Anatomical position, planes and directional terms

"The pain is on the left." Whose left? "It's above the kidney." Above in which direction, with the patient standing or lying down? Anatomy solves this with a precise shared vocabulary, so that a description means the same thing to everyone who reads it. Once you know these terms, radiology reports, operation notes and textbook descriptions become far easier to follow.

Anatomical position

Every directional term is defined relative to a standard pose called anatomical position:

• standing upright, facing forward,
• feet together and pointing forward,
• arms by the sides,
• palms facing forward, with the thumbs pointing away from the body.

Directions stay tied to this pose even when the patient is lying down or curled up. "Superior" always means toward the head, whatever position the body is actually in.

Left and right always mean the patient's left and right, never the observer's. That is why anatomical diagrams and medical images are usually labeled: facing a patient, or looking at a standard chest X-ray or CT slice, the patient's right side is on the viewer's left.

The three planes

Anatomists and radiologists slice the body along three planes at right angles to each other:

CT and MRI images are usually displayed as axial slices, viewed as if looking up from the patient's feet. That is another reason the patient's right appears on the left of the screen.

Directional terms

These terms come in opposing pairs.

A few details are worth knowing:

• Proximal and distal are mainly used for limbs, but also along tubes such as the bowel, blood vessels and airways, where proximal means closer to the origin.
• In the hand, the palm side is the palmar surface and the back of the hand is the dorsal surface. In the foot, the sole is the plantar surface and the top is the dorsal surface.
• In four-legged animals, "dorsal" and "ventral" mean up and down; in humans they are used as synonyms for posterior and anterior.

Movements

Movements at joints have their own paired terms:

• Flexion bends a joint and usually reduces the angle between two parts; extension straightens it.
• Abduction moves a limb away from the midline; adduction brings it back toward it.
• Medial and lateral rotation turn a limb around its long axis, toward or away from the midline.
• Pronation turns the palm to face backward (or down); supination turns it to face forward (or up).
• In the foot, inversion turns the sole inward and eversion turns it outward; dorsiflexion lifts the foot up and plantarflexion points it down.

Regions of the abdomen

Because abdominal pain is so common, clinicians divide the abdomen into areas.

The simpler system uses four quadrants, formed by a vertical and a horizontal line through the navel:

• Right upper quadrant: most of the liver, the gallbladder, part of the duodenum and the right kidney.
• Left upper quadrant: the stomach, spleen, the tail of the pancreas and the left kidney.
• Right lower quadrant: the appendix, the start of the large bowel (cecum) and, in women, the right ovary.
• Left lower quadrant: the sigmoid colon and, in women, the left ovary.

The more detailed system uses nine regions: right and left hypochondriac (under the ribs), epigastric (upper middle), right and left lumbar or flank, umbilical (around the navel), right and left iliac or inguinal, and hypogastric or suprapubic (above the pubic bone, over the bladder).

Body cavities and the retroperitoneum

The trunk contains the thoracic cavity, with the lungs in their pleural cavities and the heart and great vessels in the central mediastinum, and the abdominopelvic cavity below the diaphragm. Many abdominal organs hang inside a lining called the peritoneum, but some sit behind it in the retroperitoneum. Students often remember the retroperitoneal organs with the mnemonic SAD PUCKER: the suprarenal (adrenal) glands, aorta and inferior vena cava, most of the duodenum, most of the pancreas, the ureters, the ascending and descending colon, the kidneys, the esophagus and the rectum.

Knowing which structures are retroperitoneal explains why, for example, kidney pain is often felt in the back and flank rather than the front of the abdomen.

How Tangent uses these ideas

Stat!'s Tangent game shows a front view of the body in anatomical position, with R on the left of the screen and L on the right, exactly as described above. Structures that lie at the back of the body are drawn dashed and marked as behind. Every guess is scored by how close it lies to the hidden structure, taking depth from front to back into account as well as position on the map. Thinking in planes and directions, "the target is deep and lateral to my last guess", is the fastest way to home in. The Tangent guide explains the scoring and strategy.