About Bones
The adult human skeleton contains about 206 bones, although the exact count can vary. Bones are living organs with blood vessels, nerves, marrow, and cells that constantly maintain and remodel tissue. They support the body, protect organs, store minerals, and provide attachment points for muscles.
Bones Are Living Tissue
Bone is constantly being built, repaired, and reshaped.
Learn MoreBone Structure
Bones combine a hard outer layer with lighter internal structures.
Learn MoreBone Marrow
Marrow inside certain bones helps make blood cells.
Learn MoreJoints
Joints are places where bones meet.
Learn MoreBone Growth
Children and teenagers have growth plates near the ends of many long bones.
Learn MoreMineral Storage
Bones store most of the body's calcium and phosphorus.
Learn MoreProtection
The skeleton protects many delicate organs.
Learn MoreX-Rays and Bones
Bone absorbs more X-rays than most soft tissue.
Learn MoreBones Are Living Tissue
Specialized cells called osteoblasts build bone matrix, while osteoclasts remove old bone. This remodeling helps repair tiny damage, respond to physical stress, and control mineral storage.
Bone Structure
Compact bone forms a strong shell, while spongy bone contains a lattice of thin supports called trabeculae. The design provides strength without making the skeleton unnecessarily heavy.
Bone Marrow
Red bone marrow contains stem cells that produce red blood cells, many white blood cells, and platelets. Yellow marrow contains more fat and serves different metabolic roles.
Joints
Some joints hardly move, while others such as shoulders, hips, knees, and fingers allow wide ranges of motion. Cartilage, ligaments, capsules, and fluid help joints move smoothly and stay stable.
Bone Growth
Growth plates contain cartilage where new bone length is added. They gradually close as the skeleton reaches adult size. Different bones finish growing at different times.
Mineral Storage
These minerals make bone tissue hard and also perform important jobs elsewhere in the body. Hormones help balance mineral levels between bone, blood, kidneys, and the digestive system.
Protection
The skull surrounds the brain, vertebrae protect the spinal cord, ribs shield the heart and lungs, and the pelvis helps protect organs in the lower abdomen.
X-Rays and Bones
That difference makes bones appear bright on many radiographs. X-rays are useful for examining fractures, alignment, joints, growth plates, and some bone diseases.