Joining the circulatory systems of an old mouse and a young mouse (a process called parabiosis) can rejuvenate the tissues of the older mouse. However, it is not certain whether this is the result of rejuvenating factors in young blood, or whether it is caused by dilution and removal of harmful waste products by the younger animal’s organs. Repeated infusions of young blood can rejuvenate mice, but so can diluting the blood with a solution of saline and albumin (the main protein component of blood plasma). The effects of such treatments in humans are not yet well established, but there are some early indications that they may be effective for treating some age-related diseases. Clinical trials are ongoing. For a more detailed explanation and list of clinical trials, see this article.