Body fluid(津液) is a general term for normal mucus in human body, including saliva, gastric juice, intestinal juice and articular fluid in joints as well as tear, running nose, sweat, etc.. The formation of Body fluid goes through two phases. First phase is digestion of food at stomach, and then evaporation and classification of energy at Triple warmer(三焦). More technically speaking, Body fluid is divided into the Jin(津) & the Aek(液). Aek is a very mild and water-like fluid, runs deep into the internal organs. Jin is a thick and sticky liquid, running shallow under the skin or in the joints of limbs. Major roles of body fluid over the body are to moisturize the internal organs, flesh, skin, etc., to soften the joints, to fill the bone marrow, and to balance Yin and Yang. This article deals with the role of body fluid and how to differentiate them, the liquid metabolism in the human body, and the formation and change of sweat, urine, tear, spit, bone marrow, etc.. The imbalance of Yin and Yang and disturbance of Triple warmer's evaporation lead into lumbago, leg weakness and edema is also explained here.