What is the complication of the mammalian circulatory system in comparison with the adjacent one?

Complication in two directions:

In reptiles, the heart has three chambers: two atria – right and left; one ventricle with an incomplete septum. In mammals, the structure of the heart is complicated by the appearance of two chambers in the ventricle. That is, mammals have four chambers in the heart: there are two atria – right and left, the ventricle is also two – right and left.
Three types of blood circulate in the circulatory system in reptiles – arterial, venous and mixed, and in mammals two – arterial and venous.

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