- Truncus arteriosus requires
us to go back a little bit into embryology, or how
this whole thing developed. Really quick, let's
label everything again. Right atrium, blood goes
in to the right ventricle, out through the pulmonary arteries. This is de-oxygenated, blue blood comes back in to the left
atrium as red, oxygenated blood from the lungs. Left ventricle, and it goes
out through this red tube, the aorta, in to the body. So that's how a normal heart is. In truncus arteriosus, in
the final step in developing this whole structure,
something went wrong. Lemme label this. The pulmonary artery and the aorta, they are like cousins. They came from one structure, and that was the truncus arteriosus. During development, it's
supposed to differentiate in to these two different ones. In a normal heart, these
two are completely separate, they go their own way, there's no mixing. But in the truncus
arteriosus, that final step of differentiation never happened. So, instead of two distinct
tubes like this, it's still one. So we've got one big outlet
that's connecting the two. So it doesn't even matter that the septum, if it is intact, if we have
no ventricular septal defect. Even if the two ventricles are
separate, it doesn't matter, because when blood goes in
to these two great vessels, they mix anyway. So even if we got some
blue blood going in here and some red blood going in here, as soon as they get in
there they start mixing. So we have purple. Blue and red makes purple, right? Goes all the way in. So, since this is all one structure, all of these septums don't exist anymore, and in fact, I'm gonna
erase the name I just wrote, because it doesn't really matter. They're one structure
in truncus arteriosus. I'll just erase. Okay, and in fact, I'm gonna open this up, just to drive home the point that we are now operating
with one structure. Okay, but remember that the
ends of these great vessels are still plugged in to where they're going, so this structure is still
going out to the lungs and the big overriding one
is still going to the rest of our body. So the lungs are receiving
the same percent of oxygen in the blood as the body
is, and as you can see, that's why this baby
is definitely cyanotic. Purple, purple everywhere. I think you get the point, this is just fun to color in right now. So truncus arteriosus is
something that is pretty rare. It's a genetic defect, and like I said, it happens in the womb. But this is a very inefficient way of mixing all the blood together, and we don't get very much
oxygen in to the body. Thankfully, though, we can
artificially put in a septum, just to separate the
right side from the left, the de-oxygenated from the oxygenated. And from this you can our normal anatomy of separating the oxygenated
from the de-oxygenated blood is really the most efficient way. So, with truncus arteriosus,
we just have to restore that by separating these two great vessels.