6.1 The Functions of the Skeletal System

Your skeleton isn't just a passive frame. It's a dynamic system that supports your body, enables movement, and protects vital organs. From storing minerals to producing blood cells, your bones are constantly at work.

Beyond structure, your skeleton is living tissue. It undergoes continuous remodeling, stores energy, and works with cartilage to provide both strength and flexibility where needed.

Functions of the Skeletal System

Support, Movement, and Protection

The skeletal system provides the body's structural framework. Without it, you'd have no shape, no upright posture, and no way to stand against gravity. Bones also serve as anchoring points for soft tissues like muscles, tendons, and ligaments, keeping everything stable and in place.

Bones enable movement by acting as levers. When muscles contract, they pull on bones at joints, producing a range of motion. The joint type determines how much and what kind of movement is possible.

The skeleton also shields your most vulnerable organs from external damage:

• The cranium encases and protects the brain
• The ribcage shields the heart and lungs
• The vertebral column surrounds the spinal cord

Mineral Storage and Blood Cell Production

Bones are the body's main reservoir for calcium and phosphorus, stored as hydroxyapatite crystals within the bone matrix. This mineral storage ties directly into calcium homeostasis: when blood calcium drops, osteoclasts break down bone matrix to release calcium into the bloodstream. When calcium levels are adequate, osteoblasts incorporate minerals back into new bone matrix during formation and remodeling.

Bones also store energy. Yellow bone marrow contains triglycerides (fats) that the body can tap into as an energy reserve.

One of the skeleton's most critical jobs is hematopoiesis, the production of blood cells. This happens in red bone marrow, where hematopoietic stem cells differentiate into:

• Erythrocytes (red blood cells)
• Leukocytes (white blood cells)
• Thrombocytes (platelets)

In adults, red bone marrow is found in the vertebrae, ribs, sternum, pelvis, and the proximal ends of long bones like the femur and humerus.

Bone Structure and Remodeling

Bone is not static. It undergoes continuous remodeling to repair microdamage and adapt to the mechanical forces placed on it. Three key structures support this process:

• Osteocytes are mature bone cells embedded in the matrix. They sense mechanical stress and signal where remodeling is needed.
• The periosteum is the outer membrane covering bone surfaces. It contains blood vessels and nerves that nourish the bone and play a role in growth and repair.
• The endosteum lines the inner surfaces of bone (including the medullary cavity) and is active during remodeling.

The inner cavities of bones are filled with bone marrow, which supports both hematopoiesis (red marrow) and fat storage (yellow marrow).

Bone vs. Cartilage in the Skeleton

Bone and cartilage are both connective tissues, but they fill very different roles.

Bone is hard and mineralized. It provides structural support, protects organs, enables movement, stores minerals and fat, and produces blood cells. Bone dominates wherever the body needs strength and rigidity, like the femur.

Cartilage is flexible and resilient. It cushions joints as articular cartilage, providing smooth, low-friction surfaces for movement. It also maintains the shape of structures like the nose, ears, and trachea. Cartilage persists in areas where flexibility is more important than rigidity, such as the intervertebral discs between vertebrae.

During development and growth, cartilage plays a special role: it serves as the template for bone formation through endochondral ossification. In growing bones, cartilage at the epiphyseal plates (growth plates) allows bones to elongate until the plates close at maturity.