Bones

Bone 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 person reaches full height, at which time the growth plates close. Thereafter, bones grow very slowly--in thickness far more than in length.

Bones have two shapes: flat (such as the plates of the skull and the vertebrae) and tubular (such as the thighbones and arm bones, which are called long bones). All bones have essentially the same structure. The hard outer part consists largely of proteins, such as collagen, and a substance called hydroxyapatite. Composed mainly of calcium and other minerals, hydroxyapatite stores much of the body's calcium and is largely responsible for the strength and density of bones. The marrow in the center of bones is softer and less dense than the rest of the bone and contains specialized cells that produce blood cells. Blood vessels supply blood to the bone, and nerves surround the bone. Bones have a unique way of healing (see Section 5, Chapter 62).

Bones undergo a continuous process known as remodeling (see Section 5, Chapter 60). 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, the body requires an adequate supply of calcium and other minerals and must produce the proper amounts of several hormones, such as parathyroid hormone, growth hormone, calcitonin, estrogen, and testosterone.

Bones are surrounded by a thin membrane called the periosteum. Bones can feel pain because of nerves located mostly in the periosteum and receive blood through blood vessels that enter through it.