Your body contains over two hundred bones, joined by cartilage and ligaments. Together they form your skeleton, the framework that supports and protects your muscles and internal organs.
When you touch your bones through soft skin, they feel as solid and unchanging as rock. But that’s just the outer shell. Underneath, bone tissue is porous and very much alive. Blood vessels run through it and at the center is bone marrow, where blood cells are formed.
Bone is made of calcium and other minerals – that’s why it’s hard. Like muscle, bone tissue constantly repairs and renews itself, though with bone this happens much more slowly. The process is called remodeling.
Remodeling is affected by many factors, but the three most important are:
- Estrogen and other hormones
Osteoblasts, the bone-making cells, are especially responsive to estrogen. So after menopause, when the ovaries produce less, bone formation slows down.
- Calcium supply and demand
Calcium is essential to many of the body’s behind-the-scenes chemical reactions, it’s needed for muscles to contract, to regulate blood pressure, and to control bleeding. If you don’t get enough calcium from your diet, your body is forced to draw on the supply stored in your bones.
- Mechanical forces
Physical impact stimulates bone formation. That’s why walking is a better exercise for bones than swimming: When you float in the water, you barely touch bottom, but when you walk, your feet hit against ground. The tug of muscle against bone works the same way. This is one reason that strength training affects bone density. And the stronger your muscles, the more stimulation they provide.