The human skeletal system provides protection and support for the body's organs, enables controlled movement, produces red blood cells, and regulates the amount of calcium found within the blood. The bones of the skeletal system are classified by their shapes and functions and are constantly being reshaped. There are specific cells that break down existing bone and others that build new tissue. As humans develop, cartilage skeletons are replaced with bone. Various hormones regulate the creation and destruction of bone tissue and also how much bone is allowed to grow at any given time. Bones have distinct surface features where muscles attach, bones join, and blood vessels and other materials pass through them.
At A Glance
- The main functions of the skeletal system include support, movement, protection, hematopoiesis, and storage of nutrients and minerals.
- Human bones are classified by their shapes—flat bones, short bones, long bones, and irregular bones—which are important in determining their functions.
- Bones have many different types of markings that are associated with muscle attachment points, joints, and passageways for vessels and nerves.
- Bones contain an outer, or compact, region and an inner, or spongy, region. Marrow is found within the cavities of long bones; red marrow produces blood cells, and yellow marrow primarily functions as energy storage.
Bone tissue is constantly being remodeled to better suit the body’s needs due to the actions of osteoblasts, which are bone-building cells, and osteoclasts, which break down existing bone.
- There are three primary types of bone cells—osteoblasts, osteoclasts, and osteocytes—all of which arise from generic stem cells.
Compact bone provides rigid strength but also serves as a pathway for blood and nervous signal transmission.
- Endochondral ossification replaces cartilage with bone, while intramembranous ossification does not require a cartilaginous template.
Interstitial growth produces longer bones as the cartilage lengthens and is replaced by bone tissue, while appositional growth occurs when new bone tissue is deposited on the surface of the bone, resulting in bone thickening.
- Hormones, physical activity, and nutrition influence how bones grow and are formed.
- During puberty, growth hormone levels increase, thereby increasing the rate of growth.
- Bones respond to mechanical stress by increasing in size and thickness.
- Deficiencies in certain vitamins and minerals can affect bone growth and health.
Fractures are classified by how the bone breaks, where the bone breaks, the completeness of the break, and whether or not the bone penetrates through the skin, resulting in bones healing themselves in four stages: hematoma formation, soft callus formation, hard callus formation, and bone remodeling.