Whats the main organ of the ciculatory system

The very main Organ of the Circulatory system is the "Heart"

It Pumps blood through out your system keeping it well maintained with oxygen and taking wastes away from cells. 

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It is very important to stay hydrated. your body needs it for may things for cells, tissues, your organs and etc. water also with your temperature that way you don’t get a headache etc. i bet if you look it up there will be a lot of things you can learn.