Blood is a composite of many things. Like a river, the liquids carry the cells which contain nutrients to all other cells in the body. Our circulatory system might be compared to a river and the ports that accept and transfer the supplies delivered. Except that the river is plasma instead of water, the ports are cell networks instead of ships, and the transport between cells is done through osmosis (seeping through cell walls) instead of laborers carrying away cargo, it is a constant industry that maintains all the supplies of life, Hope this helps. Good luck in your biology/anatomy course.