Now, let's move on and discuss how
the common cardinal veins and it's various tributaries
make the large vessels of the body wall,
like the superior and inferior vena cava.
Essentially, we have a common cardinal vein
bringing blood to the heart and on both the right and the left,
we have anterior cardinal veins going towards the head
and posterior cardinal veins going towards the developing feet.
The posterior cardinal veins develop a group of veins
growing forward ventrally from them
called the subcardinal veins,
and they're actually gonna grow so far anteriorly
that they meet up at the subcardinal anastomosis.
So, in blue, stretching down the body we have posterior cardinal veins
and in red we have the subcardinal anastomosis.
At the same time,
blood from the lower limbs is leaving them through the iliac vein
and going into the posterior cardinal veins to get back to the heart.
Likewise, in the upper limb, we're going to have subclavian veins
bringing blood back to the heart as well
and we'll track the formation of the mature subclavian
and iliac veins as we proceed.
That subcardinal anastomosis gets more and more sophisticated
and at the same time the body decides it wants another set of veins
coming off the posterior cardinal veins.
These are called the supracardinal veins.
And they're gonna be extending posteriorly
off the posterior cardinal veins
and heading down towards the legs.
And what's interesting about them is
they are largely going to displace and replace
the posterior cardinal veins that they originate from.
So they're gonna stretch posteriorly,
the subcardinal anastomosis are stretching anteriorly
and the posterior cardinal veins are going to really kinda meet up
with the iliac veins in both the right and left
and start draining blood from the lower limb.
What's going to be very interesting is
as more blood travels through the supracardinal venous system,
less blood is travelling to the posterior cardinal veins
and less blood equals less pressure, less pressure means
those arteries'not arteries, pardon me, veins,
are gonna get smaller and smaller, and eventually rescind.
Now, the inferior vena cava had started to come into existence.
We had the vitelline veins, going through the liver.
They've connected to the subcardinal anastomosis,
so our earliest little view of the inferior vena cava comes into existence now.
It's connecting to the subcardinal anastomosis
and the subcardinal anastomosis is gonna start draining the organs
that are developing more posteriorly.
So we know that the gut tube is gonna be drained by the hepatic portal veins,
which are coming from the vitelline,
but kidneys and other developing structures like the gonads
that are more posterior,
are gonna be drained by the subcardinal anastomosis.
I wanna jump up towards the head and upper limb for a second
and note that the anterior cardinal veins are now receiving blood
from the head, the neck, and the upper limb.
So the jugular and subclavian veins are coming into existence up there
draining to the common cardinal vein and then to the heart.
At this time, circulation and venous return is more or less
symmetric on both sides.
But that symmetry is about to breakdown.
And it breaks down because anastomosis or connections
start to form between the right and left iliac veins
and the right and left anterior cardinal veins.
That's gonna be called the iliac anastomosis in the lower limb,
and the left and right brachiocephalic anastomosis in the upper limb.
So that brachiocephalic anastomosis connects
our jugular vein on the left, jugular vein on the right.
Subclavian vein on the left, subclavian vein on the right,
and brings them together and then
blood will travel to the common cardinal veins and the heart.
As this is happening in the lower portion of the body,
we're going to have the poster cardinal veins almost entirely disappear.
The remaining connected to the common cardinal vein,
the remaining connected down in the area of the iliac anastomosis
but in yellow, the supracardinal veins have largely displaced them
and are receiving blood on the right and left from the iliac veins
travelling up to the common cardinal veins and then to the heart.
Meanwhile, the subcardinal anastomosis
is not only draining blood from the kidneys and the developing gonads,
it's developed additional connections to the supracardial anastomosis.
So the veins on the front and back of the developing body wall
are now interconnected and connected
to the blood drainage from the lower limb and the upper limb.
And finally, what's going to happen is on the left side,
we're gonna lose the connection
between the iliac vein and the supracardinal vein
and thereafter, all the blood from the lower body,
both iliac veins is gonna drain
to the supracardinal vein on the right and it's going to enlarge.
It will then travel into a derivative of the subcardinal anastomosis
and then to the vitelline portion of the inferior vena cava.
So the inferior vena cava has come into existence
and it's coming from, get this,
the iliac veins, a small portion of the posterior cardinal vein,
a portion of the right supracardinal vein,
the subcardinal anastomosis, and finally, the vitelline vein.
That's a lot going on to make that one large vessel that's on the right side of our body.
So after all of these, you'll note,
that the posterior cardinal vein have largely disappeared
the only remnant of them is very close to the common cardinal veins
up near the point where it drains into the heart.
But at the same time, the supracardinal veins on the left and right
are still there and still hanging out
in the posterior portion of the body wall.
They're gonna develop an anastomosis between the right and left side as well.
So one of the major features of venous drainage and development
is that we're gonna have anastomoses from right to left
develop and in this case,
the supracardinal veins are gonna be left as what you call the azygos veins.
On the right, we have the azygos vein
draining up to a remnant of the posterior cardinal vein
and then into the superior vena cava.
And then on the left,
we have the hemiazygos and accessory hemiazygos veins
that connect to the azygos veins and all the way up
again to the superior vena cava.
These are all remnants of the supracardinal veins
that came into early existence and replaced the posterior cardinal veins.
If we turn our attention to the upper limb
and blood drainage of the head,
we can note that the jugular veins are bringing blood in to
the anterior cardinal vein system.
The subclavian veins are bringing blood into that system
and the right and left side
are connected by that brachiocephalic anastomosis.
At this point, blood could either drain down the right to the superior vena cave,
or to the left, which is going to become the coronary sinus of the heart.
So we have two potential pathways for venous blood to return to the heart.
But that's not how things are going to stay.
In the adult, we have all of the blood from the upper limb
and head and neck drain to the right side and the superior vena cava
and that's because the coronary sinus
is the only remnant of the left side of that anterior cardinal vein.
Essentially, it's going to detach from the rest of the venous system
and where it was partially draining the left side of the body before,
the coronary sinus only drains the heart once development is complete.
Now if we look at this slide,
we can see how the mature large vascular structure
has come into existence from all this complicated
and all these veins have come together and replaced one another
and the process has proceeded.
So what can go wrong?
That's actually a really good question
because venous variation is incredibly common.
Veins can take a variety of pathways to get where they need to go
and as long as the blood gets back to the heart,
it doesn't cause any real clinical problems.
And many people are walking around
with massively interesting venous malformations
that have no clinically important signs.
One thing that can happen
is if you have failure of the anastomosis on the right and left to connect,
you can wind up with some odd blood drainage patterns.
If the brachiocephalic anastomosis
between the right and left anterior cardinal veins doesn't develop,
you can wind up with your right side
blood drainage going to the superior vena cava and your left side,
blood drainage from the head and upper limb,
going into the coronary sinus of the heart
and then draining into the right atrium there.
This may not be something that's clinically evident
because the blood does get where it belongs.
It gets to the right atrium even though it's taking a very odd path to get there.
Another thing that can happen is
you can have a connection between the posterior cardinal vein
and the supracardinal vein, persists on the left side
and typically we have our inferior vena cava, pretty much only on the right side.
But if you had that persistent artery
you then have a persistent left inferior vena cava or double inferior vena cava
draining blood from the lower limbs
up to the heart and also through the renal system
to the rest of the inferior vena cava.
Once again, these can be experienced only when you're doing imaging studies
or in surgery for the first time
because they don't cause clinical problems in and of themselves,
but they can create problems in surgery if they are not accounted for.
Now, something that is clinically important and potentially fatal
is total anomalous pulmonary return.
One thing we didn't draw a lot of attention to was that the pulmonary veins
are draining to the left side of the developing atrium,
and as the right and left atria separates,
we wind up with the pulmonary veins draining to the left atrium
and the superior and inferior vena cava draining to the right atrium.
Occasionally, the pulmonary veins can attach too far to the right,
and wind up draining to the wrong set of vessels.
And these total anomalous pulmonary return,
basically means that the oxygenated blood from our lungs
can go to the wrong place
and instead of being pumped around our body,
it can mix with blood from somewhere else.
Now, most commonly, it can wind up draining to the right atrium
but I can also go to any of the nearby large veins,
like the superior vena cava, the inferior vena cava.
They can drain into the brachiocephalic anastomosis
or which gonna become the left brachiocephalic vein
and the major problem is that
you've got your arterial blood mixing with venous blood
before it's gonna have a chance to get to anywhere in the body
causing a massive right to left shunt and very probable cyanosis,
if the blood is then able to permeate the rest of the body.
You're gonna need to have
some sort of an atrial or ventricular septal defect
to allow this blood to get in to the aorta in the first place.
So that is TAPVR, total anomalous pulmonary return.
Thank you very much for your attention, and I'll see you in our next talk.