Our bones are predominantly made from calcium because calcium, in combination with phosphate as hydroxyapatite crystals, provides the rigid structure and strength necessary for bone function, while also serving as the body’s major mineral reservoir[1][2][3].
The primary mineral found in bone is hydroxyapatite ($ \mathrm{Ca_{10}(PO_4)_6(OH)_2} $), which makes up about 65% of bone by weight and is responsible for bone's hardness and resistance to compression[1][2][3]. Calcium's properties—its abundance in the environment, ability to form insoluble salts, and suitability for physiological storage—make it ideal for this role[4][3].
Key reasons for calcium’s predominance in bone:
- Provides rigidity and strength: Calcium phosphate crystals embedded within a collagen framework make bones hard and strong, allowing them to support and protect the body[1][5][6].
- Serves as a reservoir: Bone acts as a depot for calcium, which can be released into the blood as needed to support essential bodily functions like muscle contraction, nerve conduction, and blood clotting[7][8][3][9].
- Biological feasibility and availability: Calcium is abundant in Earth’s crust and biologically accessible, making it a practical choice during evolutionary development of skeletons[4].
- Chemical suitability: Calcium’s low solubility as hydroxyapatite means it can be stored in large amounts without disturbing cellular function, while its presence as a signaling molecule in other physiological processes is tightly regulated[4][3].
Other minerals or elements (like iron or silicon) are used in the skeletons of some non-vertebrate species, but calcium’s unique chemical, biological, and evolutionary advantages explain its overwhelming predominance in vertebrate bones[4].
A caveat: While calcium is critical for bone health, the bone’s structural matrix relies on both collagen (for flexibility) and mineralization (for hardness), and bone health also depends on factors like phosphorus, vitamin D, and physical activity[1][3].
Diagram of four bone cell types: Osteocyte (maintains tissue), Osteoblast (forms matrix), Osteogenic cell (stem cell), and Osteoclast (resorbs bone).
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- https://www.niams.nih.gov/health-topics/what-bone
- https://www.ncbi.nlm.nih.gov/books/NBK109827/
- https://lpi.oregonstate.edu/mic/health-disease/bone-health-in-brief
- https://www.reddit.com/r/askscience/comments/10rzdvf/why_are_the_overwhelming_majority_of_skeletal/
- https://kidshealth.org/en/parents/bones-muscles-joints.html
- https://cancer.ca/en/cancer-information/cancer-types/bone/what-is-bone-cancer/the-bones
- https://medlineplus.gov/ency/article/002062.htm
- https://nutritionsource.hsph.harvard.edu/calcium/
- https://orthoinfo.aaos.org/en/staying-healthy/calcium-nutrition-and-bone-health/
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