Skeletal System: Structure, Function, and Bone Physiology Study Guide

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The skeletal system performs several essential roles in the human body, contributing to structure, movement, and homeostasis.

Support: Provides a rigid framework that supports the body and cradles soft organs.
Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).
Movement: Serves as levers for muscles to act upon, enabling movement.
Mineral Storage: Stores minerals, especially calcium and phosphorus, which can be released into the bloodstream as needed.
Blood Cell Production: Houses bone marrow, which produces red blood cells, white blood cells, and platelets (hematopoiesis).

Bones are classified based on their shape and structure. Each type serves specific functions in the body.

Type	Description	Example
Long bones	Longer than they are wide; mostly compact bone	Femur, humerus
Short bones	Cube-shaped; mostly spongy bone	Carpals, tarsals
Flat bones	Thin, flattened, often curved	Sternum, skull bones
Irregular bones	Complex shapes	Vertebrae, hip bones
Sesamoid bones	Develop within tendons	Patella
Sutural (Wormian) bones	Small bones within cranial sutures	Skull sutures

Long bones have a characteristic structure that supports their function in movement and weight-bearing.

Diaphysis: The shaft or central part of the bone.
Epiphyses: The ends of the bone, usually wider than the diaphysis.
Metaphysis: The region between the diaphysis and epiphysis, including the growth plate.
Medullary cavity: The hollow space within the diaphysis containing bone marrow.
Articular cartilage: Covers the surfaces of the epiphyses where bones articulate.
Periosteum: The outer fibrous covering of the bone.
Endosteum: The inner lining of the medullary cavity.

Bones contain two main types of tissue: spongy (cancellous) and compact (cortical) bone.

Spongy bone: Found mainly in the epiphyses of long bones and inside flat bones. It consists of a network of trabeculae, providing structural support and housing bone marrow.
Compact bone: Forms the dense outer layer of bones, especially in the diaphysis of long bones. It provides strength for weight-bearing.

Distinguishing Feature: Spongy bone has a porous, lattice-like structure, while compact bone is dense and solid.

Osteocytes: Mature bone cells that maintain bone tissue. They reside in small spaces called lacunae.
Osteoblasts: Bone-forming cells that secrete bone matrix.
Osteoclasts: Large cells that break down bone matrix, aiding in bone resorption and remodeling.
Osteoprogenitor (osteogenic) cells: Stem cells that differentiate into osteoblasts.

Bone matrix: The intercellular substance of bone tissue, consisting of collagen fibers and inorganic mineral salts (mainly hydroxyapatite).
Periosteum: The tough, fibrous outer membrane covering the bone except at joint surfaces.
Endosteum: The thin membrane lining the medullary cavity.

Bones grow in thickness by the addition of new bone tissue by osteoblasts in the periosteum.

Long bones grow in length at the epiphyseal (growth) plates through endochondral ossification.

Intramembranous ossification: Bone develops directly from mesenchymal tissue. This process forms flat bones such as those of the skull.
Endochondral ossification: Bone forms by replacing a hyaline cartilage model. This process forms most bones, including long bones.

Parathyroid hormone (PTH): Increases blood calcium levels by stimulating osteoclast activity and calcium reabsorption in the kidneys.
Calcitonin: Lowers blood calcium levels by inhibiting osteoclasts and promoting calcium deposition in bones.
Calcitriol (active vitamin D): Increases calcium absorption from the gut.

These hormones act primarily on bones, kidneys, and the intestines.

Hypersecretion of GH in childhood: Leads to gigantism (abnormally increased growth).
Hyposecretion of GH in childhood: Causes dwarfism (reduced growth).
Hypersecretion of GH in adulthood: Results in acromegaly (enlargement of extremities and facial bones).

Type	Description
Open (compound)	Bone breaks through the skin
Closed (simple)	Bone does not penetrate the skin
Displaced	Bone ends are out of alignment
Non-displaced	Bone ends remain aligned
Complete	Bone is broken all the way through
Incomplete	Bone is not broken all the way through
Comminuted	Bone is broken into three or more pieces
Greenstick	Bone breaks incompletely, common in children
Compression	Bone is crushed, common in vertebrae
Spiral	Ragged break caused by twisting forces
Impacted	Broken bone ends are forced into each other

Osteomalacia: Softening of bones due to vitamin D deficiency in adults, leading to bone pain and weakness.
Osteoporosis: Decreased bone mass and density, increasing fracture risk, especially in postmenopausal women and the elderly.
Paget's disease: Chronic disorder causing abnormal bone remodeling, resulting in enlarged and misshapen bones.

Disorder	Main Features
Osteomalacia	Soft bones, pain, due to vitamin D deficiency
Osteoporosis	Low bone mass, fragile bones, increased fracture risk
Paget's disease	Disorganized bone remodeling, enlarged bones

A patient with osteoporosis may suffer a hip fracture after a minor fall due to decreased bone density.

Additional info: Some details, such as the full list of fracture types and the summary of bone disorders, were expanded for academic completeness.