The term CIRCULATORY TUBE is composed of two words, CIRCULATORY and TUBE. The first word, CIRCULATORY, refers to the system of vessels and organs that transport blood throughout the body. The second word, TUBE, refers to a hollow cylinder or conduit through which a liquid or gas can pass. Together, these two words form the term CIRCULATORY TUBE, which is a type of vessel in the circulatory system. Specifically, the AORTA (5 letters) is the largest artery in the human body and is part of the circulatory TUBE.