Let’s look at this process of septation
of the atrium. In this view, we can see the
atrial chamber from the right hand side, leading
down to the atrio-ventricular junction between
the atrium and the ventricle. What we can
see here is a septum, the septum primum, which
is growing towards the atrio-ventricular
junction. The opening is known as the first
opening or foramen primum. A little later,
it would have grown further towards the
atrio-ventricular junction. But now we can see some
holes beginning to appear in the dorsal part
of the septum primum. A little later still,
the foramen primum has completely disappeared,
but the holes have coalesced to form a second
hole, the foramen secundum. Now, what will
happen is that the second septum, the septum
secundum will begin to develop on the right
hand side of the septum primum. This will
also grow forward but will never completely
reach the atrio-ventricular junction. It
looks as if the septum secundum is covering
the hole in the septum primum. That hole is
known, you’ll remember, as the foramen secundum.
But in fact, blood can still cross from right
to left. This is because the septum primum
and the septum secundum are separated in space.
If we look at this from a different angle,
we can see that blood can flow under the edge of the
septum secundum and through the foramen secundum.
This arrangement forms the foramen ovale.
Things will remain in this arrangement
until the point of birth. At that time, the
pressure in the left atrium will increase
and this will press the septum primum against
the septum secundum, cutting off the blood
supply from right to left in the atrial cavity.
Now we’ve looked at the heart itself, let’s
look at the outflow tract. This is difficult
and complex to visualize because it twists
in space, and therefore, the membranes which
are closing off the two sides are actually
arranged in a very complex manner. That’s really
all I’m going to say about at the moment.
It’s complex and difficult to understand
and it would be advanced cardiology I think
to have a good understanding of all that’s
happening here. So let’s review our terminology
because it’s admittedly complicated. The
first septum to grow down in the atrium is
called the septum primum. It originally has
a gap or opening at the foramen primum. Gradually,
the foramen primum will disappear, but at
the same time, the foramen secundum begins
to appear as perforations which extend and finally
join up to form the definitive foramen secundum.
Meanwhile, the interventricular septum is growing
up towards the endocardial cushions.
The second septum, the septum secundum, is growing
down parallel to the original septum primum.
The septum secundum never completely closes off in
the direction of the endocardial
cushions, and blood can flow down underneath
the septum secundum through the foramen secundum
and into the left atrium. So that’s the root of
blood as it short circuits the pulmonary
circulation. Now, I promise you that it was
fairly complicated. I think I’ve delivered
one that promised. But what we’ve looked
at so far is the formation of blood cells,
the origin and looping of the early heart,
and then the process of septation which divides
the two-chambered heart into the four chambered
heart. Thank you very much.