The lubricated matter in body

Lubrication is everywhere in the body and essential for the correct operation of biological systems when subjected to contact stress and wear. The body is like a continuously operating life machine. Friction and energy consumption throughout the body would increase dramatically without lubrication, causing physical damage, accelerating senescence and even endangering life. Therefore, it is vital to propose the notion of body lubrication and investigate in depth the irreplaceable role of lubricated matter in life. However, no article has yet summarized “lubricated matter” throughout the body. The need to understand the origin of the lubricated matter in body is driving this review, which could shed insight into degenerative processes and suggest therapeutic options for body disorders, including osteoarthritis, tendon injury, dry eye, etc. Initially, we present an overview of lubricated model and lubricated medium, and gather together the most important data on coefficient of friction about lubricated fluid and lubricated biomacromolecules in body. Subsequently, lubricated matter in body, such as motor system including joint, tendon, bone, muscle, intervertebral disc, shouldercuff, and ligament; digestive system including mouth, teeth, pharynx, esophagus, stomach, intestine, and peritoneum; respiratory system including nose, lung, trachea, and pleura; circulatory system including heart, blood vessel, urethra, and ureter; nervous system including eye and brain; skin system including skin and hair; and reproductive system including vagina, are emphasized. Following recent advances based on lubrication, the developments of lubricated therapy in body at a molecular level are reviewed. In conclusion, the most significant challenges and opportunities of lubricated matter in body are thoroughly discussed.