1 Anatomical Terminology and Body Organization
Learn the standard reference position, directional vocabulary, body planes and cavities, regional maps, and levels of organization used to describe the human body.
Reference posture and orientation
Anatomical descriptions use a shared reference posture so that locations and relationships remain clear even when a person is lying down or turned. The terms refer to the person’s body, not to the observer’s viewpoint.
The standard reference position
In , a person stands upright and faces forward, with feet flat and directed forward. The arms rest at the sides, the palms face forward, and the thumbs point away from the body. Directional descriptions use this position even when the person is actually in another posture.
A person lying face up is supine; a person lying face down is prone.
Directional relationships
Directional terms describe one structure in relation to another; a structure is not simply “,” but to another structure. These relationships remain clear if the arm moves or the person is viewed from another angle.
(cranial): Toward the head or upper part of the body. The chest is to the abdomen.
Inferior (caudal): Toward the feet or lower part of the body. The stomach is inferior to the heart.
Anterior (ventral): Toward the front. The breastbone is anterior to the heart.
Posterior (dorsal): Toward the back. The spine is posterior to the breastbone.
: Toward the body’s midline. The nose is to the eyes.
Lateral: Away from the midline. The ears are lateral to the eyes. In , the thumb is lateral to the little finger.
Proximal: Closer to a limb’s attachment to the trunk or to a point of origin. The elbow is proximal to the wrist.
Distal: Farther from a limb’s attachment to the trunk or from a point of origin. The fingers are distal to the wrist.
Superficial: Closer to the body surface. The skin is superficial to the muscles.
Deep: Farther from the body surface. The ribs are deep to the skin.
Planes and body sections
A plane is an imaginary flat surface passing through the body. Planes describe body sections and the orientation of medical images.
A divides the body into right and left portions. The midsagittal (median) plane divides it into equal right and left halves; a parasagittal plane divides it into unequal portions.
A frontal (coronal) plane divides the body into anterior and posterior portions.
A transverse (horizontal) plane divides the body into and inferior portions.
An oblique plane passes through the body at an angle rather than along one of these standard planes.
A transverse image of the abdomen shows a cross-section through the body at a particular level.
Body cavities and their linings
Body cavities are internal spaces that contain and protect organs. The two major cavities are the (posterior) and the (anterior).
The contains the central nervous system and has two continuous parts:
The cranial cavity, within the skull, contains the brain.
The vertebral (spinal) cavity, within the vertebral column, contains the spinal cord.
The is divided by the diaphragm into the thoracic cavity and the abdominopelvic cavity. The thoracic cavity contains the lungs and the mediastinum. Each lung is enclosed by a pleural cavity. The mediastinum is the central region between the lungs; it contains the heart, which is enclosed by the pericardial cavity, as well as other structures.
The abdominopelvic cavity has two useful subdivisions, although no wall separates them. The abdominal cavity contains many digestive organs. The pelvic cavity contains the urinary bladder, rectum, and internal reproductive organs.
Serous membranes help organize some ventral cavities. A has a parietal layer lining a cavity wall and a visceral layer covering an organ. A small amount of fluid between the layers reduces friction as organs move. The pleura, pericardium, and peritoneum are serous membranes associated with the lungs, heart, and many abdominopelvic organs, respectively.
Body regions and regional terms
The body has two broad regions. The includes the head, neck, and trunk. The appendicular region includes the upper and lower limbs and their attachments to the trunk.
More specific regional terms identify surface locations:
Cephalic: head; cervical: neck.
Thoracic: chest; abdominal: abdomen; pelvic: pelvis.
Brachial: upper arm; antebrachial: forearm.
Femoral: thigh; crural: lower leg.
In precise anatomical usage, the arm is the portion between the shoulder and elbow, and the forearm is between the elbow and wrist. The thigh is between the hip and knee, while the leg is between the knee and ankle.
Mapping the abdominopelvic area
The abdominopelvic area can be mapped using either four quadrants or nine regions. The are named from the person’s perspective: right upper (RUQ), left upper (LUQ), right lower (RLQ), and left lower (LLQ). A vertical and a horizontal line crossing at the navel form the quadrants.
The nine-region scheme provides a more detailed map. Listed below in upper, middle, and lower rows, the regions run from the person’s right to left:
Upper row: right hypochondriac, epigastric, left hypochondriac.
Middle row: right lumbar, umbilical, left lumbar.
Lower row: right iliac (inguinal), hypogastric (pubic), left iliac (inguinal).
Both systems provide concise ways to report locations, such as the site of pain or a physical finding.
Levels of body organization
The body is organized from simpler components to more complex structures:
Chemical level: Atoms and molecules form the materials of the body.
Cellular level: Molecules make up cells, the smallest living structural units.
Tissue level: Groups of similar cells and their surrounding material perform related functions.
Organ level: Two or more tissue types form a structure with specific functions.
Organ-system level: Organs work together to carry out broader functions.
Organism level: All organ systems function together as one human being.
For example, cells build tissues, several tissues form the heart, and the heart works with blood vessels and blood as part of the cardiovascular system. Each level contributes to the organization of the whole body.