Bone, although strong, is a constantly changing tissue that has several functions. Bones serve as rigid structures to the body and as shields to protect delicate internal organs. They provide housing for the bone marrow, where the blood cells are formed. Bones also maintain the body's reservoir of calcium. In children, some bones have areas called growth plates. Bones lengthen in these areas until the child reaches full height, at which time the growth plates close. Thereafter, bones grow in thickness rather than in length, based on the body's need for additional bone strength in certain areas.
Bones have two shapes:
Some bones have combinations of these. All bones have essentially the same structure. The hard outer part (cortical bone) consists largely of proteins, such as collagen, and a substance called hydroxyapatite, which is composed mainly of calcium and other minerals. Hydroxyapatite is largely responsible for the strength and density of bones. The inner part of bones (trabecular bone) is softer and less dense than the hard outer part but still contributes significantly to bone strength. A reduction in the amount or quality of trabecular bone increases the risk of fractures (breaks). Bone marrow is the tissue that fills the spaces in the trabecular bone. Bone marrow contains specialized cells (including stem cells) that produce blood cells. Blood vessels supply blood to the bone, and nerves surround the bone.
Bones undergo a continuous process known as remodeling (see Osteoporosis). In this process, old bone tissue is gradually replaced by new bone tissue. Every bone in the body is completely reformed about every 10 years. To maintain bone density and strength, the body requires an adequate supply of calcium, other minerals, and vitamin D and must produce the proper amounts of several hormones, such as parathyroid hormone, growth hormone, calcitonin, estrogen, and testosterone. Activity (for example, weight-bearing exercises for the legs) helps bones strengthen by remodeling. With activity and optimal amounts of hormones, vitamins, and minerals, trabecular bone develops into a complex lattice structure that is lightweight but strong.
Bones are covered by a thin membrane called the periosteum. Bone injuries are painful because of pain-sensing nerves located mostly in the periosteum. Blood enters bones through blood vessels that enter through the periosteum.