Body Organization and Anatomical Terminology

Why the Vocabulary Comes First

Every description you will ever give of a patient depends on words that mean exactly one thing. "The wound is on his left arm" is useless if the person reading it does not know whose left you meant. Anatomical terminology exists to remove that ambiguity, and once you have it, the rest of the body becomes much easier to describe.

Learn this page first. Every other page in this section assumes you already have it.

Anatomical Position

Anatomical position is the reference posture that every directional term is defined against. The patient is standing upright, facing forward, arms at the sides, palms turned forward, feet flat and pointing forward.

The patient does not have to be in that position for the terms to apply. You describe a patient lying face down on a sidewalk exactly as if he were standing in anatomical position, because otherwise the words would change meaning every time you rolled him.

The single most important consequence: left and right always mean the patient's left and right. A laceration on the side of the chest facing you as you kneel at his side is his left if you are on his left. Your own orientation never enters into it.

The Planes

A plane is an imaginary flat cut through the body. Three of them matter.

The midsagittal plane is the sagittal cut placed exactly in the middle, dividing the body into equal left and right halves. Anything closer to that line is medial, and anything farther from it is lateral. Your nose is medial to your ear. Your ear is lateral to your nose.

Directional Terms

These are the words that let you write a report someone else can follow.

Proximal and distal describe positions along a limb, so use them for arms and legs. Describing a chest wound as proximal tells the receiving hospital nothing.

Positions You Will Actually Document

A patient found sitting up and leaning forward on braced arms is in the tripod position, which is a finding rather than a position you put someone in. A patient chooses it because it takes the weight of the shoulders off the chest and lets the accessory muscles pull harder.

Body Cavities

The body's organs sit inside enclosed spaces, and knowing which space something occupies tells you what an injury there will threaten.

• Cranial cavity. Inside the skull, holding the brain. Rigid walls, which is why swelling inside it has nowhere to go.
• Spinal cavity. Inside the vertebral column, holding the spinal cord.
• Thoracic cavity. The chest, from the base of the neck down to the diaphragm, holding the heart, lungs, great vessels, trachea and esophagus. Protected by the ribs, sternum and thoracic spine.
• Abdominal cavity. From the diaphragm down to the pelvis, holding the stomach, intestines, liver, spleen, gallbladder and pancreas. No bony protection across the front.
• Pelvic cavity. The lowest portion, holding the bladder, rectum and the internal reproductive organs.

The mediastinum is the space in the middle of the thoracic cavity between the two lungs. The heart, great vessels, trachea and esophagus all live there.

The retroperitoneal space sits behind the abdominal lining rather than inside the abdominal cavity. The kidneys, the ureters, the pancreas, the abdominal aorta and much of the duodenum are retroperitoneal. Bleeding into that space is hidden bleeding, because the abdomen does not distend the way it would with free blood in the cavity.

The Four Abdominal Quadrants

Draw one vertical line and one horizontal line crossing at the umbilicus. That gives you four quadrants, and it is how abdominal findings are reported.

The small intestine spreads across all four quadrants, and the bladder sits at the midline below all of them.

Two associations are worth memorizing because they show up constantly. Right upper quadrant pain points at the liver and gallbladder. Right lower quadrant pain points at the appendix.

Levels of Organization

The body is built in layers, and each layer is made of the one below it.

1. Cells. The smallest living unit.
2. Tissues. Groups of similar cells doing one job. Four types: epithelial (covering and lining), connective (support, including bone, blood and fat), muscle (movement), and nervous (signaling).
3. Organs. Two or more tissue types working as a unit, such as the heart or the stomach.
4. Organ systems. Organs cooperating toward one function, such as the respiratory system.
5. Organism. All systems together.

Homeostasis

Homeostasis is the body holding its internal conditions steady while the outside world changes. Temperature, blood glucose, pH, fluid volume and oxygen levels are all defended within narrow ranges.

The defending is done by negative feedback. A sensor detects that something has drifted, a control center compares it against a set point, and an effector pushes it back. Blood pressure falls, baroreceptors detect it, the brain signals the heart to speed up and the vessels to constrict, and pressure comes back up.

Almost everything you will treat is a failure of homeostasis, or the body's attempt to defend it. The fast heart rate in blood loss is not the injury. It is the compensation, and reading it that way is most of clinical thinking.

Terms of Movement

You will use these describing an injury or a limitation.

• Flexion decreases the angle at a joint. Extension increases it.
• Abduction moves a limb away from the midline. Adduction moves it back toward the midline.
• Rotation turns a part around its own axis.
• Supination turns the palm up. Pronation turns the palm down.
• Inversion turns the sole inward. Eversion turns it outward.

What to Take Away

Anatomical position defines every direction word, and left always means the patient's left. Three planes, a short list of directional terms, five cavities, four quadrants. Get those cold and the rest of this section reads like plain English instead of a foreign language.