By Julia Michelle | Updated Mar 24, 2022

The human skeleton is a complex framework divided into the axial and appendicular components. The axial skeleton comprises the skull, vertebral column, ribs, and sternum, while the appendicular skeleton includes the limbs, shoulder girdle, and pelvic girdle. Bone tissue comes in four primary shapes—long, short, flat, and irregular—and is composed of collagen fibers fortified with calcium and phosphate. Collagen confers flexibility, and the mineral matrix provides tensile strength.

1. Structure

The skeleton acts like the steel framework of a building, offering rigidity that shapes the body and supports the weight of muscles and organs. Without this framework, the chest cavity would collapse, compressing the lungs, heart, and other vital organs.

2. Movement

Movement results from the coordinated work of three systems:

• Neural impulses that activate muscles.
• Muscles that contract to generate force.
• The skeletal framework that provides levers and anchor points.

Each muscle has an origin (the stationary bone) and an insertion (the bone that moves). The length of the lever arm influences the mechanical advantage, which is why shorter limbs require less muscular power to achieve the same movement.

3. Protection

Protection is arguably the most critical function. The skull encases the brain; the vertebral column safeguards the spinal cord; and the rib cage shields the heart and lungs while remaining flexible enough to accommodate respiration. Flat plates of the skull join at sutures that allow growth and eventual fusion in early childhood.

4. Blood Cell Production

Red and white blood cells are produced in the red marrow of bones. At birth, all bone marrow is red. As individuals age, approximately half of the marrow converts to yellow, fatty marrow. In adults, yellow marrow dominates long bones, whereas red marrow persists in flat bones such as the pelvis, skull, and sternum, as well as in vertebral bodies and the epiphyses of long bones. In cases of severe blood loss, the body can re‑convert yellow marrow back to red to increase hematopoiesis.

5. Storage

Bones serve as reservoirs for calcium and phosphate, essential minerals for muscle contraction, nerve signaling, and blood clotting. Hormonal signals from the endocrine system regulate the release of calcium from bone into the bloodstream when serum levels fall, and deposit excess calcium back into bone when levels rise. Adequate dietary calcium and vitamin D support this balance and help prevent osteoporosis.