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Structural Organization of the Human Body: An Overview

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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.

Overview

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It is convenient to consider the body's structures in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms, molecules, organelles, cells, tissues, organs, organ systems, and organisms.

To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, atoms, and molecules. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles such as protons, electrons, and neutrons. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.

A cell is the smallest independently functioning unit of a living organism. Even bacteria, which are extremely small, independently-living organisms, have a cellular structure. Each bacterium is a single cell. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells.

A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid together with a variety of tiny functioning units called organelles. In humans, as in all organisms, cells perform all functions of life. A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function. An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet the physiological needs of the body.

Organs that "belong" to one system can also have functions integral to another system. In fact, most organs contribute to more than one system. The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 1.2 : S tructural Organization of the Human Body

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