Body systems and organs and their functions
The human body is a large structural unit made up of many smaller parts. There are four kinds of smaller units - cells, tissues, organs and systems.
The cell is the structural unit or the brick with which the human body is built. The tissue is an organization of many smaller cells and these are held together by some form of substance between them.
Organ is complex unit. It is an organization of different kinds of tissues arranged in away to perform a special function.
Systems are more complex. A system is an organization of different kinds of organs that performs complex functions for the body.