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- 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.