An organ is a body structure composed of tissues that have related functions. The human body has levels of organization. The cell is the basic structural and functional unit of a living organism which communicates with different and similar cells in order to work together and carry out all physiological processes. Cells with similar structures are grouped together in a tissue to perform a shared function. A group of tissues forms an organ that together with other organs make organ system. The integrated activity of all the organ systems makes the whole organism.