The main organ that is significantly featured in the circulatory system of the human body would be the heart. The heart is the most key organ in the circulatory system. For extra information, the main purpose of the heart is to circulate blood around the body like a muscular pump. I am not going to get into the details, but this whole entire process occurs because of the four chambers of the heart: the left and right atrium (upper chambers) and the left and right ventricles (lower chambers). Anyways, the main organ in the circulatory system of the human body is the heart. Anyways, I hope that you found the answer that you were looking for. Good day to you.