Human skeleton
The human body’s internal framework is the skeleton. At birth, it contains roughly 270 bones, but by adulthood, about 206 remain after some fuse together (not counting accessory bones). Skeletal bone mass accounts for roughly 14% of total body weight—around 10 to 11 kilograms for an average person—and peaks between ages 25 and 30. The skeleton splits into two main parts: the axial skeleton and the appendicular skeleton.
The axial skeleton consists of the spinal column, rib cage, skull, and associated bones. The appendicular skeleton attaches to the axial skeleton and includes the shoulder girdle, pelvic girdle, and the bones of the arms and legs. The skeleton has six key jobs: support, movement, protection, blood cell production, mineral storage, and endocrine regulation.
Compared to many other primates, the human skeleton shows less sexual dimorphism, though subtle differences between sexes appear in the skull, teeth, long bones, and pelvis. Generally, within a given population, female bones are smaller and less robust than male ones. The female pelvis is also shaped differently to aid childbirth. Unlike most primates, human males lack a penile bone.
**Divisions**
**Axial** The axial skeleton (80 bones) includes the vertebral column (32–34 bones, depending on how many segments make up the sacrum and coccyx), the non-cartilage parts of the rib cage (12 pairs of ribs plus the sternum), and the skull (22 bones; some sources add 7 associated bones). This part of the skeleton maintains upright posture, transferring weight from the head, trunk, and upper limbs down to the hip joints. Many ligaments support the spine, and the erector spinae muscles also help with support and balance.
**Appendicular** The appendicular skeleton (126 bones) comprises the pectoral girdles, upper limbs, pelvic girdle (pelvis), and lower limbs. Its functions are to enable locomotion and protect major organs involved in digestion, excretion, and reproduction.
**Functions**
**Support** The skeleton provides a framework that holds the body up and keeps its shape. The pelvis, along with its ligaments and muscles, forms a floor for pelvic structures. Without the rib cage, costal cartilages, and intercostal muscles, the lungs would collapse.
**Movement** Joints between bones allow movement—some, like ball-and-socket joints, permit a wide range, while others, like the pivot joint in the neck, are more limited. Skeletal muscles, attached to bones at various points, power movement. Muscles, bones, and joints work together as the main mechanical system for motion, coordinated by the nervous system. It is thought that a reduction in human bone density during prehistoric times—linked to the shift from hunting to agriculture—decreased agility and dexterity.
**Protection** The skeleton shields many vital internal organs. The skull protects the brain; the vertebrae protect the spinal cord; and the rib cage, spine, and sternum protect the lungs, heart, and major blood vessels.
**Blood cell production** The skeleton is where haematopoiesis occurs—blood cell development in the bone marrow. In children, this happens mainly in the marrow of long bones like the femur and tibia. In adults, it occurs mostly in the pelvis, cranium, vertebrae, and sternum.
**Storage** Bone matrix stores calcium and participates in calcium metabolism, while bone marrow stores iron in ferritin and is involved in iron metabolism. Bones are not pure calcium; they consist of a mix of chondroitin sulfate and hydroxyapatite, with hydroxyapatite making up about 70% of a bone. Hydroxyapatite itself is 39.8% calcium, 41.4% oxygen, 18.5% phosphorus, and 0.2% hydrogen by mass. Chondroitin sulfate is a sugar composed mainly of oxygen and carbon.
**Endocrine regulation** Bone cells release a hormone called osteocalcin, which helps regulate blood sugar (glucose) and fat storage. Osteocalcin boosts both insulin secretion and sensitivity, increases the number of insulin-producing cells, and reduces fat stores.
**Sex differences** Anatomical differences between human males and females are more pronounced in soft tissues than in the skeleton. The human skeleton is less sexually dimorphic than that of many other primates, but subtle differences in skull shape, teeth, long bones, and pelvis appear across populations. Female bones are generally smaller and less robust than male ones within a given population. It is unclear how much of these differences are genetic versus environmental.
**Skull** Several gross features of the human skull show sexual dimorphism, including the median nuchal line, mastoid processes, supraorbital margin, supraorbital ridge, and chin.
**Dentition** Sexual dimorphism in human teeth centers on the canine teeth, though it is not nearly as pronounced as in some other species.
- Bone mass percentage of body weight
- ~14%
- Average bone mass
- 10–11 kg
- Age of maximum bone mass
- 25–30 years
- Major divisions
- Axial and appendicular
- Major functions
- Support, movement, protection, blood cell production, mineral storage, endocrine
Lore & Background
The human skeleton performs six major functions: support, movement, protection, production of blood cells, storage of minerals, and endocrine regulation. The axial skeleton (80 bones) is formed by the vertebral column (32–34 bones), the rib cage (12 pairs of ribs and the sternum), and the skull (22 bones; some sources add 7 associated bones). The upright posture of humans is maintained by the axial skeleton, which transmits weight from the head, trunk, and upper extremities down to the lower extremities at the hip joints. The appendicular skeleton (126 bones) is formed by the pectoral girdles, the upper limbs, the pelvic girdle, and the lower limbs. Their functions are to make locomotion possible and to protect the major organs of digestion, excretion, and reproduction. The skeleton provides the framework which supports the body and maintains its shape. The pelvis, associated ligaments and muscles provide a floor for the pelvic structures. Without the rib cages, costal cartilages, and intercostal muscles, the lungs would collapse. Movement is powered by skeletal muscles, which are attached to the skeleton at various sites on bones. Muscles, bones, and joints provide the principal mechanics for movement, all coordinated by the nervous system. The skeleton helps to protect many vital internal organs: the skull protects the brain, the vertebrae protect the spinal cord, and the rib cage, spine, and sternum protect the lungs, heart, and major blood vessels. The skeleton is the site of haematopoiesis, the development of blood cells that takes place in the bone marrow. In children, haematopoiesis occurs primarily in the marrow of the long bones such as the femur and tibia. In adults, it occurs mainly in the pelvis, cranium, vertebrae, and sternum. The bone matrix can store calcium and is involved in calcium metabolism, and bone marrow can store iron in ferritin and is involved in iron metabolism. Bone cells release a hormone called osteocalcin, which contributes to the regulation of blood sugar (glucose) and fat deposition. Osteocalcin increases both insulin secretion and sensitivity, in addition to boosting the number of insulin-producing cells and reducing stores of fat.
Reader's Guide
The human skeleton is not as sexually dimorphic as that of many other primate species, but subtle differences between sexes in the morphology of the skull, dentition, long bones, and pelvis exist. In general, female skeletal elements tend to be smaller and less robust than corresponding male elements within a given population. The human female pelvis is also different from that of males in order to facilitate childbirth. Unlike most primates, human males do not have penile bones. A variety of gross morphological traits of the human skull demonstrate sexual dimorphism, such as the median nuchal line, mastoid processes, supraorbital margin, supraorbital ridge, and the chin. Human inter-sex dental dimorphism centers on the canine teeth, but it is not nearly as pronounced as in the other great apes. Long bones are generally larger in males than in females within a given population. Muscle attachment sites on long bones are often more robust in males than in females. The human pelvis exhibits greater sexual dimorphism than other bones, specifically in the size and shape of the pelvic cavity, ilia, greater sciatic notches, and the sub-pubic angle. The Phenice method is commonly used to determine the sex of an unidentified human skeleton by anthropologists with 96% to 100% accuracy in some populations. Women's pelvises are wider in the pelvic inlet and are wider throughout the pelvis to allow for child birth. The sacrum in the women's pelvis is curved inwards to allow the child to have a 'funnel' to assist in the child's pathway from the uterus to the birth canal. One of the most common skeletal disorders is osteoporosis. Also common is scoliosis, a side-to-side curve in the back or spine, often creating a pronounced 'C' or 'S' shape when viewed on an x-ray of the spine. This condition is most apparent during adolescence, and is most common with females. Arthritis is a disorder of the joints. It involves inflammation of one or more joints. The most common form of arthritis, osteoarthritis, can affect both the larger and smaller joints of the human skeleton. The cartilage in the affected joints will degrade, soften and wear away. Osteoporosis is a disease of bone where there is reduced bone mineral density, increasing the likelihood of fractures. Osteoporosis is defined by the World Health Organization in women as a bone mineral density 2.5 standard deviations below peak bone mass, relative to the age and sex-matched average, as measured by dual energy X-ray absorptiometry, with the term 'established osteoporosis' including the presence of a fragility fracture. Osteoporosis is most common in women after menopause, when it is called 'postmenopausal osteoporosis', but may develop in men and premenopausal women in the presence of particular hormonal disorders and other chronic diseases or as a result of smoking and medications, specifically glucocorticoids. Osteoporosis usually has no symptoms until a fracture occurs. Osteoporosis treatment includes advice to stop smoking, decrease alcohol consumption, exercise regularly, and have a healthy diet. Calcium supplements may also be advised, as may vitamin D. When medication is used, it may include bisphosphonates, strontium ranelate, and osteoporosis may be one factor considered when commencing hormone replacement therapy. The Sushruta Samhita, composed between the 6th century BCE and 5th century CE, speaks of 360 bones. Books on Salya-Shastra (surgical science) know of only 300. The text then lists the total of 300 as follows: 120 in the extremities, 117 in the pelvic area, sides, back, abdomen and breast, and 63 in the neck and upwards.
Did You Know?
- The human skeleton is composed of around 270 bones at birth, which decreases to around 206 bones by adulthood after some bones fuse together.
- The bone mass in the skeleton makes up about 14% of total body weight, reaching maximum mass between ages 25 and 30.
- Unlike most primates, human males do not have penile bones.
- The Phenice method can determine the sex of an unidentified human skeleton with 96% to 100% accuracy in some populations.
- The Sushruta Samhita, composed between the 6th century BCE and 5th century CE, speaks of 360 bones, while books on Salya-Shastra know of only 300.
More in Human Anatomy 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record