Everyone knows bones are hard. They’re made from some of the same minerals found in rocks, like calcium, magnesium, and phosphates.
In fact, about two-thirds of a bone’s weight comes from minerals. Bones give us more than just structure and strength.
▲The longest and strongest bone in the human body is the femur, or thighbone.
▲ The smallest bones are the ossicles—three tiny bones in the middle ear.
Underneath the tough outside of most bones, there’s a mushy center. That part is just as important as the soft organs, like the heart or lungs.
Blood and Bones
About 25 trillion red blood cells race through your bloodstream all the time. They’re delivering oxygen to all parts of your body. Red bone marrow pumps out about 200 billion new red blood cells each day. That’s just to replace the ones that wear out. ▼
▲ Many white blood cells are made in the bone marrow. But not all of them. White blood cells fight off infections from bacteria, viruses, and other bodily invaders.
▲ This is a cross section of compact, or hard, bone. It shows osteons in circular layers. They surround blood vessels and nerves. An osteon is made up of tiny bony tubes, which are made up of a fiberlike protein called collagen.
▲ Whenever you cut yourself, platelets help the blood to clot and form a plug. That keeps you from bleeding to death. Platelets are made in the bone marrow and lymph nodes.
▲ Some people have trouble producing enough blood cells. Leukemia is a disease that causes that problem. So those people sometimes have to get bone marrow from someone else. Bone marrow transplants help them make healthy new blood cells.
◀ Five Kinds of Bones
1. Long bones—like the humerus in the arm—are longer than they are wide.
2. Flat bones, like the ribs and breastbone, are thin and broad.
3. Irregular bones come in various shapes to do special jobs. The vertebral bones in the spine are a good example.
4. Round (sesamoid) bones, such as the kneecap, are usually found near the joints and help with movement. They are called sesamoid bones because they look a little like sesame seeds.
5. Short bones, like ankle bones, are about as long as they are wide.
▲ When the skeleton first forms, it is made mainly of cartilage. Bone (in pink or blue) starts forming when an embryo is around six weeks old. As a person ages, most cartilage turns to bone. Some does not, such as the stuff between the vertebrae (backbones) and in the joints. Cartilage cushions some bones from shock. It keeps other bones from rubbing together.