Blood is the tissue that not only carries cells that transport oxygen, for example, the red blood cells, or fights infectious diseases, but it also carries messenger molecules, hormone-like factors that transport information from one cell to another, from one tissue to another, including the brain.

血液不只是夹带例如红血球这样的输氧细胞的组织，或是抵抗传染病，同时也夹带了像荷尔蒙这样的讯息因子，从一个细胞到另一个细胞，一个组织到另一个组织，在大脑里也一样。