This substance is composed primarily of calcium and other minerals, and stores much of the body's calcium it is primarily responsible for the strength of bones. The hard outer part of bones are comprised mostly of a proteins such as collagen, in addition to a substance called hydroxyapatite.
Cartilage is a type of connective tissue, and contains collagen and elastin fibers. Although mature bones consist largely of calcium-70% calcium salts and about 30% organic matrix, mostly collagen fibers-most bones in the skeleton of vertebrates, including humans, began as cartilage. Humans are born with over 300 bones but some bones, such as those in the skull and lower spine, fuse during growth, thereby reducing the number. Hence, bone is strong, hard, and slightly elastic. Living mature bone is about 60% calcium compounds and about 40% collagen. Inside every person is a skeleton, a sturdy framework of about 206 bones that protects the body's organs, supports the body, provides attachment points for muscles to enable body movement, functions as a storage site for minerals such as calcium and phosphorus, and produces blood cells. The skeletal system is a living, dynamic, bony framework of the body, with networks of infiltrating blood vessels.
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