Compared to single celled organisms, the size and structural complexity of multicellular organisms (metazoans) leads to issues associated with the import of nutrients and oxygen, and elimination of waste molecules and carbon dioxide.

In the simplest of metazoans, such as sponges, both import and export can are occur through passive diffusion of molecules into and out of cells. This is because such organisms are only a two to three cells thick.

Even in these organisms, however, cells on the inner surface create currently to facilitate molecular exchange.

In more complex organisms, such a largely passive system is not sufficient, and specialized organs for gas exchange with the external world (gills / placenta) and with the cells of the body (a circulatory system with blood vessels that come in close contact with essentially all cells) are needed.

The process of forming this circulatory system is called vasculogenesis and begins early in embryonic development. Closely coupled to vasculogenesis is the formation of the heart, which pumps fluid through the circulatory system.