Cells are the basic structural and functional units of the living organism. In the multicellular organism, cells are form tissues that are characterized by certain functions and structures. In tissues, cells are linked to each other via specific interactions (direct or indirect) mediated by adhesion molecules. In tissues, cells communicate and respond to certain environmental changes. Tissues form organs. Some tissues maintain the shape and structure of the organ, while others are directly involved in the functioning of the organ. These are complex structures that perform a certain function in the organism. Each organ is not an isolated structure as it directly interacts with organs that perform other functions. For example, most organs contain vessels and nerves although they perform other functions (i.e., digestion). Finally, organs form organ systems. These groups are composed of several organs performing similar functions in the organism. Each organ has a specific function in the group of organs. Organ systems maintain the structural and functional integrity of the organism.