Skeletal System: Bone Structure, Function, and Calcium Homeostasis

Study Guide - Smart Notes

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Support: Bones provide a rigid framework that supports the body and cradles soft organs.
Protection: Bones protect vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).
Movement: Bones act as levers for muscles to pull on, enabling movement.
Mineral Storage: Bones store minerals, primarily calcium and phosphorus, which can be released into the bloodstream as needed.
Blood Cell Formation: Hematopoiesis occurs in the red marrow of certain bones, producing red and white blood cells and platelets.
Triglyceride Storage: Yellow bone marrow stores fat as an energy reserve.

Osteoblasts: Bone-forming cells responsible for synthesizing and secreting the bone matrix.
Osteocytes: Mature bone cells derived from osteoblasts; maintain bone tissue.
Osteoclasts: Large, multinucleated cells that resorb (break down) bone matrix, releasing minerals into the blood.
Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

Long Bones: Longer than they are wide (e.g., femur, humerus).
Short Bones: Approximately equal in length and width (e.g., carpals, tarsals).
Flat Bones: Thin, flattened, and usually curved (e.g., sternum, skull bones).
Irregular Bones: Complex shapes (e.g., vertebrae, hip bones).
Sesamoid Bones: Small, round bones embedded in tendons (e.g., patella).

Diaphysis: The shaft of a long bone; composed mainly of compact bone.
Epiphysis: The ends of a long bone; contains spongy bone and red marrow.
Metaphysis: Region between diaphysis and epiphysis; includes the epiphyseal plate (growth plate) in children.
Medullary Cavity: Central cavity within the diaphysis; contains yellow marrow in adults.
Periosteum: Dense connective tissue covering the outer surface of bone.
Endosteum: Thin membrane lining the medullary cavity.

Organic Component: Mainly collagen fibers, providing flexibility and tensile strength.
Inorganic Component: Hydroxyapatite crystals (calcium phosphate), providing hardness and resistance to compression.

Intramembranous Ossification: Bone develops directly from mesenchymal tissue; forms flat bones of the skull, mandible, and clavicle.
Endochondral Ossification: Bone forms by replacing hyaline cartilage; forms most bones of the skeleton, especially long bones.

Primary Ossification Center: The first area of a bone to start ossifying, usually in the diaphysis.

Secondary Ossification Center: Develops in the epiphyses after birth.

Epiphyseal Plate (Growth Plate): Hyaline cartilage plate where bone growth in length occurs in children and adolescents.

Length: Occurs at the epiphyseal plate via interstitial growth of cartilage, followed by ossification.
Width (Appositional Growth): Osteoblasts in the periosteum add new bone tissue to the outer surface, increasing diameter.

Continuous process of bone resorption (by osteoclasts) and deposition (by osteoblasts).
Essential for bone health, repair, and calcium homeostasis.

Bone Resorption: Breakdown of bone matrix by osteoclasts, releasing minerals into the blood.
Bone Deposition: Formation of new bone matrix by osteoblasts.

Cells Responsible: Osteoclasts (resorption), Osteoblasts (deposition).

Hormone	What Stimulates Its Release?	Effects on Targets	Overall Effect on Blood Calcium
PTH (Parathyroid Hormone)	Low blood calcium levels	Bone: Stimulates osteoclasts to resorb bone (increases resorption) Kidney: Increases reabsorption of calcium, decreases loss in urine; stimulates formation of calcitriol Intestine (via calcitriol): Increases calcium absorption	Increases blood calcium
Calcitonin	High blood calcium levels	Bone: Inhibits osteoclast activity (decreases resorption), stimulates osteoblasts (increases deposition) Kidney: Increases excretion of calcium in urine	Decreases blood calcium
Calcitriol (active Vitamin D)	PTH stimulation, low blood calcium	Intestine: Increases absorption of calcium and phosphate Bone: Promotes bone resorption (in synergy with PTH) Kidney: Decreases excretion of calcium	Increases blood calcium

Normal Range: Approximately 8.5–10.5 mg/dL.
Hypocalcemia: Low blood calcium; increases neuromuscular excitability, can cause tetany, muscle spasms, and cardiac arrhythmias.
Hypercalcemia: High blood calcium; decreases neuromuscular excitability, can cause muscle weakness, kidney stones, and cardiac arrhythmias.

Rickets: Defective mineralization of bone in children, usually due to vitamin D deficiency.
Osteoporosis: Decreased bone mass and density, increasing fracture risk.
Osteogenesis Imperfecta: Genetic disorder causing brittle bones due to defective collagen synthesis.
Achondroplastic Dwarfism: Short-limbed dwarfism due to abnormal cartilage formation at growth plates.
Gigantism/Acromegaly: Excess growth hormone before (gigantism) or after (acromegaly) epiphyseal plate closure, causing abnormal bone growth.

Hematoma Formation: Blood clot forms at the fracture site.
Fibrocartilaginous Callus Formation: Soft callus of collagen and cartilage forms to bridge the broken bone ends.
Bony Callus Formation: Osteoblasts replace the soft callus with spongy bone.
Bone Remodeling: Spongy bone is remodeled into compact bone, restoring the bone's original shape.

Wolff's Law: Bone grows and remodels in response to the mechanical stresses placed upon it.
Epiphyseal Plate vs. Line: The plate is present in growing children; the line is the remnant in adults after growth has ceased.

Additional info: Some explanations and definitions were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.