Structural Organization of the Human Body: An Overview

Human body structures can be organized into hierarchical levels.

The chemical level is the most basic, where atoms combine to form molecules, such as proteins, lipids, and carbohydrates. Complex molecules further combine to form cells and organelles—specialized intracellular structures.

Cells are the smallest living unit of the human body. They perform important biological functions, including synthesizing and breaking down biomolecules and communicating with other cells.

Cells with similar structure and function are organized together to form tissues. All the different types of tissues perform distinct and specialized roles, such as muscle contraction or wound repair.

Two or more tissue types together form the next level, the organs. Organs are special structures that accomplish complex functions; for example, the kidneys remove waste from the blood.

A group of organs functioning together form an organ system, such as the digestive system. All organ systems collectively create a complete organism, the highest level of organization.