Tissues and organs, in a medical context, refer to different levels of biological organization. A tissue is a group of cells, all of the same kind, which share the same structure and function. An organ is a collection of tissues, some of which may be very different from one another, which work together to perform a more complex function as if the tissues were a single structural unit.
The organizational level above organs is called a system, for example, the circulatory system, the reproductive system, the digestive system or the nervous system. Systems are made up of organs, which work to perform still more complicated functions, explains Carol's Classroom. To demonstrate this idea, consider the nervous system. Individual neurons form sheets of tissue within the brain, a bodily organ. The brain in turn works with sensory nerves and motor nerves to produce the thoughts, sensations and movement which is the nervous system's main function. Simple neurons, isolated from tissue, could not create the level of complexity present in the whole system. Only by working together with the larger organizational units of the whole nervous system does the body function to produce abstract thought, the sight of a mountain or a headlong slide into third base.