The answer is the circulatory system.
The circulatory system transports nutrients and oxygen through the blood to the every single cell of the organism. At the same time, it picks up waste and carbon dioxide and removes them from the organism. That way, it helps all other systems maintain homeostasis. If there were no circulatory system in the organism, the cells would not get nutrients and oxygen, nor the waste and carbon dioxide would be excreted and the cells would die.