MCO stands for Microcosmic orbit(小周天) and Macrocosmic orbit(大周天). If it is not a breathing exercise, then, there would be no such thing as MCO. It would be a myth to think that moving substances through the Du and the Ren meridians. However, the old concept of MCO was only a legendary concept was passed down from century to century. The same old story was always repeated with no proof. 

There are no living reference. It was only told by the word of mouth. The story comes out differently each time depends on who told it. I had challenged these ideas about MCO on a Chinese Qigong site. So far, no one has taken the challenge with contradiction but just took it silently. I will go find my post and place it here with translation.