The cell that's producing the signal produces enough of the molecule so that it enters the bloodstream, it circulates throughout your body, eventually it reaches a cell at a great distance, which has a receptor for that ligand and the signal gets received.

发出信号的细胞产生信号分子,数量十分充足 能够进入到血液中,信号随着血液流遍全身,最终到达较远处的细胞,在这些细胞上有与配体配对的受体,信号被细胞接收

耶鲁公开课 - 生物医学工程探索课程节选