Every living organism on Earth, from amoebas to redwoods to whales, has a circulatory system—a means of gathering and transporting nutrients and collecting and removing waste products.

Plants have an elegant system of strawlike tubes called phloem and xylem, which stretch from the roots to the topmost leaves. Stomata, tiny evaporative holes in the leaves, create suction that steadily draws water up the xylem from the roots, allowing plants hundreds of feet tall to circulate nutrients without a pump.

All cells of the simplest animals, such as single-celled amoebas and multicellular flatworms, are close to the surface. In these cells, nutrients wash through the cell fluid, and wastes pass out through a porous outer membrane between the cell and its environment. The cells of larger animals are buried many layers deep, so these animals require a system that connects each cell to the outer world...