Bone is very complex at a microscopic level. It is not homogeneous. What is sometimes called the "bony" part of the bone is mineral, although there are plenty of other minerals in the other parts of the bone tissue. The bony sections include materials that if removed from the body would be insoluble in water - these are calcium and phosphate salts - much of the bony nodules would be classified as hydroxyapatite
A rough overall chemical formula is [Ca3(P)4)2]3.Ca(OH)2. In a lab this mineral can be dissolved into calcium phosphate, calcium fluoride, calcium carbonate, and calcium hydroxide.
Bone is roughly 30% organic material (by weight) and 70% inorganic mineral. The organic material is about 90% collagen. Other organic material is mostly protein, too, of different kinds. Some of the protein in the bone is not unlike mucus, and there are also constituents of the immune and circulatory systems - blood vessels go through the bone.
Collagen is a protein and a fiber that makes up a good deal of the body. The collagen in bone is type 1 collagen, as opposed to type 2 (found in cartilage) and type 3 (skin includes both type 1 and type 3 collagen).
The strength of the bones is correlated with their mass. A linear regression of bone strength vs bone mass shows a correlation of about 80 percent.
The mineral portion of the bone is shaped into thin sheets called lamellae. Between the lamellae are spaces called lacuna, which are most often occupied by osteocytes.
Osteoclasts are large multi-nucleate cells that move within the bone. They migrate to the surface of the bone and release acids and enzymes that dissolve the mineral bony material This is the destructive part of bone remodeling, or osteoclastogenesis.
Osteocytes are shaped like elongated starfishes. They are also called simply "bone cells" (although this is technically imprecise) or bone-corpuscles. Most of the cells in bone are osteocytes. They connect to each other.
The newer bony material is more elastic and can more easily withstand stress without fracturing. Everyday life subjects bones to microfractures. In younger people, the bone heals and forms calluses. The calluses function to maintain bone strength. Older bone does not so readily form calluses and the tiny fractures may grow. Even if they don't grow, they contribute to the brittleness of the bone.
Bone remodeling is on-going. The living skeleton replaces about 10% of its bony mass every year.
Mother Nature gave us the bone remodeling process for good reasons. The process allows changes in bone architecture to meet changing mechanical needs. These are slow changes, but they are real and can help the skeletal system adjust to changes in body weight. Normal life results in tiny microtears and rips in the bone matrix and the adsoprtion and reabsorption of minerals helps smoothout the tiny damages. The remodeling also helps keep calcium levels in the blood stable - calcium is important in the rest of the body, too, and the bones are both a source and a sink of calcium in the homeostatic process.
Bone formation or deposition of minerals is called
When a child is growing up, the rate of bone formation exceeds the rate or resorption. This is self evident - "growing up" implies an expanding skeleton. Even after the person has reached maximum height in adolescence, the bone mass continues to increase for a few years as the skeleton "fills out".
Later in life, past age 30 or 40 or so, bones get smaller. The resorption rate exceeds the adsorption rate and bone density declines, even in healthy people with no clinical illness or disease. If the decline in density is dramatic enough, a diagnosis of osteoporosis can be assigned, under current diagnostic guidelines.
The body needs calcium for many functions (almost all physiological processes involve calcium, which is one reason the body's homeostatic processes control the calcium concentration in the blood to a narrow window.
The skeleton functions as a storage system for the body's calcium - a sink and a source in engineering terms. Indeed 99% of the body's calcium can be found in the bones and teeth (as can 85% of phosphorus.) http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/thyroid/calcium.html
Hormones involved in the homeostatic process include parathyroid hormone, calcitonin, and vitamin D. If these look familiar, they should, because all are employed in attempts to control osteoporosis. The kidneys remove excess calcium and phosphorus from the blood, and both the kidneys and the digestive system can adjust operations in response to needs and shortages of these elements. The bones, acting as source and sink, also participate in the homeostatic process. Does this mean the homeostatic process causes osteoporosis? Scientists are not ready to say that, but it is important to look at the body as a whole when trying to understand bone density.
Other Bone Diseases: