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BONES! BONES! BONES!  Like a house, the human body has a framework. But instead of wood, the body's framework is made of all the bones in our skeletal system. There are long bones (arms and legs), short bones (fingers and toes), flat bones (skull and sternum), and even tiny bones (in the middle ear).

Some bones, like the ribs, permit respiration and protect vital organs such as the heart from harm. Others, like the spinal vertebrae, form the framework to keep us upright as well as surround and protect the spinal cord. Unlike a house's framework, however, the body's framework is alive - bones are living tissue. From birth until mid- to late- adolescence, bones grow as we do. They reshape themselves throughout our lives. Some of the bone cells that carry on this growth and "remodeling" work are called **osteoblasts** and **osteoclasts**. Bones grow at special areas called **growth plates**.

The outer layer of hard "compact" bone (the cortex) consists of a system of tunnels that looks like a miniature collection of hollow pipes. These tunnels keep bones lightweight, yet provide the strength necessary to support the body. The tunnels also allow for the exchange of nutrients and waste products. Collagen, a protein, gives bones their elasticity while calcium salts make them hard. Bone marrow, located in the center of our long bones, makes blood cells. To develop properly and grow strong, our bones need **calcium**, **vitamin D**, and **regular exercise**.

Sources of calcium such as milk, yogurt, ice cream, and broccoli provide the nutrients for continuous bone building and remodeling. Vitamin D helps our bodies absorb this calcium through the gut. Vitamin D comes either from a chemical action of sunlight on our skin or from fortified milk.

Finally, regular exercise makes bones stronger by stressing them. When bones are stressed, they respond by fortifying themselves. This cycle of stress and response to stress promotes strengthening. When we don't get enough calcium, vitamin D, and exercise, our bones deteriorate. For example, young children who are deprived of calcium and vitamin D may develop a type of bone weakness called **rickets**. Older people, particularly women, may develop a form of weak bones called **osteoporosis**.

Prevention of bone weakness is crucial - once weakness occurs, it may not be reversible. So to have healthy bones and keep our body's living framework strong and supple, we must eat foods **containing calcium every day, exercise on a regular basis** and get either a moderate amount of exposure to sunlight.

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