All cells must obtain nutrients and get rid of waste products through their cell membranes. In single-celled organisms the cell membrane is in contact with the cell's surroundings so diffusion and other processes accomplish this exchange. However, large, multicellular organisms consist of trillions of individual cells, most of which are not located near the surface of the organism, yet must exchange materials in the same manner as in single-celled creatures. Therefore, large organisms consist of many different systems that assist in the exchange of materials such as food, wastes, and gases between the cells of the organisms and the external environment. The primary systems involved are the circulatory, respiratory, digestive, and excretory systems.

These systems must be coordinated and regulated in such a way that they supply the needs of the individual cells. Two other systems, the nervous and endocrine systems, are involved in managing these functions. This chapter will look at the systems involved in the exchange of materials and coordination of activities within the human body.