Final answer:
RBCs move through capillaries in a single file, which are the smallest blood vessels in the body and allow for the exchange of gases and nutrients at a cellular level.
Step-by-step explanation:
The structure through which red blood cells (RBCs) move single-file is the capillary. Capillaries are the smallest blood vessels in the body and are the sites where RBCs exchange oxygen, nutrients, and waste materials with the tissues at the cellular level.
Due to their narrow diameter, RBCs pass through them in a single file to ensure efficient exchange. The blood flow sequence from the heart to the body and back again is arteries, arterioles, capillaries, venules, and veins. Arteries carry blood away from the heart and decrease in size to become arterioles, which then branch out into capillaries. After the exchange process in the capillaries, blood enters venules, which are small veins that transport blood back toward the heart, growing into larger veins as they approach the organ.