Bones are strong, rigid structures made of proteins and minerals such as calcium.
Bones have a hard, dense outer layer that gives them strength. The inside of bones is porous (full of tiny holes) to let blood flow through. The inside of bones also contains the bone marrow. The outside of bones is covered by a thin sheet of tissue called the periosteum, which helps nourish the bone.
Bones give your body form and structure and allow you to move around.
Joints are where two bones meet.
Joints would fall apart unless they were held together by ligaments.
Joints have several different ligaments that hold them together. Ligaments and the way the ends of your bones are shaped let joints move only in certain directions. For example your knee joint can bend and straighten but not move sideways.
Some joints, such as your shoulder, can move a lot. Other joints, such as those between your ribs and spine, move only a little.
Like ligaments, muscles connect two bones across a joint. When muscles contract, they pull the joint open or closed.
When you see a skeleton you might think bones are not alive, like pipes or sticks of wood. However:
Children's bones grow by getting longer from the ends. There is a soft part at the end of a child's bones called a growth plate. Growth plates form new bone that makes bones longer. During adolescence, children's growth plates turn into hard bone so their bones stop getting longer.
But even adult's bones constantly break down and reform in a process known as remodeling (see Osteoporosis Osteoporosis Osteoporosis is a condition that weakens your bones. Your bones become thinner and more fragile. This is called a loss of bone density. If you have osteoporosis, your bones break more easily... read more ).
For bones to grow and remodel, they need a steady supply of calcium, other minerals, vitamin D, and certain hormones.
Most bones have a porous center (meaning it's full of tiny holes) that contains bone marrow. Marrow is made of specialized cells (including stem cells) that produce blood cells What is in blood? Blood is the red fluid in your arteries and veins. It provides the oxygen, water, and nutrients that your tissues and organs need to survive. You have about 5 liters of blood in your body. Your... read more .
Problems with bones include:
Bone tumors may be cancerous or noncancerous (benign). Some cancerous tumors spread to your bones from other organs.
Several types of cancer, including multiple myeloma Multiple Myeloma You have several types of blood cells: Red blood cells carry oxygen White blood cells fight infection Platelets help your blood clot when you're bleeding Plasma cells are a special type of white... read more , leukemia Leukemias read more , and lymphoma Lymphomas read more , can develop in the bone marrow.