Bone anatomy, bone types, axial and appendicular divisions, joint classifications, bone physiology, and remodeling — the skeletal system is both the body's framework and a dynamic living tissue. These memory tricks connect the anatomy and physiology.
Proven Mnemonics & Acronyms — fast to learn, hard to forget.
0 correct0 missed
What does this stand for?
cards correct this round
0 correct0 wrong
questions correct
Skeletal System Functions
SPMSB — Support · Protection · Movement · Storage · Blood cell production
Five functions of the skeletal system — anatomy meets physiology
What bones actually do — five functions that go far beyond support
Support: framework for body — bones provide the rigid scaffold that holds you upright and gives muscles something to pull against. Protection: skull (brain), vertebrae (spinal cord), ribs (heart and lungs), pelvis (reproductive organs). Movement: bones act as levers — muscles attach via tendons and pull on bones to create movement at joints. Storage: calcium reservoir (99% of body's calcium in bones), phosphate storage. Mineral release regulated by PTH and calcitonin. Blood cell production: hematopoiesis in red bone marrow — RBCs, WBCs, platelets. Adults: red marrow in axial skeleton, pelvis, proximal long bone epiphyses.
Six structural components of a long bone — each with a specific role
Long bone anatomy — from shaft to ends, inside and out
Diaphysis: the shaft — compact bone surrounding medullary cavity. Epiphysis: the ends — spongy bone covered by articular cartilage. Epiphyseal plate (growth plate): hyaline cartilage in growing bones — site of lengthening. Replaced by epiphyseal line when growth stops. Periosteum: tough fibrous membrane covering outer bone surface (not articular cartilage) — anchors tendons/ligaments (Sharpey's fibers), contains osteoprogenitor cells for repair, blood vessels and nerves. Endosteum: thin membrane lining medullary cavity and spongy bone trabeculae — contains osteoclasts and osteoblasts. Medullary cavity: hollow center — yellow marrow (fat) in adults, red marrow in children.
Four bone cell types — build, maintain, and resorb bone
The four bone cells — what each does and how they work together
Osteoprogenitor cells: stem cells in periosteum and endosteum — divide to produce osteoblasts. Activated during growth and fracture repair. Osteoblasts: bone-BUILDING cells — secrete osteoid (unmineralized matrix, mainly collagen) → calcium phosphate deposits → mineralized bone. Become trapped → become osteocytes. Osteocytes: mature bone cells in lacunae (small cavities) — connected by canaliculi (tiny channels). Maintain bone matrix, sense mechanical stress, communicate with osteoblasts and osteoclasts. Most numerous bone cell. Osteoclasts: large multinucleated cells — bone DESTRUCTION. Secrete acid and enzymes → dissolve bone matrix → releases Ca2+ into blood. Stimulated by PTH.
Osteoprogenitor
Stem cells → osteoblasts. In periosteum and endosteum.
Osteoblasts
Build bone. Secrete osteoid → mineralized. Become trapped → osteocytes.
Osteocytes
Most numerous. In lacunae. Connected by canaliculi. Sense mechanical stress.
SLIFS — Short · Long · Irregular · Flat · Sesamoid
Five bone shapes — each suited to its mechanical role
Five bone shapes — shape reflects function in every case
Long bones: longer than wide — humerus, femur, radius. Lever for movement. Contain medullary cavity. Short bones: roughly cube-shaped — carpals (wrist), tarsals (ankle). Provide stability with limited motion. Mostly spongy bone. Flat bones: thin, flat, curved — skull, sternum, ribs, scapula. Protection and large surface for muscle attachment. Sandwich of compact bone around spongy bone (diploë in skull). Irregular bones: complex shapes — vertebrae, hip bones, facial bones. Sesamoid bones: embedded in tendons — patella (kneecap) is largest. Protect tendons from friction and alter mechanical advantage.
Long
Humerus, femur, tibia, fibula, radius, ulna, phalanges. Levers for movement.
Skull, ribs, sternum, scapula. Protection + large muscle attachment surface.
Sesamoid
In tendons. Patella largest. Protect tendons and improve mechanical advantage.
Axial vs Appendicular
Axial = Axis of body (80 bones) · Appendicular = Appendages (126 bones)
206 bones total — 80 axial + 126 appendicular
Two divisions of the skeleton — which bones belong to each
Axial skeleton (80 bones): forms the central axis — skull (22), vertebral column (26), thoracic cage (25 ribs + sternum = 37). Protects central nervous system and thoracic organs. Appendicular skeleton (126 bones): bones of the limbs and the girdles that attach them. Pectoral girdle (4 bones: 2 clavicles + 2 scapulae), upper limbs (60), pelvic girdle (2 hip bones = os coxae), lower limbs (60). The pelvic girdle includes two hip bones (each formed by fusion of ilium, ischium, pubis) — forms with sacrum and coccyx to make bony pelvis. Together 206 bones in the adult (300 in newborn — many fuse).
Skull (22)
Cranial bones (8) protect brain. Facial bones (14) form face structure.
Most mobile — shoulder and hip. All movements including rotation.
Bone Remodeling
Osteoclasts eat · Osteoblasts build · Wolff's Law — bone adapts to stress
Continuous remodeling — old bone removed, new bone deposited based on mechanical demand
Bone remodeling — why bones change shape and strength throughout life
Bone is continuously remodeled — old bone is constantly removed and new bone deposited. ~10% of skeleton replaced each year in adults. Osteoclasts resorb bone → osteoblasts fill cavity with new bone (takes ~3-4 months). Wolff's Law: bone responds to mechanical stress by increasing density along lines of stress — explains why exercise strengthens bones and immobilization weakens them. Hormonal control: PTH stimulates osteoclasts (raises blood Ca2+), calcitonin stimulates osteoblasts (lowers blood Ca2+), estrogen promotes osteoblast activity (menopause → estrogen drops → osteoporosis). Growth hormone and thyroid hormone also influence remodeling. Fracture repair: hematoma → soft callus → hard callus → bone remodeling.
Wolff's Law
Bone deposits where stress applied, resorbs where not. Weight-bearing exercise builds bone density.
PTH
Low blood Ca2+ → PTH → osteoclasts active → bone resorption → Ca2+ released.
Osteoporosis
Resorption > deposition. Post-menopausal (low estrogen). Trabecular bone most affected.
Anatomical movements at synovial joints — always described in anatomical position
Joint movement terminology — the vocabulary of musculoskeletal motion
Flexion: decreases joint angle (bending elbow). Extension: increases joint angle (straightening elbow). Hyperextension: beyond anatomical position. Abduction: moves limb AWAY from midline. Adduction: moves limb TOWARD midline. Rotation: bone rotates around its long axis. Medial/lateral (internal/external). Circumduction: combination movement — limb traces a cone. Dorsiflexion: foot pulled toward shin. Plantar flexion: foot pointed down (standing on tiptoe). Inversion: sole faces medially. Eversion: sole faces laterally. Supination: forearm rotates — palm faces up. Pronation: palm faces down. Opposition: thumb to fingers.
Abduction/Adduction
Away/toward midline. ABduction = Away. ADDuction = ADD to midline.
Supination/Pronation
Supination = hold a bowl of soup (palm up). Pronation = pour it out (palm down).
Dorsiflexion
Foot toward shin. Walking on heels. Tested for nerve function (L4-L5).
Inversion/Eversion
Sole medial/lateral. Ankle sprains usually involve inversion (sole turns in).
🎓 Common Exam Questions
Q: What are the functions of the skeletal system?
A: Six functions: (1) Support — framework that supports body weight and holds organs in place. (2) Protection — skull protects brain, vertebrae protect spinal cord, ribcage protects heart and lungs. (3) Movement — bones act as levers; muscles attached via tendons generate movement at joints. (4) Mineral storage — bones store calcium (99% of body calcium), phosphorus, and other minerals; released into blood when needed. (5) Blood cell production (hematopoiesis) — red bone marrow in spongy bone of flat bones and proximal epiphyses of long bones produces RBCs, WBCs, and platelets. (6) Energy storage — yellow marrow in medullary cavity of long bones stores fat (adipose tissue) as energy reserve. Clinical: red marrow replaced by yellow marrow with age — in severe anemia, yellow marrow can revert to red to increase production.
Q: Describe the gross anatomy of a long bone.
A: Diaphysis: shaft, compact bone surrounding medullary cavity (yellow marrow in adults). Epiphyses: ends of bone, covered by articular cartilage (hyaline), contain spongy (cancellous) bone with red marrow. Epiphyseal plate/line: growth zone in children (hyaline cartilage); fuses in adulthood leaving epiphyseal line (remnant). Periosteum: fibrous outer covering of bone (except at articular surfaces). Contains osteogenic cells (for growth and repair), attaches tendons and ligaments (Sharpey fibers). Has blood vessels and nerves. Endosteum: thin membrane lining medullary cavity and trabeculae of spongy bone. Contains osteoblasts and osteoclasts for remodeling. Nutrient foramen: opening for nutrient artery supplying bone interior. Clinical: periosteal pain explains why fractures hurt even before displacement — periosteum is highly innervated.
Q: What are the four bone cells and what do each do?
A: Osteogenic (osteoprogenitor) cells: mitotically active stem cells in periosteum and endosteum → differentiate into osteoblasts when bone formation needed. Osteoblasts: bone-building cells. Secrete osteoid (unmineralized bone matrix — collagen type I + proteoglycans). Calcium phosphate crystallizes → hydroxyapatite → bone. As matrix surrounds them, they become osteocytes. Osteocytes: mature bone cells trapped in lacunae. Maintain bone matrix, sense mechanical stress (mechanotransduction), communicate via canaliculi. Longest-lived bone cells. Signal osteoblasts and osteoclasts to coordinate remodeling. Osteoclasts: bone-resorbing cells. Large, multinucleated (from macrophage lineage). Secrete HCl (dissolves mineral) and lysosomal enzymes (digest matrix) via ruffled border. Regulated by PTH (activates), calcitonin (inhibits), estrogen (inhibits — explains post-menopausal bone loss).
Q: What are the types of joints and their range of motion?
A: By structure: Fibrous joints: bones connected by fibrous connective tissue — no joint cavity. Sutures (skull — immovable), syndesmoses (fibula-tibia — slight movement), gomphoses (teeth in sockets — immovable). Cartilaginous joints: bones connected by cartilage. Synchondroses (hyaline cartilage — epiphyseal plates, first rib-sternum — immovable), symphyses (fibrocartilage — intervertebral discs, pubic symphysis — slight movement). Synovial joints: most joints — freely movable. Characteristics: articular cartilage, joint cavity, synovial membrane (secretes synovial fluid), joint capsule. Types by movement: Ball-and-socket (hip, shoulder — most movement), Hinge (elbow, knee, ankle — flexion/extension), Pivot (radioulnar, atlantoaxial — rotation), Condyloid (wrist — flexion/extension + abduction/adduction), Saddle (carpometacarpal of thumb — same as condyloid + circumduction), Gliding/Plane (between carpal bones — sliding).
Q: What is bone remodeling and how is it regulated?
A: Bone remodeling: continuous process of resorption (osteoclasts) and deposition (osteoblasts) — balances mineral homeostasis and structural adaptation. Coupling: osteoclasts resorb → release growth factors from matrix → stimulate osteoblasts to refill. Regulation: PTH (parathyroid hormone): low Ca2+ → PTH → stimulates osteoclasts → releases Ca2+ from bone → raises blood Ca2+. Also activates vitamin D → increases intestinal absorption. Calcitonin (thyroid C cells): high Ca2+ → calcitonin → inhibits osteoclasts → lowers blood Ca2+. Estrogen/testosterone: inhibit osteoclasts — protect bone density. Loss → osteoporosis. Mechanical stress (Wolff Law): bone remodels according to mechanical forces applied — compression increases bone density; disuse causes atrophy. Vitamin D: essential for calcium absorption in intestine. Deficiency → rickets (children), osteomalacia (adults). Bisphosphonates (alendronate): bind hydroxyapatite → inhibit osteoclasts → used for osteoporosis.