04 The Skeletal System

The skeletal framework

The skeletal system is made up of bones, cartilage, joints, and supporting connective tissues. It forms a strong but living framework that works closely with the muscular and circulatory systems. In adults, the skeleton typically contains 206 bones.

Bone structure and remodeling

Bone is connective tissue with a flexible collagen framework strengthened by mineral salts, especially calcium phosphate. Collagen helps bone resist pulling forces, while mineral gives it hardness and helps it resist compression.

Most bones contain two forms of tissue. Compact bone is dense and forms the strong outer layer. Spongy bone is a lighter, lattice-like tissue inside bones; its spaces contain marrow and help distribute forces.

A typical long bone, such as the femur, has a shaft called the diaphysis and expanded ends called epiphyses. The shaft surrounds a medullary cavity. A tough, blood-vessel-rich covering called the periosteum coats most of the outer surface. Smooth articular cartilage covers the bone ends where they meet at joints.

Bone is continually remodeled. Osteoclasts break down bone, osteoblasts build it, and osteocytes help maintain and regulate bone tissue.

Joint structure and movement

A joint, or articulation, is where two or more bones meet. Joints connect the skeleton and vary in how much movement they permit. Structurally, they are classified as fibrous, cartilaginous, or synovial.

• Fibrous joints join bones with dense connective tissue and allow little or no movement. Skull sutures are an example.

• Cartilaginous joints join bones with cartilage and allow limited movement. Some joints between vertebrae are examples.

• Synovial joints have a fluid-filled joint cavity and allow the greatest range of movement. The knee, shoulder, and hip are examples.

In a synovial joint, articular cartilage reduces friction, synovial fluid lubricates the surfaces, and a capsule encloses the joint. Ligaments help hold the bones together and limit excessive movement. Joint structure reflects function: the shoulder permits broad movement, while the hip’s deep, stable joint helps support body weight. Cartilage and joint fluid make movement smoother, but joints still depend on surrounding muscles and ligaments for support and control.

What the skeleton does

The skeleton supports the body by providing a framework for soft tissues. It also protects organs: the skull protects the brain, and the rib cage helps protect the heart and lungs.

Bones enable movement by providing rigid structures that muscles can pull on. The skeleton also stores minerals, especially calcium and phosphate, which can be released when the body needs them, and houses bone marrow, where blood cells are produced.

How bones and muscles produce movement

Skeletal muscles attach to bones by tendons and usually cross one or more joints. When a muscle contracts, it pulls on a bone; the bone acts as a lever, and the joint acts as a pivot. Muscles can pull but cannot push, so many movements use opposing muscle groups.

At the elbow, the biceps contracts to bend the arm while the triceps relaxes. To straighten the arm, the triceps contracts and the biceps relaxes. This interaction also helps maintain posture and stabilize joints.

Regular movement places mechanical stress on bone, which can contribute to the process by which bone adapts and remodels. The skeleton provides the framework and levers for movement, muscles supply the force, and joints guide the motion.

Bone marrow and blood cells

Red bone marrow carries out hematopoiesis: the production of red blood cells, many white blood cells, and platelets. These cells enter the circulation and serve the wider body. Red blood cells transport oxygen, white blood cells contribute to immune defense, and platelets help blood clot.

Red marrow is found in parts of spongy bone, including areas of the pelvis, vertebrae, ribs, sternum, and the ends of some long bones. Yellow marrow is richer in fat and mainly stores energy.