Skeletal system: function

If bones only held you up, a skeleton would be a simple frame. In fact, bones do five jobs, and several of them might surprise you.

First, bones give support. They provide the body's shape and hold the soft organs in place; without them you would collapse like a jellyfish on a beach. Second, bones protect. The skull encloses the brain, the rib cage shields the heart and lungs, and the vertebrae surround the spinal cord. Third, bones help you move. Tendons attach muscles to bones, and when a muscle contracts it pulls on a bone, which pivots at a joint like a lever. You will see how in the muscular system lessons.

Fourth, bones make blood cells. Red marrow inside certain bones, such as the sternum, ribs, vertebrae, and pelvis, produces red blood cells, white blood cells, and platelets, millions of new red blood cells every second. Yellow marrow, found in the centers of long bones, stores fat for energy. Fifth, bones store minerals. About 99 percent of the body's calcium is stored in bones. Your nerves, muscles, and heart also need calcium, so when the amount in the blood drops, bones release some, and when there is extra, bones take it in. This is an early look at homeostasis, which you will study at the end of the unit.

Bone is constantly being rebuilt. Cells called osteoclasts break down old bone, and cells called osteoblasts build new bone. This process, called remodeling, replaces much of an adult's skeleton roughly every ten years. In children and teens, bones grow longer at growth plates, areas of cartilage near the ends of long bones, until they harden in the late teens. When a bone breaks, a blood clot forms first. Next a soft cartilage patch forms, then it hardens into bone, and finally remodeling reshapes the repair. Healing takes weeks to months.

You can help your bones. Eat enough calcium and get enough vitamin D. Do weight-bearing activity such as walking, running, and jumping, because bones respond to stress by becoming denser. Avoid smoking. Bone mass peaks in early adulthood, so the bone you build now is the bank you draw on later in life, when bones can weaken in a condition called osteoporosis.

Common mistake: thinking calcium only matters for children. Bones are a calcium reserve for the whole body for life.