Over large distances, efficient supply of materials is provided by mass transport

Mass transport is a system which transports necessary materials around an organism.

It consists of a transport medium, vessels, a means of moving the transport medium and something that controls the direction of flow.

In mammals this corresponds to blood, blood vessels, the heart and valves.

A mass transport system is needed if an organism is too large (low surface area to volume ratio) to get the materials it needs from diffusion.