Tissue and organ systems are different hierarchical levels of biological organization. Dr. Ingrid Lobo explains for Nature Education that biological organisms are systems of complexity. The basic functional unit of life is a single cell, and cells similar in function assemble to form tissue. An organ is a collection of different tissues joined as a functional unit; a group of organs working together to perform a specific task is an organ system.
According to biology tutorials from Hartnell College, animal tissue is divided histologically into four basic types: epithelial, connective, muscle and nervous tissue. Epithelial tissues are formed by epithelial cells, which cover organ surfaces and are linked by semi-permeable, tight junctions. Connective tissue is fibrous and composed of cells separated by extracellular fluid. Connective tissue gives shape to organs and holds them in place, such as bone and blood. Muscle tissue is the active contractile tissue of the body, functioning to produce force and cause motion. Nervous tissue is specialized to react to stimuli and conduct impulses to various organs in the body. Nervous tissue makes up the brain and spinal cord of the central nervous system and the branching peripheral nerves of the peripheral nervous system. Two or more tissue types in a collective structural unit serving a common function comprise an organ. Multiple organs working together to execute a specific bodily function make up an organ system.
According to Anatomy & Physiology, a learning initiative, the human body is made up of 11 organ systems: the integumentary, skeletal, muscular, lymphatic, respiratory, digestive, nervous, endocrine, cardiovascular, urinary and reproductive systems.