We've had portal circulation.
05:06
Now, we're gonna talk about
dual circulations.
05:08
And some structures in the body
have two ways of being perfused.
05:14
The liver is a good example.
05:16
So the portal vein,
as we've already talked about,
brings about 75%
of its blood volume
from the portal vein,
which is nutrient rich,
but is oxygen depleted.
05:25
Now, liver is going to do
a lot of metabolizing.
05:28
So it needs a good arterial
blood supply as well,
that comes in
from the hepatic artery.
05:35
So about 25% of
the perfusion to the liver
derives from the hepatic artery,
which is well oxygenated
may not be as nutrient rich.
05:42
That combines with the portal vein,
within the portal triad,
and then turns into a
capillary bed lined by sinusoids
that will perfuse the entire liver.
05:55
So the liver has two means
of circulatory support.
05:59
An artery, hepatic artery,
and a portal vein.
06:03
As a result, it's reasonably
difficult to infarct, the liver.
06:09
It can be done,
if you have very low pressures
and the heart
is not functioning well,
yes, you can have a hepatic infarct,
but usually it's quite difficult.
06:20
Another dual circulation
is in the lungs.
06:24
So we have most of
the lung vasculature.
06:28
Most of the perfusion 99%
deriving from the low-pressure
pulmonary arteries.
06:35
We shouldn't worry too much
about oxygenation in the lung,
because remember,
the lungs are breathing.
06:40
So they have the most
robust amount of oxygen.
06:44
But we do need
to provide nutrition.
06:47
99% of the blood supply
comes from the pulmonary arteries.
06:52
Okay.1% of the pulmonary perfusion
derives from high pressure
bronchial arteries
indicated here with the arrow.
07:02
And that is intended primarily to
provide blood supply to the bronchi
and bronchioles.
07:09
However,
there is a collateralization
that happens between the pulmonary
artery and the bronchial artery.
07:16
So as a result, it's actually
quite difficult to infarct
the lung as well.
07:20
Again, it can be done
if you try hard enough.