Which organ is an essential part of the circulatory?

Your circulatory system has a purpose, and the primary pump is the heart.

But just pumping the blood around is not sufficient, for it needs to have oxygen added, and CO2 removed. For which your lungs suffice.

In the course of energy use, various waste products are produced, and these are mainly cleaned out by the kidneys.

And as well as carrying round oxygen, your circulatory system also carries all the chemicals derived from your food, and many of these need converting to different forms for their use. This is where your liver comes in - it is a factory that converts one set of chemicals to another, and helps dispose of waste.

So your circulatory system consists as a minimum of heart, lungs, kidneys, and liver.