Organization of the Human Body

Human anatomy and physiology is the study of the structures and functions of the human body.

Levels of Organization and Body Cavities


The body can be studied at seven structural levels: chemical, organelle, cell, tissue, organ, organ system, and organism.

  • Chemical: The chemical level involves interactions among atoms and their combination into molecules.
  • Organelle: An organelle is a small structure contained within a cell that performs one or more specific functions.
  • Cell: Cells are the basic functional units of life. All cells share many characteristics, but they differ in structure and function.
  • Tissue: A tissue is a group of cells with similar structures and functions.
  • Organ: An organ is composed of two or more tissue types that together perform one or more common function.
  • Organ system: An organ system is a group of organs classified as a unit because of a common function or set of functions.
  • Organism: An organism is any living thing considered as a whole. Organisms can have anywhere from a single cell to trillions of cells.

Body Cavities

The human body has many cavities, some of which open to the exterior. A cavity is a fluid-filled space in the body that holds and protects internal organs. The ventral cavity (front of the body) contains three major cavities:

  • The thoracic cavity is surrounded by the rib cage and separated from the abdominal cavity by the diaphragm. It is divided into right and left halves by a structure called the mediastinum. It contains the esophagus, trachea, thymus gland, heart, and both lungs, along with other structures.
  • The abdominal cavity is bounded by the abdominal muscles below the thoracic cavity and contains the stomach, intestines, liver, spleen, pancreas, and kidneys.
  • The pelvic cavity is enclosed by the bones of the pelvis and contains the urinary bladder, part of the intestines, and the internal reproductive organs. The abdominal and pelvic cavities are sometimes referred to as the abdominopelvic cavity.

The dorsal cavity is the back of the human body, and it is subdivided into two cavities: cranial and spinal.

  • The cranial cavity contains the brain.
  • The spinal cavity contains the spinal cord.

Body Cavities Example

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