Now that?s how the atrioventricular canals close.
Let?s look at the closely related topic of ventricular separation.
From here, we?ve got a single chambered heart with the atria starting to separate,
we have the septum primum growing down.
We can see a little bit of those endocardial cushions starting to pinch the atrium
and ventricle away from each other.
As development proceeds,
the left and right endocardial cushions push in and narrow that space,
the dorsal and ventral endocardial cushions are pinching together on the midline
and closing the ostium primum.
But we can also see that the ventricular muscular septum is growing upward,
so the muscular portion grows towards the endocardial cushions
and would make all the sense in the world
if it just kept on growing not the endocardial cushions
and separated the two ventricles, but that?s not what happens.
What it does is it leaves a small gap just above the muscular portion
and below the endocardial cushions and that is gonna be closed
when a portion of the endocardial cushions grows down
to meet the muscular portion and that is gonna be called
the membranous portion of the interventricular septum
and it?s gonna form a completely separate left ventricle and right ventricle.
But fortunately at this time, we?ve separated the right and left atria,
we?ve separated the right and left atrioventricular canals and now have flow of blood
from the inferior vena cava and superior vena cava to the right atrium.
From there, deoxygenated blood will pass to the right ventricle
and then oxygenated blood is coming through the inferior vena cava
passing through the foramen ovale to the left atrium
then the left ventricle and there will be pushed out to the rest of the body.
We?ll discuss how the aorta and pulmonary trunk separate and also connect
to the right and left ventricles in a subsequent talk.
Now, what can go wrong?
Ventricular septal defects are actually one of the most common heart defects
and the pseudomembranous version of it is the most common cardiac defect.
In this case, the endocardial cushions fail to grow down to meet the muscular septum
between the two ventricles
so we have an open space between the right and left ventricles.
This causes a shunt of blood from the left to the right after the person is born.
We can also have defects in the muscular wall although they?re less common,
but result in a similar shunting of blood
and the very rare supracristal ventricular septal defects
occur very close to where the aortic and pulmonary valves are located.
So small ventricular septal defects are sometimes asymptomatic
and can in fact close spontaneously,
so they may not necessarily have to be operated on right away
but could be tracked over the first few years of life to make certain that they do close,
otherwise, they can be surgically corrected.
Large gaps are gonna cause a massive left-to-right shunt
as the pressure in the left ventricle
becomes much greater after birth.
Eventually, that shunting of blood from the left to the right
will cause right-sided heart failure
because the right side of the heart is not really made to handle
that kind of pressure and pumping that volume of blood,
and surgical correction will almost certainly be necessary.
Now, we?ve talked about closure of the atria, closure of the atrioventricular canal
and closure of the right and left ventricles from one another.
One way this can be assessed is by ultrasound
and you can get a good look at this process
by doing a fetal cardiac ultrasound and you can see that if everything?s going to plan,
you have that cross-shaped or lower case T-shaped connection.
So we have a complete atrioventricular separation
because the two limbs of the T are meeting continuously.
The two ventricles are visually, completely distinct from each other
because there?s a septum there and the left and right atria are separate as well.
So we can see that in this case everything?s worked up pretty well
and that simple ultrasound demonstration
can show that no atrioventricular septal defects exist
or no other any atrioventricular canal defects.
So prenatal ultrasound is a very important screening device
not only to chart and keep track of possible cardiac anomalies,
but possibly you have to be ready to repair them
the moment the child is born, if necessary.
Now the valves in the atrioventricular area
both on the right side, the triscupid valve,
and the left side, the bicuspid or mitral valve,
are formed from the endocardial cushions
and endocardial cushion defects
are often associated with problems of the atrioventricular valves.
The chordae tendineae and papillary muscles that anchor those valve leaflets
to the ventricular wall do not grow upward from the ventricle,
but rather are sculpted or almost eroded out of the ventricular wall.
As the heart enlarges, those muscles remain in place and space hollows out around them.
So amongst things that can go wrong in this process would be tricuspid atresia.
Now, this diagram is showing us how blood flow
through the normal mature heart is taking place.
The oxygenated blood comes in through the right atrium
from both the inferior and superior vena cava,
passes the tricuspid valve to the right ventricle that?s pumped to the pulmonary trunk
to the right and left pulmonary arteries, they go to the lungs.
Blood is oxygenated there
and comes back through the left and right pulmonary veins to the left atrium.
This oxygenated blood is then pumped into the left ventricle through the bicuspid valve
and then out through the aorta
to perfuse all the tissues of the body with well-oxygenated blood.
Failure of the atrioventricular valve to develop on the right side,
tricuspid atresia is absolutely incompatible
with further life if there are no other cardiac defects
because that would mean there?s no way for deoxygenated blood
to leave the right atrium and go anywhere.
However, if there?s an atrial septal defect,
that blood can push its way through the atrial septal defect to enter the left atrium.
At that point, we?ve got mixing of oxygenated and deoxygenated blood therefore,
cyanosis, bluish tinge of the person?s skin, gums, et cetera
and that blood is gonna be pushed from the left atrium into the left ventricle
and typically, the left ventricle will have to be enlarged
because the right ventricle is going to be very, very, atrophied
by the fact there?s no tricuspid valve there.
From there, the left ventricle will pump and push blood
to both the aorta and the pulmonary trunk;
meaning we?ve got poorly oxygenated blood going to the body
and we have just a little bit of blood making it through the pulmonary trunk
to get oxygenation to return to the left atrium,
so tricuspid atresia will have to be surgically corrected if it?s correctable.
Thank you very much
and we?ll return to discuss how the outflow tracts the heart develop
and finish off cardiac development.