The Functions of the Skeletal System

Core Concepts

Support: the body's framework

Bones provide the rigid framework that maintains the body's shape and holds the soft tissues in place. The skeleton is organized into two divisions worth knowing now: the axial skeleton (the skull, vertebral column, and rib cage — the central axis of the body) and the appendicular skeleton (the bones of the limbs plus the pectoral and pelvic girdles that attach them to the axial skeleton). Cartilage, present at joints and in places like the nose and ears, adds flexible support where rigidity would be a problem.

Protection: armor for vital organs

Many bones are shaped specifically to shield delicate structures:

• The skull encloses and protects the brain.
• The vertebral column surrounds and protects the spinal cord.
• The rib cage (ribs, sternum, and thoracic vertebrae) protects the heart and lungs.
• The pelvis cradles and protects the lower abdominal and pelvic organs.

This is why injuries to these regions are treated as potentially serious: the bone that protects can also, when fractured, threaten the organ behind it.

Movement: bones as levers

Bones don't move by themselves — they are moved by muscles. Skeletal muscles attach to bones via tendons, and where bones meet, joints allow motion. A classic way to think about it is the lever system: the bone is the lever, the joint is the fulcrum, and the muscle contraction supplies the effort. The elbow is a commonly taught example: the biceps pulls on the forearm near the elbow, lifting the hand (the load) at the far end. Because the effort arm is short relative to the load arm, the elbow trades force for speed and range — you can move your hand fast, but you can't lift as heavy a weight as you could with a longer effort arm. Different joints use different lever arrangements, which is why some movements are strong and slow and others are fast and precise.

Mineral storage and homeostasis: the body's calcium bank

Bone matrix is hardened by mineral crystals — mainly calcium phosphate as hydroxyapatite — deposited around a collagen framework. This makes bone strong (collagen resists tension) and rigid (minerals resist compression). About 99% of the body's calcium is stored in the skeleton (a commonly cited figure), with the remaining 1% in blood and tissues — yet that small blood fraction is critical, because calcium ions are required for muscle contraction, nerve impulse transmission, and blood clotting.

Bone therefore functions as a reservoir: when blood calcium falls, bone releases calcium into the blood; when blood calcium rises, bone takes it up. This exchange is controlled by hormones — parathyroid hormone (PTH) and the active form of vitamin D (calcitriol) raise blood calcium, and calcitonin lowers it — which are studied in detail in the calcium-homeostasis topic later in this chapter.

Blood cell production: hematopoiesis in red marrow

Red bone marrow is the tissue that produces blood cells — red blood cells (erythrocytes), most white blood cells (leukocytes), and platelets — a process called hematopoiesis. In children, red marrow fills most bones; in adults, it is largely confined to the flat bones (sternum, ribs, pelvis, skull, vertebrae) and the ends (epiphyses) of some long bones such as the femur and humerus. This location matters clinically: a bone-marrow biopsy is typically taken from the pelvis or sternum, precisely because those bones retain red marrow in adults.

Triglyceride storage: yellow marrow as an energy reserve

As children grow into adults, much of the red marrow in the shafts of long bones is replaced by yellow bone marrow, which consists largely of fat cells (adipocytes) storing triglycerides. Yellow marrow is an energy reserve, and under certain conditions (e.g., severe blood loss) some yellow marrow can convert back to red marrow to boost blood cell production. The fat in the marrow also contributes to the body's overall energy stores.

Bone as living tissue: remodeling

Bone is continuously remodeled by three cell types you will meet later in this chapter: osteoblasts (bone-building), osteoclasts (bone-resorbing), and osteocytes (mature bone cells that sense mechanical load). Remodeling lets bone repair micro-damage, adjust calcium release, and adapt to stress — a principle commonly taught as Wolff's law: bone grows and remodels in response to the forces placed on it. This is why weight-bearing exercise strengthens bone and prolonged immobility leads to bone loss. As with all models, Wolff's law is a useful simplification, not a complete explanation.

How It Works / Step-by-Step Process

Blood calcium regulation (the commonly taught loop, previewed here and detailed later in the chapter):

1. Blood calcium dips below the body's set point.
2. The parathyroid glands release parathyroid hormone (PTH).
3. PTH stimulates osteoclasts to break down bone matrix, releasing calcium into the blood; it also promotes activation of vitamin D (calcitriol), which increases calcium absorption from food.
4. Blood calcium returns toward normal; PTH release drops.
5. When blood calcium rises too high, calcitonin (from the thyroid gland) promotes calcium deposition into bone, lowering blood calcium.

This loop is why a skeleton problem can look like a blood problem, and vice versa — the skeleton and blood are linked through one mineral.

Common Confusions