In the human body, there are four organizational levels. The smallest living units are the cells, some 75 to 100 trillion of them falling into more than 100 different types. Similar cells, along with the nonliving material, called matrix, in which they are embedded, are grouped to form tissues, each kind designed to carry out some one function. Related tissues are joined together into organs adapted to perform particular tasks. Last come the body's systems, groups of organs responsible for a series of interrelated functions. The body as a whole has been described as a community of cells, a social order in which each of 75 trillion individuals has some assigned place to occupy, some specific role to play.