Living tissues are composed of similar cells and many different cell types.
The body has many levels of organization.
Cells are the basic structural and functional unit of life.
A cluster of cells similar in structure, performing the same particular function forms a tissue.
Tissues in turn group together and form the organs.
In turn, the organ system is formed when a group of organs is dedicated to performing a function.
For example, the respiratory system consists of organs such as the nose, pharynx, lungs, alveoli, etc which are organs performing the function of respiration, hence contributing to the organ systems of the respiratory processes.