The human body is organized at several levels of scale that can each be examined

 * The human body has many levels of structural organization: atoms,
cells, tissues, organs, and organ system. 
* The simplest level is the chemical level, which includes tiny
building blocks such as atoms. 
* Cells are the smallest functional units of life. 
* Tissues are groups of similar cells that have a common function.
* An organ is a structure that is composed of at least two or more
tissue types and performs a specific set of functions for the body.
* Many organs working together to accomplish a common purpose is
called an organ system.

[Introduction to Tissues | Boundless Anatomy and Physiology | Study Guides](https://www.nursinghero.com/study-guides/boundless-ap/introduction-to-tissues)