Cells carry on the many functions needed to sustain life. They grow and divide, thereby producing more cells. This requires that they take in nutrients, which they use to power the work that cells do and to make the materials that a cell or an organism needs. Specialized cells perform specialized functions in multicellular organisms. Groups of specialized cells cooperate to form a tissue, such as a muscle.
Different tissues are in turn grouped together to form larger functional units, called organs.
Each type of cell, tissue, and organ has a distinct structure and set of functions that serve the organism as a whole. The human organism has systems for digestion, respiration, reproduction, circulation, excretion, movement, control and coordination, and for protection from disease. Disease represents a breakdown in structures or functions of an organism. Some diseases are the result of intrinsic failures of the system. Others are the result of infection by other organisms.