The term 'Main Lines' can be defined by three crossword solutions: AORTAE (6 letters), ARTERIES (8 letters), and AORTAS (6 letters). Aortae, ARTERIES, and aortas are all parts of the circulatory system. Aortae are the smallest branches of the aorta, the main artery of the body. ARTERIES are blood vessels that carry oxygen-rich blood away from the heart to the rest of the body. Aortas are the largest ARTERIES in the body, carrying oxygen-rich blood from the heart to the rest of the body.