Bones, Muscles, and Joints

What Are Bones and What Do They Do?

Bones provide support for our bodies and help form our shape. Although they're very light, bones are strong enough to support our entire weight.

Bones also protect the body's organs. The skull protects the brain and forms the shape of the face. The spinal cord, a pathway for messages between the brain and the body, is protected by the backbone, or spinal column. The ribs form a cage that shelters the heart and lungs, and the pelvis helps protect the bladder, part of the intestines, and in women, the reproductive organs.

Bones are made up of a framework of a protein called collagen, with a mineral called calcium phosphate that makes the framework hard and strong. Bones store calcium and release some into the bloodstream when it's needed by other parts of the body. The amounts of some vitamins and minerals that you eat, especially vitamin D and calcium, directly affect how much calcium is stored in the bones.

Bones are made up of two types of bone tissues:

1. Compact bone is the solid, hard outside part of the bone. It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves.
2. Cancellous (KAN-suh-lus) bone, which looks like a sponge, is inside compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae (truh-BEH-kyoo-lee). This is where bone marrow is found.

In this soft bone is where most of the body's blood cells are made. The bone marrow contains stem cells, which produce the body's red blood cells and platelets, and some types of white blood cells. Red blood cells carry oxygen to the body's tissues, and platelets help with blood clotting when someone has a cut or wound. White blood cells help the body fight infection.

Bones are fastened to other bones by long, fibrous straps called ligaments (LIG-uh-mentz). Cartilage (KAR-tul-ij), a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other.

How Do Bones Grow?

The bones of kids and young teens are smaller than those of adults and contain "growing zones" called growth plates. These plates consist of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone. These growth plates are easy to spot on an X-ray. Because girls mature at an earlier age than boys, their growth plates change into hard bone at an earlier age.

Bone-building continues throughout life, as a body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells:

1. osteoblasts (AHS-tee-uh-blastz), which make new bone and help repair damage
2. osteocytes (AHS-tee-o-sites), mature bone cells which help continue new born formation
3. osteoclasts (AHS-tee-o-klasts), which break down bone and help to sculpt and shape it

What Are Muscles and What Do They Do?

Muscles pull on the joints, allowing us to move. They also help the body do such things as chewing food and then moving it through the digestive system.

Even when we sit perfectly still, muscles throughout the body are constantly moving. Muscles help the heart beat, the chest rise and fall during breathing, and blood vessels regulate the pressure and flow of blood. When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy.

Humans have three different kinds of muscle:

1. Skeletal muscle is attached by cord-like tendons to bone, such as in the legs, arms, and face. Skeletal muscles are called striated (STRY-ay-ted) because they are made up of fibers that have horizontal stripes when viewed under a microscope. These muscles help hold the skeleton together, give the body shape, and help it with everyday movements (known as voluntary muscles because you can control them). They can contract (shorten or tighten) quickly and powerfully, but they tire easily.
2. Smooth, or involuntary, muscle is also made of fibers, but this type of muscle looks smooth, not striated. We can't consciously control our smooth muscles; rather, they're controlled by the nervous system automatically (which is why they're also called involuntary). Examples of smooth muscles are the walls of the stomach and intestines, which help break up food and move it through the digestive system. Smooth muscle is also found in the walls of blood vessels, where it squeezes the stream of blood flowing through the vessels to help maintain blood pressure. Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don't tire easily.
3. Cardiac muscle is found in the heart. The walls of the heart's chambers are composed almost entirely of muscle fibers. Cardiac muscle is also an involuntary type of muscle. Its rhythmic, powerful contractions force blood out of the heart as it beats.