Human Body: organization

Big Picture

The body is organized on several different levels, from organ systems to cells. Groups of cells form tissues, and groups of tissues form organs. Organs then form organ systems. All parts of the body work together to maintain homeostasis, or keep internal conditions stable. Failure of homeostasis can be fatal.

Key Terms

Cell: Basic unit of structure and function.

Tissue: A cluster of cells with similar functions.

Organ: A group of tissues working together

Organ System: A group of organs working together.

Homeostasis: Process where the body tries to keep internal conditions stable.


  • All systems of the body work together to maintain homeostasis.
  • Hormones and the endocrine system play a large role in maintaining homeostasis.
  • If homeostasis fails, the person can get sick and die.

Levels of Organization

Humans are organized at different levels in order of most general to most complex:

  • organism, organ system, organ, tissue, cell

The cell is a basic unit of life, as it is the smallest unit capable of carrying out the functions of life.

  • We have specialized cells that are suited to perform a specific task
  • Examples: blood cells, bone cells, neurons

Connected cells performing a similar function form a tissue.
Four basic types of tissues:

  • Connective tissue connects different structures to form overall structure of the body
  • Epithelial tissue covers body surfaces to protect organs; also secretes and absorbs substances
  • Muscle tissue can contract to move bones
  • Nervous tissue carries electric nerve signals

Organs are then made up of two or more types of tissues working together.

  • Examples: heart, brain, lungs, skin

The organs can be organized into an overall organ system that performs a more complex job.

Levels of Organization
Image Credit: CK-12 Foundation,
Human Organ System
Image Credit: Images copyright alxhar, 2011, used under licenses from