The human skeleton and its 200-plus bones are obviously crucial to the body's structure and durability, lending stature and support while protecting internal soft-tissue organs. But bone also acts as a storage medium for certain minerals, which are cycled between the skeleton and body fluids under tight cellular control.
Calcium and Phosphorus
Calcium and phosphorus are the most numerous minerals in the human skeleton. Some 99 percent of the human body's reservoir of calcium -- the most numerous of all body minerals -- resides in the skeleton, including the teeth. About 85 percent of the body's phosphorus hails from the bones. Phosphorus in combination with oxygen creates phosphates. These, in turn, collaborate with calcium to form various calcium-phosphate crystals, most significantly hydroxyapatites.
Both calcium and phosphorus are critical to metabolism: Calcium, for example, helps drive blood-vessel expansion and contraction, while phosphorus is vital for the delivery of oxygen throughout the bloodstream via hemoglobin.
Bone Strength
Calcium phosphates lend the bone its hardness, but mineralized bone alone is highly brittle. Part of the magic of the skeleton's durability is the interaction of the bone minerals with the protein structures called collagen, which form about a third of the bone's total weight and enhance its flexibility. A human embryo has a skeleton made up of collagen-based cartilage, but as the individual develops, true bone forms with the deposition of calcium-phosphate minerals in integration with collagen fibers. Pure cartilage structure persists in places like the ears and nose of an adult human.
Other Minerals
Magnesium makes up only about 1 percent of bone material, but most of this mineral in the body is found in the skeleton. Its depletion in bone has been shown to make skeletal crystals larger and more brittle, according to the Linus Pauling Institute at Oregon State University. In addition to enhancing bone structure, magnesium plays a role in hundreds of metabolic reactions within the body, including the synthesis of adenosine triphosphate -- that crucial molecule that helps make energy available for metabolic function -- and the transfer of ions across cellular membranes.
Mineral Cycling
Bone is an actively engaged material. To maintain proper mineral levels in the body, parts of the skeleton may be dissolved or reabsorbed into the tissues and blood by the bone cells called osteoclasts; replacement bone structure can be initiated by similar cells known as osteoblasts. According to the authors of "The Anatomy and Biology of the Human Skeleton," the entire mineral component of the skeleton of an infant cycles through resorption and rebuilding each year, while in adults about 18 percent of bone annually undergoes this turnover. When calcium levels in the blood, which are intensely controlled by the body, are deficient, excessive resorption from bone can lead to the condition called osteoporosis.
References
- Oregon State University -- Linus Pauling Institute; Calcium; Jane Higdon, et al.; 2007
- Oregon State University -- Linus Pauling Institute; Phosphorus; Jane Higdon, et al.; 2007
- "The Anatomy and Biology of the Human Skeleton"; D. Gentry Steele, et al.; 1988
- Encyclopedia Britannica Kids: Chemical Composition of Bones (Video)
- Katharine Lady Berkeley's School: Information on the Human Skeleton
- Oregon State University -- Linus Pauling Institute; Magnesium; Jane Higdon, et al.; 2007
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