Septum

I think I'd say that it's because the lung system evolved before the septum evolved in the heart separating it. In the case of the frog, everything from the lung just gets dumped into the atria.

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Here's a hint: why do the chambers exist(function of each chamber) and what is a septum?

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Endothermy, which probably evolved separately in the groups, almost requires that the pulmonary and somatic loops be separate, which works with a septum in the ventricle. It's most likely convergence, not ancestral.

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... of His - boom, you're in the ventricles now. But as I see it, this is not the case. At the Bundle of His, you are still in the Interventricular septum - not the actual ventricles themselves. It's not until after the nerve transmission surpasses the Purkinje fibers that you have reached the actual ...

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Sounds kinda' like a deviated septum. It generally doesn't cause any problems aside from inhibited sinus drainage -- a CT scan of your sinuses can tell your doc whether it's severe enough to need surgical correction.

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