Bones are more than just the scaffolding that holds the body together. Bones come in all shapes and sizes and have many roles. In this article, we explain their function, what they consist of, and the types of cells they involve.

Bones are living, active tissues that the body is constantly remodeling.

Their functions include supporting body structure, protecting vital organs, and allowing the body to move. Also, they provide an environment for bone marrow, where the body creates blood cells, and they act as a storage area for minerals, particularly calcium.

The skeleton accounts for around 15% of body weight. At birth, humans have around 270 soft bones. As they grow, some fuse.

By adulthood, people have between 206 and 213 bones. The reason for the difference is that some people have more or fewer bones in their ribs, vertebrae, fingers, and toes.

The largest bone in the human body is the thighbone, or femur, and the smallest is the stapes in the middle ear, at around 3 millimeters long.

Bones consist mostly of the protein collagen, which forms a soft framework. The mineral calcium phosphate hardens this framework, giving it strength. The bones contain 99% of the body’s calcium.

Bones have an internal structure similar to a honeycomb, which makes them rigid yet relatively light.

An x-ray of various types of bonesShare on Pinterest
Tridsanu Thophet/EyeEm/Getty Images

Bones are composed of two types of tissue.

Compact (cortical) bone is a hard outer layer that is dense, strong, and durable. It makes up around 80% of adult bone mass and forms the outer layer of bone.

Cancellous (trabecular or spongy) bone makes up the remaining 20% of bone and consists of a network of trabeculae, or rod-like, structures. It is lighter, less dense, and more flexible than compact bone.

Bones also contain:

  • osteoblasts and osteocytes, responsible for creating bone
  • osteoclasts, or bone-resorbing cells
  • osteoid, a mix of collagen and other proteins
  • inorganic mineral salts within the matrix
  • nerves and blood vessels
  • bone marrow
  • cartilage
  • membranes, including the endosteum and periosteum

Below is a 3D map of the skeletal system. Click to explore.

Bones are not static tissue but need constant maintenance and remodeling. There are three main cell types involved in this process.

  • Osteoblasts are responsible for generating and repairing bone. They produce a protein mixture that doctors call osteoid, which is mineralized and becomes bone.
  • Osteocytes are inactive osteoblasts that are mineralized and remain within the bone they have created. They communicate with other bone cells and help support metabolic functions within the bone.
  • Osteoclasts are large cells with more than one nucleus. They use acids resulting from certain reactions to break down used bone. This process is called resorption. Osteoclasts help remodel injured bones and create pathways for nerves and blood vessels to travel through.

Bone marrow

Bone marrow is present in almost all bones where cancellous, or spongy, bone is present.

Bone marrow produces blood cells, including:

  • red blood cells, which deliver oxygen to cells
  • white blood cells, essential for the body’s immune system
  • platelets, which the body uses for clotting

The marrow produces around 2 million red blood cells every second. It also produces lymphocytes, or the white blood cells involved in the immune response.

Extracellular matrix

Bones are essentially living cells embedded in a mineral-based organic matrix. This extracellular matrix consists of organic components (mostly type 1 collagen) and inorganic components, including hydroxyapatite and other salts, such as calcium and phosphate.

Collagen gives bone its tensile strength, namely resistance to pulling apart. Hydroxyapatite gives the bones compressive strength, or resistance to compression.

Bones serve various functions that affect the whole body. Studies show that, in addition to structure and movement, bones support energy metabolism, the production of blood cells, the immune system, and brain function.

Mechanics

Bones provide a frame to support the body. Muscles, tendons, and ligaments attach to bones. Without anchoring to bones, muscles could not move the body.

Protection

Some bones protect the body’s internal organs. For instance, the skull protects the brain, and the ribs protect the heart and lungs.

Synthesis

Cancellous bone is a vital reservoir for developing red blood cells, platelets, and white blood cells. Also, the body destroys defective and old red blood cells in bone marrow.

Metabolism

The metabolic functions of bone include:

  • Storage: Bones act as a reserve for minerals, particularly calcium and phosphorous. Bone marrow adipose tissue can also store fatty acids.
  • Endocrine function: Bones produce the precursors to various hormones, including those involved in growth, insulin production, and brain development. They release hormones that act on the kidneys and influence blood sugar regulation and fat deposition.
  • Calcium balance: Bones can raise or reduce calcium in the blood by forming bone, or breaking it down in a process called resorption.
  • pH balance: Some research has suggested bones can release or absorb alkaline salts, helping blood to stay at the right pH level, but scientists need more studies to confirm this.
  • Detoxification: Bones can absorb heavy metals such as lead, mercury, and arsenic from the blood.

There are five types of bones in the human body:

  • Long bones: These are mostly compacted bones with little marrow and include most of the bones in the limbs. They tend to support weight and help movement.
  • Short bones: These have a squat, cubed shape and include bones of the wrist and ankle.
  • Flat bones: These have a flat, broad surface. They consist of two outer layers of compact bone and an inner layer of spongy bone. The bones of the skull, breastbone, ribs, and shoulder blades are flat bones. They tend to have a protective role.
  • Sesamoid bones: These are embedded in muscles and tendons near the surfaces of joints. They include the patella or kneecap. They protect tendons from wear and stress.
  • Irregular bones: These bones do not fit into the first four categories and have an unusual shape. They include the bones of the spine and pelvis. They often protect organs or tissues.

The bones of the skeleton belong to two groups: The appendicular and axial skeletons.

The appendicular skeleton comprises 126 bones, including those of the limbs, shoulders, and pelvic girdle. It provides structure and support to other parts of the body.

The axial skeleton has less range of motion than the appendicular skeleton. It comprises the bones of the skull, vertebral column, and thoracic cage.

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Electron microscope image of trabecular bone (x100 magnification).
Image credit: Sbertazzo

The body is always remodeling bone. This allows the body to fix damaged bone, reshape the skeleton during growth, and regulate calcium levels.

Remodeling is a two-part process. During formation, the body lays down new bone tissue. In resorption, osteoclasts break down and remove bone.

If one part of the skeleton comes under increased stress over time — for instance, during exercise — the sections of bone under most pressure will become thicker in response.

Osteocytes, osteoclasts, and osteoblasts play key roles, but other elements also contribute. These include parathyroid hormone, vitamin D, estrogen, and testosterone.

Osteoporosis is a bone disease that involves a reduction in bone mineral density. This increases the risk of fractures.

It most commonly occurs in females after menopause. However, it can affect males too, and it can start before menopause.

Osteoporosis occurs either when removal or resorption of bone happens too quickly, new bone forms too slowly, or for both reasons.

Risk factors include:

Screening can help prevent or slow the progression of osteoporosis. Tests can show that osteopenia, the early stage of osteoporosis, is present. At this point, a doctor may recommend dietary measures or supplements.

As bone deterioration worsens, medications are available to slow its progression.

What other bone diseases are there?

Currently, researchers are looking into ways to regenerate bone. This could help people with osteoarthritis, osteoporosis, and other conditions. It could also help mend broken bones.

Bone regeneration is a complex process. Scientists are currently looking at various aspects, including ways to:

  • speed up mineral production for the generation of new bone
  • use natural or synthetic grafts to enhance bone healing
  • scaffold new bone and allow growth to occur
  • use artificial biomaterials to achieve bone regeneration
  • stimulate nerve pathways to encourage authentic bone production
  • regenerate bones with surfaces that allow for nutrient absorption
  • use stem cells to encourage bone to regenerate

Here are some answers to questions people often ask about bones:

Why are bones important for overall health?

Bones support the body’s structure and protect vital organs, but they also play a key role in blood cell production, the immune system, the storage of calcium, the release of essential hormones, and many other functions.

What things are important for bone health?

Following a varied diet with plenty of calcium, getting enough vitamin D, and exercising are important for bone health. Avoiding smoking and limiting alcohol intake can also help prevent osteoporosis.

How do you know if your bones are unhealthy?

Various health problems can affect the bones. Signs of osteoporosis include a loss of height, an increasingly stooped posture, and fractures that happen often or easily. A screening test can show if a person has reduced bone density.

Bone pain can be a sign of bone damage, infection, or bone cancer. Bones can become soft if there is a vitamin D deficiency. This can lead to bent shins in children, known as rickets. In adults, doctors call this osteomalacia.

A nontraumatic bone fracture in adults over 50 years old may also be an early sign of undiagnosed cancer, such as metastatic breast or lung cancer or multiple myeloma.

Bones play an essential role in the structure and function of the human body.

As well as enabling movement, they maintain appropriate levels of many compounds. They regulate hormonal pathways, contribute to metabolism, support the immune system, and more.