Bones

Bones are amazing. From the bones in your feet to the bones in your skull, your body has 206 kinds of different bones. These bones give your body its shape. Just think about it. If it were not for your bones, you would be a big blob on the floor. Yuck!

Bones are surprisingly strong. They get this strength from calcium (and a few other minerals like phosphorus). If you think about how hard and strong cement is, you have an idea of how bone can be so strong. Cement, like bone, contains a lot of calcium. But how is the calcium turned into bone? There are cells called osteoblasts that do the work. When you eat anything that contains calcium -- broccoli, yogurt, whatever -- the calcium is absorbed from your small intestine into your blood. Osteoblasts pull the calcium out of the blood and deposit it as new bone.

Your bones are constantly rebuilding themselves to stay strong. Cells called osteoclasts break down bone, and then osteoblasts come and lay down new bone in its place. This happens all through your life, making sure your bones are always fresh. This process also explains why a broken bone can heal itself. Osteoblasts cluster around the break and start multiplying. They will eventually bridge the broken bone and then start filling it in with new calcium. Meanwhile, osteoclasts will break down all the bone fragments nearby to clean up the area.

Got Milk? Milk and other calcium-rich foods are good for your parents too! You probably don't think of your bones as growing once you reach adulthood. After all, you don't keep getting taller as you age. But your body continues to break down old bone and build new bone throughout your life. The tricky part comes with age. More bone is added than lost during youth. But somewhere around age 30, your bones reach their peak bone mass (bones are strongest and most dense). After that point, the tables turn, and the breakdown of bone begins to slowly outpace the building of new bone.

Inside your larger bones is another surprise – bone marrow. The marrow is able to generate new red and white blood cells, as well as new osteoblasts and osteoclasts, and send them out into the bloodstream. You have trillions of red blood cells in your body right now and, as they wear out, the bone marrow creates new ones – millions of them every second. A red blood cell lasts about four months.

Muscles attach to bones so that the bones can move. There are strong fibers at the end of muscles called tendons. The tendons attach to bone and let the muscles pull on bones when they contract. One interesting thing about muscles, tendons and bones is that they can all get stronger. If you lift a lot of weights, your muscles will get bigger and stronger. So will the tendons. So will the attachment points where the tendons connect to the bones. And so will the bones themselves. The whole system gets stronger, not just the muscle. After you die, and when all that’s left of your body is the bones, scientists will be able to see how strong you were by looking at your bones. If you exercise a lot, your bones will be stronger and the places where the tendons connect will be stronger too. Exercise is written into your bones!