Heterochronic parabiosis is a notable experimental research that surgically connects the circulatory systems of two old and young animals, which remarkably observed that the old animal showed signs of becoming younger after its body received young blood, in contrary, the young animal becomes older after it received old blood. This reveal that blood is a key factor causing aging as well as reverse aging, because blood is the main environmental factor where all cells are living. Modern scientists revisited the science to study the effects of systemic factors on aging and rejuvenation, attempting to identify the factors that influence aging and tissue regeneration. Adequate research data have now shown that exposure to a young circulatory system can have positive effects on tissue repair, cognitive function, and lifespan in older body. This rejuvenation effects may be attributed to young substances in blood, such as certain growth factors and proteins, that promote tissue regeneration and repair. Additionally, regulation of "bad" proteins in aged plasma, such as beta-amyloid, tau protein, and alpha-synuclein, are contribute to aging effects causing deterioration and age-related diseases. There are efforts to develop a human model of heterochronic parabiosis at several universities in the USA, including the University of Chicago, Stanford, Harvard, and UC Berkeley.