The Skeletal System

Osteoporosis

As people age, the replacement of bone cells and of the minerals in bone becomes less efficient. Over time, bone tissue is lost. Severe bone loss can lead to osteoporosis, a condition in which bones become brittle and are easily broken. A healthy diet, regular exercise, and medication can help maintain bone density.

Bones

Your skeleton is made of bones. There are four basic bone shapes: long, short, flat, and irregular. Long bones are tube shaped, and include bones of the arms, legs, and fingers. Short bones are found in the wrists and ankles. Flat bones include the skull and sternum. Irregular bones include the bones of the spine. A typical bone is made up of four layers: the periosteum, compact bone, spongy bone, and bone marrow. Bones are covered by a tough membrane called the periosteum (PER ee AHS tee uhm). Beneath the periosteum is a layer of compact bone. Compact bone is made of tightly packed mature bone cells called osteocytes. Inside the compact bone is a layer of spongy bone. Some of the spaces in spongy bone are filled with soft tissue called bone marrow. There are two types of bone marrow. Red bone marrow produces all blood cells. Yellow bone marrow is mostly fat, which stores energy.

Types of Joints

Immovable joints, such as those in the skull, do not allow the bones to move. Slightly movable joints, such as the joints of the spine and rib cage, allow only a small amount of movement. Freely movable joints allow the most movement. Most joints of the appendages are freely movable. Movable joints are made up of bones, cartilage, and ligaments. Pads of cartilage cushion the ends of bones and help movable joints withstand pressure and stress. Movable joints are held together by strong bands of connective tissue called ligaments. Ligaments help prevent joints from moving too far in any direction.

Arthritis

When the cartilage protecting joints is damaged, a disease called arthritis can result. There are two types of arthritis. Osteoarthritis occurs when the cartilage that covers the surfaces of bones wears away. The bones rub together, which causes pain. Rheumatoid arthritis occurs when cells of the immune system attack the tissues around joints. Rheumatoid arthritis is very painful and can result in damaged and deformed joints.

Bursitis

Bursae are fluid-filled sacs that help cushion and lubricate pressure points around joints. When joints are overused, bursae can become inflamed. Bursitis is a common disorder in athletes and musicians in which these sacs become swollen and painful.

Bone Growth

In infants, the skeleton is made mostly of cartilage. Cartilage is a type of connective tissue that cushions joints, makes up flexible body parts such as ears and noses, and serves as a template for bone formation. As bones grow, bone cells called osteoblasts deposit calcium and other minerals. Bones continue to become thicker and longer through adolescence as bone replaces cartilage. Eventually, almost all of the cartilage in the body is replaced by bone. In compact bone, new bone cells grow in tight layers around channels called Haversian canals. Haversian canals provide a passageway for blood vessels to deliver the nutrients to bone cells.

Leukemia

Leukemia is cancer of the tissues that produce blood cells. Leukemia results in large numbers of immature white blood cells. Because they are not fully developed, the cells do not function properly. The buildup of these cells also interferes with production of other types of blood cells. Bone marrow and stem cell transplants can produce healthy blood cells, but leukemia has no cure.

Appendicular Skeleton

The appendicular skeleton includes the appendages, which are the shoulders, arms, hips, and legs. The arms attach to the axial skeleton at the shoulders. The shoulder blades (scapulae) and the collar bone (clavicles) make up the pectoral girdle. The legs attach to the axial skeleton at the pelvic girdle. The pelvic girdle (hips) includes the bones of the pelvis: the ilium, ischium, and pubis.

Axial Skeleton

The axial skeleton includes the skull, spine, ribs, and sternum. The axial skeleton forms the central axis of the body. The skull protects the brain. The spine supports the trunk. The ribs and sternum protect the heart and lungs.

The Skeleton

The five important functions of the skeletal system are support, protection, movement, mineral storage, and blood cell formation. Without our skeleton, our bodies would have no definite shape. The skeleton protects the internal organs. For example, the skull protects the brain. The skeleton is also important for movement. Muscles are attached to bones and pull on bones to cause movement. Bones store important minerals, such as calcium and phosphorus. Lastly, some bones make blood cells. The skeleton is divided into two major parts: the axial skeleton and the appendicular skeleton, shown in Figure 4.

Sprain

The most common injury to a joint is a sprain. A sprain occurs when a joint is bent too far or in the wrong direction. This incorrect movement damages the ligaments of the joint. Knees are especially vulnerable to sprains because of the amount of weight that the knees support.

Fractures

The most common injury to bones is a fracture. Fractures can range from hairline stress fractures, such as those from running or jumping on hard surfaces, to compound fractures in which the broken bone breaks through the skin.

Joints

The skeletal system is linked by many types of joints, as shown in Figure 5. A joint is a place where two or more bones meet. Joints are grouped by their structure and by the way they move.

Bone Injuries and Disorders

There are many kinds of injuries and disorders that can affect bone. Three common bone injuries and disorders are discussed below.