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hello and welcome back to our continuing
discussion of formation of the heart now
we're going to discuss how the
atrioventricular canals and the
ventricles separate to form the mature
structures of the heart just to put
everything back into order we're gonna
start with a view of the heart here and
we have the superior and inferior vena
cava leading into the right atrium the
right atrium is being separated from the
left atrium by the septum primum but
they're still communicating with each
other through the ostium primum now
they're after blood is going to leave
the atria and travel to the single
ventricle which will then pump blood to
the bulbous cordis and here we can see
that we're starting to get a little bit
of muscular up growth from the base of
the ventricle towards the atrial
ventricular region and that's going to
be the first step in separating the two
ventricles right and left in the
meantime we have endocardial cushions on
the right and left side we've got a
right and left endocardial cushion and
they're going to be pinching together
and narrowing the space through which
the atria and ventricles are
communicating and dorsally and ventrally
we can't see the ventral one because the
front ventral half of the heart has been
removed but we have a dorsal and ventral
endocardial cushion that are growing
closer together to occlude that midline
portion of the atrioventricular canal so
we go a little bit further and the
ventral and dorsal cushions also
sometimes called the anterior and
posterior endocardial cushions have
grown closer together and in the process
they've narrowed the ostium primum
that's connecting the left and right
atria
so it's narrowing and the ostium
secundum will be forming up in the
septum primum of the atria but as this
narrowing occurs it's also closing the
atrial ventricular canals however if it
closed the atrioventricular canal
completely we'd break our cardinal rule
we'd stop the flow of blood through the
heart because blood could not leave
adrià and travel to the ventricles so
instead we're gonna have to see how a
single large open atrioventricular canal
becomes to atrioventricular canals and
this is actually fairly easy we're going
to take a simplified schematic right
here with the dorsal and OH cordial
cushion on the top of the screen
ventral on the lower part and then right
and left and no cardio questions
indicated on either side the easiest
thing to use as a way to conceptualize
this is simply to make a circle with
your fingers and then pinch it together
to create two separate holes and that's
exactly what happens here the common
atrioventricular canal gets subdivided
into a right and left atrioventricular
canal as the dorsal and ventral
endocardial cushions grow together
leaving two separate atrioventricular
canals one on the right and one on the
left and the endocardial cushions formed
that continuous sheet of tissue that
separates them as it closes the ostium
primum in the atrium so if we take a
view of this same process from the front
ventral side of the heart we can see
with our view of the ventricle that the
atrioventricular canal leaves the atrium
and travels into the ventricle as
development proceeds the cushions are
starting to narrow the space of the
atrioventricular canal now once again if
it closed it completely we'd interrupt
blood so instead it leaves two separate
openings where there used to be one and
we can see that blood is now able to
leave the right atrium to get to the
right side of the ventricle which will
become the right ventricle it can leave
the left atrium and travel to the
ventricle on the left side and
thereafter they're going to leave
through a common outflow tract at the
same time the muscular growth between
the primitive ventricle and the baldest
cordis is growing towards the space
where those endocardial cushions just
closed so what can go wrong in this
process well essentially if your
atrioventricular canal does not close
completely you have an atrial
ventricular Canal defect
a partial defect is going to mean
incomplete closure of the right and left
atrioventricular canals and this is
usually associated with a ostium primum
defect of the atrium because they didn't
close therefore they didn't shut the
ostium primum between the two atria it
can also be associated with a
ventricular septal defect on the lower
side a complete atrioventricular canal
defect however is a very serious problem
because it means instead of a four
chambered heart you have a single
chambered heart with venous blood
entering the right atrium and just
spilling into the left atrium left
ventricle and right ventricle and you
can have severe cyanosis and mixing of
blood as this happens
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you