While they may seem solid and unchanging, your bones are made from living tissue and constantly undergo a process called reabsorption, or resorption. During this process, old bone gets removed and replaced by new bone. As part of resorption, calcium and another mineral called phosphate are pulled from your bones and passed into your bloodstream.
The Basics
Bone is made from a combination of a compound called calcium phosphate and a protein called type-I collagen. Type-I collagen forms the basic bone framework, while calcium phosphate provides hardness and reinforces the framework. Roughly 99 percent of your body's entire calcium supply is contained in your bones and teeth. The remainder circulates in your bloodstream and helps support a number of critical body functions. Your bones also contain roughly 85 percent of your body's supply of phosphate.
Bone Resorption
About 10 percent of your bone gets replaced every year. The resorption process is initiated by cells in your bone tissue called osteocytes. These cells monitor the physical forces on your bone and determine when you need to replace tissue to maintain bone integrity. After receiving a chemical signal from osteocytes, cells on your bone surfaces, called osteoclasts, activate and secrete specialized proteins, called enzymes, that break down the mineralized portion of your bone and release calcium and phosphate into your bloodstream. At the same time, other enzymes dissolve type-I collagen.
Bone Formation
Bone resorption is followed by a process called bone formation. During this process, cells on the surfaces of your bones, called osteoblasts, secrete substances that promote the formation of a new type-I collagen framework. After osteoblasts finish creating this framework, calcium and phosphate from your bloodstream form a new hardened matrix that completes the new tissue. As with bone resorption, the formation process takes place continually on a small scale in bones throughout your body. Taken together, the processes of resorption and formation are sometimes referred to as bone remodeling.
Changing Ratios
The ratio of bone resorption to bone formation changes throughout your lifespan. During childhood and adolescence, formation occurs more frequently than resorption, and as a result your bones grow in density and strength. Once your bones reach their maximum strength and density sometime in the second half of your 20s, resorption starts occurring more frequently that formation. This shift eventually causes you to lose significant amounts of bone. In women, the most pronounced bone loss occurs in the years following menopause. In men, prominent bone loss doesn't typically appear until the eighth decade of life. Consult your doctor for more information on bone resorption, bone formation and calcium.
