Anatomy of the bones

The body has 206 bones that together form the skeleton or framework. They give the body shape and stability, serve as attachment sites for tendons, and protect the internal organs.

Different types of bones

Bones are also known as osseous structures. They can be divided into four categories:

• Long and short tubular bones. Tubular bones consist of a middle section (the shaft) and two wider ends. These ends form the head or socket of a joint. An example of a long tubular bone is the shinbone. The fingers are examples of short tubular bones.
• Flat bones. These are flat bones such as those of the shoulder blade or the skull.
• Sesamoid bones. These lie within a tendon, such as the kneecap.
• Irregular bones. These are bones that have many different joint surfaces or projections. Think of the vertebrae or the carpal and tarsal bones.

Continuous build-up and breakdown of bone tissue

Bones are composed mainly of minerals, proteins, and water. Bone is constantly being broken down and rebuilt. This happens through osteoblasts, the bone-forming cells. And osteoclasts, which remove aged or damaged bone.

At a young age, the body builds more bone than it breaks down. It has to, because at birth most of the bones still consist of cartilage. The bones lengthen and harden until they reach peak bone mass. The bones are then fully grown, have the greatest bone mass, and are at their strongest. This happens around the age of 30.

After that, breakdown gradually gains the upper hand. The bones then decline in quality and strength. One consequence of this is osteoporosis. The bones become more brittle and can break more easily. Healthy bones depend on the following factors:

• Hereditary predisposition.
• A healthy and varied diet.
• Regular physical activity (see also stronger bones through sport and strength training).
• Low stress, because stress impairs the absorption of nutrients.
• Sufficient sunlight for the production of vitamin D.

Bone from outside to inside

From the outside in, bone is built up in several layers. The outside is covered by a periosteum rich in nerve fibers. That is why it hurts so’n much when, for example, you bump your shin. Directly underneath lies a compact, hard bone layer.

A bone is not completely solid. Inside, the bone contains spongy bone tissue with many small spaces, giving it an ‘airy’ appearance, the marrow cavity. This is where the bone marrow is located. There are two types of bone marrow:

• Red bone marrow: bones are filled with red bone marrow from birth. Red bone marrow produces new red blood cells for oxygen transport, white blood cells for immunity, and platelets to allow the blood to clot.
• Yellow bone marrow: later in life, much of the red bone marrow disappears and is replaced by yellow bone marrow. This mainly stores fatty acids . In places such as the thighbone, vertebrae, ribs, and hip bones, red bone marrow does remain present.

Bone from top to bottom

From top to bottom, several parts can also be distinguished in a long tubular bone:

• Epiphysis: the epiphysis lies at the end. It ensures that the articular cartilage receives adequate nutrition.
• Metaphysis: bones grow from the metaphysis, which lies below the epiphysis.
• Diaphysis: in the shaft of the bone lies the diaphysis, where the marrow cavity is responsible for the formation of new blood cells.