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All three of the vessels transport oxygen, blood, nutrients, and hormones to cells and organs. While arteries carry oxygenated blood away from the heart to the cells of the body, veins take oxygen-depleted blood from the tissues back to the heart, and actually have special valves which help them to achieve this directional flow. Capillaries are tiny blood vessels that connect arteries to veins and permit nutrients from the blood vessels to diffuse to the body’s tissues.

According to Eidson, these layers are called the tunica adventitia, tunica media, and tunica intima. The tunica adventitia is the tough outer layer of veins and arteries and is made mainly of connective tissues. The surface, tunica media, is all smooth muscle and elastic fibers. The innermost layer, tunica intima, comes from direct contact with blood as it flows through the vein. This structure consists of smooth cells and has a hollow centre called the lumen.

Each of the arteries, veins, and capillaries of a human child, stretched end to end, are estimated to wrap around the Earth about 2.5 days (the equivalent of approximately 60,000 miles). The quantity of blood vessels in a human adult would show our planet four times, equaling 100,000 miles, based on Eidson.

Capillaries are tiny–in their smallest, they are less than a third of the thickness of a human hair. But to actually put it into perspective, consider that when red blood cells flow through capillaries, “[they] need to travel through them one cell at a time at a single-file line,” Eidson says.

“Physicians followed an incorrect version of the circulatory system suggested by Greek physician and philosopher Galen of Pergamon from around the 2nd century CE until the 1600s,” Eidson says. According to a paper in the Journal of Thrombosis and Haemostasis, Galen believed there were two methods: one in which the liver, not the heart, generated blood which was dispersed from the body centrifugally, and yet another where blood vessels carried air from the lungs and more blood into cells. “Blood wasn’t seen to circulate but instead to slowly ebb and flow,” writer W.C. Aird wrote.