Bring kids to reflect on the fact that all the trillions of cells of the body need constant nourishment (food and oxygen for energy metabolism, materials for building cell structure, etc.) Likewise, they have the constant need of getting rid of wastes (carbon dioxide from energy metabolism and various other wastes). How is this achieved? (Think Time) The circulatory (blood) system provides for both of these needs. All tissues of the body are permeated with capillary beds (networks of tiny blood vessels), which bring blood close to every cell. This permits the diffusion of needed materials from the blood into cells and wastes from cells into the blood. There is a constant flow of blood through each capillary bed from larger blood vessels coming from the heart to larger vessels on the other side leading back to the heart. An animated depiction of a capillary bed is seen in the following video.