Human Anatomy: The Structure of the Body

The anatomical position is a fixed reference stance from which every paired directional term takes a single, unambiguous meaning.

Each cardinal plane cuts the body along a different axis, and the section a plane produces determines which structures the cut brings into view.

The internal organs are housed in the dorsal and ventral body cavities, lined by serous membranes, with the abdominopelvic space mapped into named regions.

The body is built in six nested levels of organization from chemical to organism, and every organ is assembled from just four primary tissue types.

A bone is a living organ whose dense compact shell, lattice-like spongy interior, and marrow together give it strength, lightness, and the ability to remodel.

The skeleton divides into an axial part along the body's midline (skull, spine, ribcage) and an appendicular part of the limbs and the girdles that anchor them.

Joints are classified by how much they move, and the freely movable synovial joint permits a named vocabulary of movements — flexion, extension, abduction, adduction, and rotation.

A skeletal muscle attaches from a fixed origin to a movable insertion and, working against an antagonist across a joint, pulls bone into motion like a lever.

The heart is two side-by-side pumps of two chambers each; four valves keep blood moving one way and a dividing wall keeps the oxygen-poor and oxygen-rich streams from ever mixing.

Blood runs two loops — a short pulmonary circuit to the lungs and a long systemic circuit to the body — through arteries that carry it away from the heart, capillaries where exchange happens, and veins that return it, each vessel's wall built for its task.

Air travels a branching set of conducting airways — trachea, bronchi, bronchioles — that only carry it, until it reaches the alveoli, thin-walled sacs whose vast combined surface lets oxygen cross into the blood.

The digestive tract is one continuous tube from mouth to anus through which food passes in sequence, while accessory organs — salivary glands, liver, gallbladder, pancreas — add secretions through ducts without food ever passing through them.

The nervous system divides into a central part — the brain and spinal cord — and a peripheral part of nerves, all built from neurons and organized into gray matter of cell bodies and white matter of fibers.

The brain's major regions and the spinal cord together receive and route signals, and a reflex arc traces the shortest path from stimulus through the cord to a rapid response.

Palpable surface landmarks — bony points, muscle edges, and pulse sites — let an anatomist locate deep structures from the outside using the same directional language learned in Unit I.

Across every system in the course, a structure's shape is physical evidence of the job it does, so reading form carefully lets you predict function even for a part you have never seen named.