Long Bones

Long bones are the main bones in our limbs and play a crucial role in our body's movement. They have a unique structure, consisting of a tube-like shaft of compact bone surrounding yellow bone marrow with ends made up of spongy bone surrounding red bone marrow.

The end of the long bone, called the epiphysis, is made up of spongy bone with a thin layer of compact bone around it. This structure allows the bones to withstand forces from different directions, and the airy ends also help to reduce the weight of the bone, which helps the soft tissue move the bone. The epiphysis is covered with articular cartilage, which provides a smooth surface where the bones can move next to each other without friction.

Between the epiphysis and the shaft is the metaphysis, which is mostly made up of spongy bone but gradually transitions into compact bone. The epiphyseal plate or growth plate is located between the epiphysis and the metaphysis. This is the place where bones grow. In children, this plate is mostly made of cartilage, but hardens and becomes compact bone by the end of puberty. As we age, the epiphyseal plate slowly fades into the epiphyseal line.

The central part of the long bone is called the diaphysis. This part gives the bone tremendous strength along its axis and is made of compact bone. At a young age, the center of the diaphysis and the epiphyses contains red bone marrow, which produces red and white blood cells. After the age of seven, the red bone marrow in the center of the diaphysis is replaced by yellow bone marrow, which is a storage site for triglycerides (fats).

Compact bone requires a lot of oxygen and nutrients, which are supplied by the numerous blood vessels running in and out of the bone. The diaphysis also contains several surface features, such as ridges and bumps, which allow for muscle attachments and serve as points for rotations.

Figure 1: The structure of long bones.