Drawing of a human femur.

Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue. Because bones come in a variety of shapes and have a complex internal and external structure, they are lightweight, yet strong and hard, in addition to fulfilling their many other functions. One of the types of tissue that makes up bone is the mineralized osseous tissue, also called bone tissue, that gives it rigidity and a honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage. There are 206 bones in the adult human body^[1] and about 270 in an infant.^[2]

Functions

Bones have nine main functions:

• Protection — Bones can serve to protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs.
• Shape — Bones provide a frame to keep the body supported.
• Blood production — The marrow, located within the medullary cavity of long bones and interstices of cancellous bone, produces blood cells in a process called haematopoiesis.
• Mineral storage — Bones act as reserves of minerals important for the body, most notably calcium and phosphorus.
• Fat Storage — The yellow bone marrow acts as a storage reserve of fatty acids
• Movement — Bones, skeletal muscles, tendons, ligaments and joints function together to generate and transfer forces so that individual body parts or the whole body can be manipulated in three-dimensional space. The interaction between bone and muscle is studied in biomechanics.
• Acid-base balance — Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts.
• Detoxification — Bone tissues can also store heavy metals and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for excretion.^{[citation needed]}
• Sound transduction — Bones are important in the mechanical aspect of hearing.

Characteristics

The primary tissue of bone, osseous tissue, is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxylapatite (this is the osseous tissue that gives bones their rigidity). It has relatively high compressive strength but poor tensile strength, meaning it resists pushing forces well, but not pulling forces. While bone is essentially brittle, it does have a significant degree of elasticity, contributed chiefly by collagen. All bones consist of living cells embedded in the mineralized organic matrix that makes up the osseous tissue.

Masto-structure

Gross anatomy

See also: Human skeleton and List of bones of the human skeleton

Individual bone structure

Section through head of a femur, showing outer compact layer and soft trabecular center with red bone marrow and a spot of yellow bone marrow (white bar = 1cm)

Bone is not a uniformly solid material, but rather has some spaces between its hard elements.

Compact bone or (Cortical bone)

The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces. This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult skeleton. Compact bone may also be referred to as dense bone.

Trabecular bone

Filling the interior of the organ is the trabecular bone tissue (an open cell porous network also called cancellous or spongy bone) which is composed of a network of rod- and plate-like elements that make the overall organ lighter and allowing room for blood vessels and marrow. Trabecular bone accounts for the remaining 20% of total bone mass, but has nearly ten times the surface area of compact bone.

Cellular structure

There are several types of cells constituting the bone;

• Osteoblasts are mononucleate bone-forming cells which descend from osteoprogenitor cells. They are located on the surface of osteoid seams and make a protein mixture known as