SKELETAL, MUSCULAR, AND NERVOUS SYSTEMS

The Skeletal System

Marathon runners travel more than 26 miles from start to finish. Running that far challenges the body both physically and mentally. Those who finish can take pride in achieving such a difficult goal. Running a marathon requires muscle strength and endurance. It also requires a sturdy skeletal system to support those muscles.

The skeletal system is a framework of bones that provides structure to your body and protects your internal organs. The skeleton also serves as a reservoir for the storage of minerals, produces blood cells, and allows movement of the body. A typical human skeleton is made up of 206 bones.

The skull contains bones that surround and protect the brain, while bones of the spine support the skull and allow movement in the back. The ribs protect important internal organs. Bones in the hand allow for many tasks, and bones in the feet support the body when walking or running.

Each bone is composed of living, growing tissue. If a bone breaks, one type of bone cell breaks down the damaged tissue while another type of bone cell begins to rebuild the bone. Calcium and vitamin D are nutrients that help build strong bones. You may think of bones as hard, solid objects. Although the outside of a bone is hard, the inside is filled with soft tissue called marrow. Bone marrow produces new blood cells.

Two types of bone marrow, red and yellow, run through the center of many long bones, such as those in the legs and arms. Red bone marrow produces red blood cells, which transport oxygen throughout the body. It also produces certain types of white blood cells, which help to fight disease. Yellow bone marrow is made up mostly of fat cells that are a source of stored energy.

Bones come in different shapes and sizes, and are based on their functions. For example, short, slender bones in your fingers allow your fingers to move and grasp things. Bones are classified as:

• long bones, found in the legs and arms

• short bones, found in fingers and toes

• flat bones, found in the skull and pelvis

• irregular bones, found in the backbone and ears

Joints

Joints are places where two or more bones meet. Tough strands of connective tissue called ligaments connect bones at most joints. Bones at joints are covered in flexible cartilage, which is a tough, elastic connective tissue. Cartilage covers bones to prevent them from rubbing against each other.