The skeletal system is the framework of bones, cartilage, and connective tissues that gives your body its shape, protects your organs, and performs several functions you might not expect, like producing blood cells and storing minerals. An adult human skeleton contains 206 bones, divided into two main groups that work together to keep you upright, moving, and alive.
Six Core Functions of the Skeleton
Most people think of bones as structural supports, and that’s accurate but incomplete. The skeletal system handles six major jobs:
- Support: Bones provide the rigid framework that holds your body upright and gives it shape.
- Movement: Bones act as levers. When muscles pull on them through tendons, you move.
- Protection: The skull shields the brain, the rib cage guards the heart and lungs, and the vertebral column encases the spinal cord.
- Blood cell production: Red bone marrow inside certain bones produces red blood cells, white blood cells, and platelets.
- Mineral storage: Nearly 99% of the calcium in your body and 86% of its phosphorus is stored in your bones. When blood calcium drops, bone tissue releases what the body needs for muscle contractions and nerve signaling.
- Fat storage: Yellow bone marrow stores fat that the body can convert to energy when needed.
The mineral storage function is especially important. Calcium ions are essential for every muscle contraction you make, including your heartbeat, and for transmitting signals between nerve cells. Your skeleton acts as a living reservoir, constantly releasing and absorbing minerals to keep blood levels stable.
Axial and Appendicular Divisions
The 206 bones of the adult skeleton split into two divisions. The axial skeleton forms the central axis of your body: skull, vertebral column, and rib cage. It contains 80 bones. The appendicular skeleton makes up the remaining 126 bones and includes everything that attaches to that central axis, your arms, legs, shoulders, and hips.
The axial skeleton breaks down further. The skull alone accounts for 28 bones, including the tiny auditory bones inside each ear. The vertebral column has 26 bones: 7 cervical vertebrae in your neck, 12 thoracic vertebrae in the mid-back, 5 lumbar vertebrae in the lower back, plus the sacrum and coccyx (tailbone) at the base. The rib cage adds 24 ribs and one sternum (breastbone).
The appendicular skeleton handles most of your interaction with the world. Your hands alone contain 27 bones each, and your feet have 26, which means more than half of all the bones in your body are in your hands and feet.
Why Babies Have More Bones Than Adults
A newborn has roughly 275 to 300 bones. That number drops to 206 by adulthood because many smaller bones fuse together as a child grows. Much of an infant’s skeleton starts as cartilage, a tough but rubbery tissue that gradually hardens through a process called ossification. Bone fusion continues through puberty and sometimes slightly beyond it, which is one reason growth plates can still be seen on X-rays of teenagers.
Four Types of Bones
Not every bone looks like the long leg bone you picture in anatomy class. Bones come in four principal shapes, each suited to a different job:
- Long bones are found in your arms, forearms, thighs, and lower legs. They act as levers for movement and bear most of your weight.
- Short bones are roughly cube-shaped and appear in the wrists and ankles, where they provide stability and allow complex, fine movements.
- Flat bones are thin and broad. Most bones in the skull are flat bones, as are the shoulder blades and ribs. They protect organs and provide large surfaces for muscle attachment.
- Irregular bones have complex shapes that don’t fit the other categories. The vertebrae and several skull bones fall into this group.
What Bones Look Like Inside
A cross-section of a typical long bone reveals two distinct types of tissue. The outer layer is compact bone, a dense, hard material that forms what appears to be a solid mass. Under a microscope, compact bone is organized into tightly packed cylindrical units. Each unit has a central canal carrying blood vessels, surrounded by concentric rings of bone matrix with living bone cells nestled in tiny pockets between the rings. Small channels radiate outward from these pockets, delivering nutrients through the hard material.
Beneath the compact outer shell is spongy bone, which looks porous but is far from weak. Spongy bone consists of a lattice of thin plates and bars arranged along the lines of mechanical stress, similar to the braces that support a building. The spaces between those plates are filled with bone marrow. This design makes spongy bone lighter than compact bone while still providing impressive structural strength. If the forces on a bone change over time (from a new exercise habit, for example), the lattice can actually reorganize itself to handle the new stress pattern.
How Bone Constantly Rebuilds Itself
Your skeleton is not a fixed structure. It tears itself down and rebuilds continuously through a process called remodeling, which relies on two key cell types working in sequence. First, bone-resorbing cells break down old or damaged bone and remove it. Then bone-forming cells move in and lay down fresh bone matrix, which hardens as minerals are deposited into it. Over time, some of these bone-forming cells become embedded in the new bone and transform into a third cell type that monitors the surrounding tissue for damage or changes in stress.
This cycle has four phases: activation (cells are recruited to a damaged area), resorption (old bone is dissolved), reversal (the demolition crew dies off and builders arrive), and formation (new bone is produced and mineralized). The balance between breakdown and buildup determines whether your bones stay strong, get stronger, or weaken over time.
The Connective Tissues That Complete the System
Bones alone don’t make a functional skeleton. Three types of connective tissue tie everything together. Cartilage is a strong, flexible tissue that acts as a shock absorber in joints, cushioning the ends of bones where they meet. Ligaments are bands of tissue that connect bones to other bones, stabilizing joints and preventing excessive movement. Tendons are cords that connect muscles to bones, transmitting the force of a muscle contraction into actual movement at a joint.
Bone Marrow and Blood Cell Production
The soft tissue filling the interior spaces of bones comes in two varieties. Red bone marrow is the body’s blood cell factory, producing red blood cells that carry oxygen, white blood cells that fight infection, and platelets that help with clotting. In children, red marrow fills most bones. In adults, it concentrates in specific locations: the pelvis, vertebrae, ribs, sternum, and the ends of the large long bones.
Yellow bone marrow takes over the spaces that red marrow vacates as you age. It consists mostly of fat cells and serves as an energy reserve. In cases of severe blood loss or certain diseases, yellow marrow can convert back to red marrow and resume blood cell production.
Osteoporosis and Bone Loss
The most common threat to the skeletal system is osteoporosis, a condition in which the balance of bone remodeling tips toward breakdown. Bone becomes porous, fragile, and prone to fractures. Osteoporosis and its milder precursor, osteopenia, affect over half of all Americans aged 50 and older. Without treatment, men and women lose 1 to 3% of their bone mass each year after age 50.
Early bone loss produces no obvious symptoms. By the time signs appear, significant damage has already occurred. Common late-stage indicators include back pain from collapsed vertebrae, gradual loss of height, a stooped posture, and fractures from minor falls or impacts that wouldn’t normally break a bone. Women who are white or of Asian descent face the highest risk, particularly after menopause, but osteoporosis affects people of all races and sexes.
Adequate calcium intake is one of the most straightforward protective measures. For most people with bone loss or at risk for it, the recommended target is around 1,200 milligrams of calcium per day. Weight-bearing exercise also stimulates the bone-forming cells to lay down new tissue, which is why walking, running, and resistance training are consistently linked to stronger bones throughout life.