The skeletal system includes the bones, cartilage, ligaments, tendons, and other connective tissues that stabilize and interconnect the bones. There are five primary functions of this body system:
- Support. The skeletal system acts as an anchor for the rest of the body, providing a structural framework for the attachment of soft tissues and organs
- Storage of Minerals and Lipids. Minerals and lipids play a vital role in physiological function, and the homeostatic regulation of these is the key to their function. The calcium bone salts are an invaluable tool to maintain normal calcium and phosphate levels in the blood. The bones of the skeleton store energy in the form of lipids in areas of yellow bone marrow.
- Blood Cell Production. Red blood cells (erythrocytes), white blood cells (leukocytes) and other blood elements are produced from hematopoietic stem cells in red marrow which fills the internal cavity of many bones.
- Protection. Many soft tissues and organ systems are surrounded by skeletal elements. The ribs protect the heart and lungs, the cranium protects the brain, the vertebrae encase the spinal cord and the pelvis cradles the digestive and reproductive organs.
- Leverage. Many bones have a role in translating the force generated by skeletal muscle into mechanical leverage against other bones.
Classification of Bones
The bones of the human skeleton are comprised of two basic kinds of osseous tissue which differ in texture.
- Compact Bone appears very smooth and homogeneous.
- Spongy Bone Also known as cancellous bone is composed of small bars of bone and has much open space.
The bones in the human body can be further separated into six broad categories according to their relative gross anatomy.
- Long Bones are categorised by their tubular shaft (diaphysis) with a rounded end (epiphysis) on each end. They are always longer than they are wide, and grow from each end, elongating the shaft. The femur, the long bone to the thigh, is the largest and heaviest bone in the human body.
- Short Bones are short and boxy being approximately as wide as they are tall. The carpal (wrist) and tarsal (ankle) bones are the only short bones in the human body.
- Flat Bones have thin wide surfaces and serve mostly as protective structures. The cranial (scalp) bones are an example of flat bones and they consist of two layers of compact bone between which there may be found spongy material, for example, the scapula and the parietal bone (in the skull).
- Irregular Bones have complex shapes other than that of the other bones. The vertebrae are irregular bones.
- Sesamoid Bones are generally small, flat and have an apex at one end. These bones form by the thickening of a muscle tendon until it finally ossifies. The patella is the biggest sesamoid bone in the body, however sesamoid bones can arise spontaneously in the hand and foot.
Bone Markings and Features
Elevations or projections appear on bones where ligaments and tendons attach and where other bones articulate at joints. Depressions, grooves and tunnels indicate the path of a nerve or vessel alongside or penetrating the bone. Some of the various features of bones are described in the following table.
|General Description||Anatomical Term||Definition|
|Projections formed where tendons and ligaments attach||Crest||A narrow, prominent ridge of bone|
|Epicondyle||An eminence on or superior to a condyle|
|Line||A linear elevation, less prominent than a crest|
|Process||Any bony prominence|
|Protuberance||A projection of bone|
|Spine||A projecting "spine like" part; sharp, slender and usually pointed|
|Trochanter||A large, rough, irregularly shaped, blunt projection|
|Tuberocity||A small, sometimes rough, rounded, blunt projection|
|Tubercle||A small, smooth eminence|
|Projections formed where bones articulate with each other||Capitulum||A small round articular head|
|Condyle||A smooth, rounded articular process|
|Facet||A small, smooth, flat articular surface|
|Head||The expanded articular surface of an epiphysis.|
|Malleolus||A rounded process|
|Ramus||An armlike bar of bone|
|Trochlea||A spool like articular process, acts like a pulley|
|Depressions and grooves formed for the passage of vessels and nerves||Fissure||A narrow, slit-like opening|
|Foramen||A round, sometimes oval opening through bone|
|Notch||An indentation at the edge of a bone structure|
|Other bone markings||Fossa||A shallow depression in bone, sometimes serving as an articular surface|
|Meatus||A canal-like passageway in bone|
|Sinus||A mucous lined cavity in bone, filled with air|
The process of bone formation is ossification; this is the formation on bone from other tissues. The major forms of ossification exist, endochondrial and intramembranous.
Endochondrial bone formation is the ossification of existing cartilaginous structures. During development most bones begin as cartilage frames of the final bone. This cartilage is hardened during ossification and results in the boney structures observed in an adult. This process can be described through five generalised steps.
- Step 1. As the cartilage enlarges the chondrocytes near the center of the bone increase greatly in size. The internal matrix is reduced to a series of struts that take up calcium salts and ossify. The chondrocytes are then surrounded by cartilage and, deprived of nutrients, die, leaving large cavities in the calcified structure.
- Step 2. Blood vessels are now able to penetrate into these spaces, and cells on the outside of the bone begin to differentiate into osteoblasts (cells of the bone), and bone begins to form on the outside of the bone.
- Step 3. The Blood vessels are then able to fully penetrate the bone forming the Primary ossification center.
- Step 4. As the bone enlarges, osteoclasts (destructive bone cells) begin to erode the center of the bone, forming the medullary cavity.
- Step 5. The final step in this process is the formation of articular cartilage to protect the newly formed bone during articulation.
Intramembranous bone formation is the formation of bone without a cartilage scaffold. This is the main process by which bone repair occurs, and is also used to form the bones of the face. This process can be summarised into the following.
- Mesenchyme cell in the membrane become osteochondral progenitor cell
- Osteochondral progenitor cell specialize to become osteoblast
- Osteoblast produce bone matrix and surrounded collagen fiber and become osteocyte
- As the result of the process trabeculae will develop
- Osteoblast will trap trabeculae to produce bone
- Trabeculae will join together to produce spongy cell
- Cells in the spongy cell will specialize to produce red bone marrow
- Cells surrounding the developing bone will produce periosteum
- Osteoblasts from the Periosteum on the bone matrix will produce compact bone
Bones have a variety of functions:
- Protection — bones can serve to protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs.
- Structure — bones provide a frame to keep the body supported.
- Movement — bones provide leverage system for, 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.
- Sound transduction — bones are important in the mechanical aspect of overshadowed Hearing (sense)|hearing.
- Blood production — the Bone marrow|marrow, located within the medullary cavity of long bones and interstices of cancellous bone, produces blood cells in a process called hematopoiesis.
- Mineral storage — bones act as reserves of minerals important for the body, most notably calcium and phosphorus.
- Growth factor storage — mineralized bone matrix stores important growth factors such as insulin-like growth factors, transforming growth factor, bone morphogenetic proteins and others.
- Fat storage — the yellow bone marrow acts as a storage reserve of fatty acids.
- Acid-Base (chemistry)|base balance — bone buffers the blood against excessive pH changes by absorbing or releasing Alkali salts|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.
- Endocrine system|Endocrine organ — bone controls phosphate metabolism by releasing Fibroblast growth factor 23|fibroblast growth factor – 23 (FGF-23), which acts on kidneys to reduce phosphate reabsorption. Bone cells also release a hormone called osteocalcin, which contributes to the regulation of blood sugar (glucose) and body fat|fat deposition. Osteocalcin increases both the insulin secretion and sensitivity, in addition to boosting the number of beta cell|insulin-producing cells and reducing stores of fat.
- Calcium balance—The process of bone resorption by the osteoclasts releases stored calcium into the systemic circulation and is an important process in regulating calcium balance. As bone formation actively fixes circulating calcium in its mineral form, removing it from the bloodstream, resorption actively unfixes it thereby increasing circulating calcium levels. These processes occur in tandem at site-specific locations.
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