1 Anatomical Terminology and Body Organization

Reference position and directions

Anatomical position provides a consistent reference for describing the body, even when a person is lying down, turned, or viewed in a medical image. A person’s right and left always refer to that person’s own sides, not the viewer’s.

Directional relationships

Directional terms describe the location of one structure relative to another. Their meaning is relational: a structure can be superior to one part and inferior to another.

• Superior (cranial): Toward the head or above; the chest is superior to the abdomen.

• Inferior (caudal): Toward the feet or below; 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.

• Medial: Toward the body’s midline; the nose is medial to the eyes.

• Lateral: Away from the body’s midline; the ears are lateral to the eyes.

• Proximal: Closer to a limb’s attachment to the trunk; the elbow is proximal to the wrist.

• Distal: Farther from a limb’s attachment to the trunk; the fingers are distal to the wrist.

• Superficial: Nearer the body surface; the skin is superficial to the muscles.

• Deep: Farther from the body surface; the bones are deep to the skin.

Prone means lying face down, and supine means lying face up. These positions do not change the use of directional terms: a person’s front remains anterior, including when the person is prone.

Planes and sections

A plane is an imaginary flat surface that passes through the body. Anatomical planes help describe sections and interpret scans.

• The sagittal plane divides the body into right and left portions. The midsagittal (median) plane divides it into equal halves, while a parasagittal plane divides it into unequal portions.

• The frontal (coronal) plane divides the body into anterior and posterior portions.

• The transverse (horizontal) plane divides the body into superior and inferior portions. An image made in this plane is often called a cross-section.

• An oblique plane passes through the body at an angle rather than along one of the three standard planes.

A plane describes an imagined division. A section is the surface revealed when a structure is actually or virtually cut along a plane.

Body regions

The body has two broad regions. The axial region includes the head, neck, and trunk; the appendicular region includes the limbs and the girdles that attach them to the trunk.

More specific regional names include the cephalic region (head), cervical region (neck), thoracic region (chest), abdominal region, and pelvic region. On the limbs, the brachial region is the upper arm, the antebrachial region is the forearm, the femoral region is the thigh, and the crural region is the lower leg. In anatomical usage, “arm” often means specifically the part between the shoulder and elbow, while “leg” means the part between the knee and ankle.

Locating areas of the abdomen and pelvis

The four-quadrant scheme names the right upper (RUQ), left upper (LUQ), right lower (RLQ), and left lower (LLQ) quadrants. The nine-region scheme gives finer localization. Read from the person’s right to left and from top to bottom, its regions are:

• Top row: right hypochondriac, epigastric, left hypochondriac.

• Middle row: right lumbar, umbilical, left lumbar.

• Bottom row: right iliac (inguinal), hypogastric (pubic), left iliac (inguinal).

Quadrants offer a quick way to describe a location, while the nine-region scheme allows finer localization. These divisions are reference areas, not separate anatomical compartments.

Body cavities and their linings

Cavities are internal spaces that contain and protect organs. The two major body cavities are the dorsal (posterior) cavity and the ventral (anterior) cavity.

The dorsal cavity consists of the cranial cavity, which contains the brain, and the vertebral (spinal) cavity, which contains the spinal cord. These two cavities are continuous with one another.

The ventral cavity includes the thoracic cavity above the diaphragm. The thoracic cavity contains two pleural cavities, which surround the lungs, and the mediastinum, the central region between them. The mediastinum includes the pericardial cavity, which surrounds the heart.

Below the diaphragm, the abdominopelvic cavity has abdominal and pelvic portions. The abdominal portion contains many digestive organs; the pelvic portion contains the urinary bladder and internal reproductive organs. No physical partition separates these portions, although distinguishing them is useful for description.

Membranes lining ventral cavities

Serous membranes line some ventral cavities and cover their organs. The parietal layer lines a cavity wall, and the visceral layer covers an organ. A small amount of fluid between the layers reduces friction as organs move. The membrane names correspond to the spaces they line: pleura, pericardium, and peritoneum.

Organization and regulation

Body structures are organized from simpler to increasingly complex levels:

Chemical level → cellular level → tissue level → organ level → organ-system level → organism level

Atoms and molecules make up cell structures and materials. Cells are the smallest living units. Groups of cells working together form tissues, and two or more tissue types combine to form an organ. Organs working together form an organ system; all the systems together make up the organism. For example, different tissues form the heart, which works with blood vessels as part of the cardiovascular system.

Structure and function are connected: the shape and arrangement of a body part help it perform its roles. Boundaries between systems are not always absolute because many organs contribute to more than one system. The systems interact to maintain homeostasis, the regulation of internal conditions within ranges compatible with life.

One common regulatory pattern is negative feedback, in which a response counteracts a change. For example, sweating helps lower body temperature when it rises. The body is therefore both a hierarchy of organized parts and an integrated whole.