The skeletal system forms the framework of your body. It includes all 206 bones in the adult body, along with the joints, cartilage, muscles, and connective tissue that hold everything together. This system gives your body its shape, supports your weight, protects your organs, and makes movement possible.
Bones: The Core of the Framework
Bones are the primary structural support of the human body. An adult typically has 206 bones, though that number can range up to 213 depending on individual variation. Babies are born with 275 to 300 smaller, softer bones, many of which start as cartilage. As a child grows, these bones gradually fuse and harden through a process called ossification. The five separate skull bones a newborn has, for example, close into a solid skull within the first year or two of life. Bone fusion continues through puberty and sometimes slightly beyond it.
Bone itself is a composite material. About 60% of bone tissue by weight is mineral (primarily a calcium-based crystal that gives bone its hardness), 30% is protein (over 90% of which is collagen, providing flexibility), and the remaining 10% is water. This combination makes bone both rigid and slightly elastic, able to resist compression without snapping like glass. Human thighbone, for instance, can withstand compressive forces exceeding 200 megapascals along its length, which is roughly comparable to some grades of concrete. It’s strongest when loaded lengthwise and weaker when force comes from the side.
Two Divisions of the Skeleton
The skeleton is divided into two main groups. The axial skeleton, with 80 bones, runs along your body’s central axis: the skull, spine, ribcage, and sternum. Its primary job is protecting vital organs like the brain, heart, and lungs. The appendicular skeleton accounts for the remaining 126 bones in the shoulders, arms, hips, and legs. This division handles most of your movement and interaction with the world around you.
Joints: Where Bones Meet
Joints are the connections between bones, and they vary widely in how much movement they allow. There are three structural types. Fibrous joints, like the sutures in your skull, have no space between the bones and allow little to no movement. They exist purely to lock bones together. Cartilaginous joints, like those between vertebrae in your spine, permit slight movement and use cartilage as a cushion. Synovial joints, like your knees, shoulders, and elbows, are the most mobile. They have a fluid-filled space between the bones that allows free-range motion, though this mobility comes at a cost: the more movable a joint is, the more vulnerable it is to injury.
Cartilage: Cushion and Shape
Cartilage is a tough, rubbery tissue that plays several roles in the body’s framework. There are three types, each suited to a different job.
- Hyaline cartilage covers the ends of bones at joints, creating a smooth, low-friction surface that resists compression. It’s also found in your nose, trachea, and ribs.
- Elastic cartilage is more flexible and gives shape to structures like the outer ear, the epiglottis, and the eustachian tubes.
- Fibrocartilage is the toughest variety, built to handle both tension and compression. It forms the discs between your vertebrae, the menisci in your knees, and the points where tendons and ligaments attach to bone.
Muscles and Connective Tissue
Bones alone can’t move or stay properly aligned. More than 600 muscles in your body work alongside the skeleton, handling everything from walking and lifting to breathing and digesting food. Muscles attach to bones through tendons and pull on them to create movement at joints, while ligaments connect bones to each other and stabilize the joints themselves.
Wrapping around and through all of this is fascia, a thin but continuous layer of connective tissue that separates muscles and organs, reduces friction between moving structures, and supports the body as a whole. Fascia surrounds and penetrates skeletal muscle, organs, joints, nerves, and blood vessels. It functions as a body-wide structural support system and also plays a role in proprioception, your sense of where your body is in space.
How Your Skeleton Renews Itself
Your framework isn’t static. Bone tissue is constantly being broken down and rebuilt in a process called remodeling. Specialized cells dissolve old or damaged bone over a period of about 30 to 40 days, and a different set of cells fills in new bone over roughly 150 days. A full remodeling cycle in dense outer bone takes a median of 120 days, while the spongy interior bone takes closer to 200 days. The entire surface of your spongy bone is completely replaced over a period of about two years.
This process serves two purposes: repairing the tiny stress fractures that accumulate from daily activity, and keeping the mineral content of your bones balanced. When the cycle works normally, old bone is replaced by equally strong new bone. When it falls out of balance, whether from aging, hormonal changes, or nutritional deficiencies, bones can become thinner and more fragile over time.
Beyond Structure
The skeletal system does more than hold you upright. The marrow inside certain bones produces new blood cells, including red blood cells that carry oxygen and white blood cells that fight infection. Bones also serve as the body’s primary storage bank for calcium and other minerals, releasing them into the bloodstream when other systems need them and absorbing them back when supplies are adequate. So while the skeleton is most obviously a framework, it quietly keeps several other critical systems running as well.