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Bones are hard, right? That’s because they’re full of hard minerals, like calcium, magnesium, and phosphates.
You can find those in rocks, too! In fact, about two-thirds of a bone’s weight comes from minerals. Bones give the body structure and strength. But they do more than that.
▲ The longest and strongest bone in the human body is the femur. You can also call it the thighbone.
▲ The smallest bones are the ossicles. They’re three tiny bones in the middle ear.
Under the hard 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 body all the time. They’re bringing oxygen to all parts of you. Red bone marrow makes about 200 billion new red blood cells daily. 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. These can come from bacteria, viruses, and other bodily invaders.
▲ This is a cross section of compact, or hard, bone. It shows osteons in circular layers. They wrap around blood vessels and nerves. An osteon is made up of tiny bony tubes. Those tubes are made up of collagen, a fiberlike protein.
▲ How do you stop bleeding? Whenever you cut yourself, platelets help your blood to clot. That makes it too sticky to run. That’s why you don’t bleed to death from a papercut. Platelets are made in the bone marrow and lymph nodes.
▲ Some people have trouble making enough blood cells. Leukemia is a disease that causes that problem. So people with leukemia sometimes have to get bone marrow from other people. Bone marrow transplants help them make healthy new blood cells.
◀ Five Kinds of Bones
1. Long bones are longer than they are wide. The humerus, in the arm, is a long bone.
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 an example.
4. Round (sesamoid) bones, like the kneecap, are usually found near the joints and help with movement. They’re called sesamoid bones because they look like sesame seeds.
5. Short bones are about as long as they are wide. Ankle bones are a type of short bone.
▲ When the skeleton is first forming, it is almost all cartilage. Bone (in pink or blue) starts to form when an embryo is around six weeks old. As we age, most cartilage turns to bone. Some does not. That’s cartilage between your vertebrae (backbones) and in your joints. Cartilage protects some bones from shock. It keeps other bones from rubbing against each other.
