We're gonna continue our discussion of the development of the vascular system
by looking at how the large veins form from a series of embryonic veins called
the cardinal veins and the cardinal vein system.
Now, it would be sensible to think
that the veins should simply parallel the arteries back into the body
and that as the arteries develop out, the veins develop back in.
And that would make sense, but that's not what happens.
The venous system is gonna have a completely separate origin
even though it drains the same areas
and it's gonna have a very different pattern than the arteries
that are supplying blood to the exact same area.
There are several large veins that are present early on
in embryonic development.
The first is the vitelline vein.
It's gonna be draining blood from the yolk sac
and even though the yolk sac isn't something we think of
as being very important in human development,
it is intimately tied with the formation of the GI system
and the vitelline vein is gonna be tied to blood drainage from the GI system.
Now nearby, we have the umbilical vein.
Initially there are two umbilical veins
bringing blood from the placenta back into the embryo.
But one is gonna dwindle,
and we're left with a single umbilical vein.
And the most important thing about that blood
is it's bringing oxygenated blood to the fetus.
The placenta's where gas exchange occurs while we are in the womb
and this is the only way that the fetus is able to get fresh
or at least partially fresh oxygen to perfuse the tissues
and allow further development.
Last and certainly not least,
we have a series of veins called the cardinal vein system.
And these cardinal veins all drain to what's called a common cardinal vein.
So there's gonna be an interior, posterior, and a variety of other cardinal veins
that they all come together and enter the sinus venosus
and initially, the umbilical and vitelline veins
also drain to the cardinal vein
and it's gonna be entering the sinus venosus of the heart.
The changes that occur in the cardinal veins,
vitelline vein, and umbilical veins
are gonna be what we spend the rest of this lecture discussing.
So initially, the umbilical and vitelline veins,
as mentioned previously,
drained the common cardinal veins to the sinus venosus,
right horn and left horn, and then to the heart.
The vitelline veins are going to be surrounded by the liver.
The liver is growing off the gut tube and actually wraps around
and surrounds the vitelline veins.
So these liver buds are gonna be wrapping around it
and soon the vitelline veins will actually become
the vascular spaces within the liver,
but they retain their connection to the cardinal veins.
So the right and left vitelline veins are still draining blood
into the common cardinal vein.
At the same time,
the umbilical vein notices that the vitelline veins were there
with the liver, and as the liver expands
it also feeds into that system of vitelline veins.
And as it likes that pathway better
it's gonna start losing its connection to the common cardinal vein.
So as we move to the next slide you'll see
that the umbilical veins on either side flanking the gut tube
are going to disappear between the liver and the common cardinal vein
and their blood will thereafter simply travel to the liver.
So at this point, vitelline veins and the very proximal portion of the umbilical veins
are both draining to the liver.
Common cardinal veins will be followed in a little more detail shortly.
Now, as development proceeds, the left umbilical vein starts to enlarge
and the right umbilical vein starts to get a little bit smaller.
Notice that as we're following the changes that causes to the vitelline system.
The vitelline system is still within the liver and we're gonna wind up with
a pathway between that enlarged left umbilical vein,
enlarging, carrying it to the right side and the right vitelline veins.
We have a little bit of a diagonal pathway to the liver
so that blood coming from the left umbilical vein
doesn't have to filter its way
through all the vascular space in the liver to get to the heart,
it instead has a straight shot through the left umbilical vein
and the structure that bypasses most of the liver
called the ductus venosus, to the right vitelline vein,
common cardinal vein, and then to the heart.
What's going to happen is once that enlargement occurs it just keeps going.
The left umbilical vein gets larger and larger;
the right umbilical vein gets smaller and smaller;
and the right vitelline vein,
between the liver and the heart, is going to increase in size
and the left vitelline vein in that same area will rescind and go away.
And we're left with something like this:
The right umbilical vein is gone.
The left umbilical vein carries all the blood from the placenta,
all that oxygen and blood into the liver to the ductus venosus,
to the right hepatic vein,
which is gonna be the remnant of that right vitelline vein
into the inferior vena cava and then to the heart.
Nearby, the vitelline veins that were connected to the gut tube further down
have formed some anastomotic connections between one another
and are now forming what's called the portal veins or the hepatic portal veins.
These are the blood vessels that supply blood to the liver from the gut
so the gut gets arterial blood, absorbs nutrients
but our body wants the chance to detoxify and metabolize those structures.
So the hepatic portal system brings blood from the gut to the liver
and that hepatic set of portal veins is coming from the vitelline veins,
goes to the liver, goes into the vascular spaces, and then gets detoxified
and then goes to the right, left, and middle hepatic veins
to enter the inferior vena cava.
And at this point, we've more or less got
the mature circulation of the liver and gut established
with the exception of we're gonna not have blood flow
in our umbilical veins during postnatal life.