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Friday, October 26, 2007

Physiology -Chap 6.3- Outline

Bones and Homeostasis

  • Bone Remodeling: Continuous replacement of old bone tissue
    • Involves bone resorption: Removal of minerals and collagen fibers from bone by osteoclasts
    • Involves bone
      deposition: Addition of minerals and collagen fibers to bone by osteoblasts
      • Resorption causes destruction of EC-M
      • Deposition causes formation of EC-M
    • Takes place at different rates in different regions
    • Benefits:
      • If new bone is placed under stress it thickness becoming stronger than old bone
      • Shape can be altered for proper support based on stress patterns
      • More resistant to fracture

    *Remodeling and Orthodontics*

  • Orthodontics: Dentistry concerned with prevention and correction of poorly aligned teeth
  • Braces
    • Places stress on bone that forms the sockets that anchor the teeth
    • In response the osteoclast/blasts remodel the sockets so the teeth align
      • Osteoclasts attach to bone surface at endosteum and forms a leak-proof seal
      • Releases lysosomal enzymes and acids
      • Osteoclasts carve a tunnel into bone
        • The minerals carved out are resorbed
      • Osteoclasts leave and osteoblasts come in and rebuild

    Factors Affecting Bone Growth and Bone Remodeling

  • Factors in growth
    • Minerals: Calcium and phosphorus needed when bones are growing
    • Vitamins: Vitamin C in needed for synthesis of collagen and for differentiation of osteoblasts into osteocytes
      • Also needed: K, B12, and A
    • Hormones:
      • Childhood:
        • Insulin-like growth factors (IGF)
          • Produced by liver and bone tissue
          • Stimulate *blasts, promote cell division, and enhance synthesis of proteins
        • Thyroid hormones also promote bone growth by stimulating *blasts
      • Puberty:
        • Sex
          • Include: Estrogen and testosterone
          • Responsible for increased *blast activity and synthesis of bone EC-M and the 'growth spurt'
      • Adulthood:
        • Sex hormones:
          • Slow resorption of old bone and promotes deposition of new bone

    Fracture and Repair of Bone

  • Fracture: Break in any bone
    • Named for sensitivity, shape, or position of line:
      • Open fracture: Broken ends protrude through skin
      • Comminuted fracture: Bone splinters at site of impact and smaller bone fragments lie between the two main fragment
      • Greenstick fracture: Partial fracture in which one side is broken and other bends
      • Impacted fracture: One end is forcefully driven into the interior of other
      • Pott's fracture: Fracture of the distal end of the lateral leg bone, with serious injury of the distal tibial articulation
      • Colles' Fracture: Fracture of the distal end of the lateral forearm bone in which the distal fragment is displaced posteriorly
    • Stress fracture: Series of microscopic fissures in bone that forms without any evidence of injury to other tissues
  • Repairing a bone:
    • Formation of Fracture Hematoma
      • Blood vessels crossing the fracture line (FL) are broken
      • Clots are formed as blood leaks (Fracture hematoma)
      • Nearby bone cells die
        • Causes swelling and inflammation
    • Fibrocartilaginous Callus Formation
      • Fibroblasts invade fracture site and produce collagen
      • Cells from periosteum develop into chondroblasts and produce fibrocartilage
        • Lead to development of fibrocartilaginous
          • Repair tissue consisting of collagen that bridges broken ends
    • Bony Callus Formation
      • Osteogenic cells develop into osteoblasts which produce spongy bone trabeculae
      • Fibrocartilage is converted to spongy bone and callus referred to bony callus
    • Bone Remodeling
      • Dead portion of original fragment is resorbed
      • Compact replaces spongy around periphery of the fracture

    *Treatments for Fracture

  • For bones to unite properly the ends must be brought to alignment
    • Called reduction
      • Closed reduction: Fractured ends of bone brought into alignment by manual manipulation
        • Skin remains intact
      • Open reduction: Brought together during a surgery

    Bone's Role in Calcium Homeostasis

  • Small changes in calcium concentration could be fatal
    • High=Heart may stop
    • Low=Breathing may stop
  • Bone buffers the calcium level
    • Releasing when too low
    • Absorbing when too high
  • Regulated by hormones:
    • Parathyroid
      hormone (PTH)
      • Secreted by parathyroid glands
      • Increases blood calcium level
      • Operates with a negative feedback system
      • Acts on kidneys to decrease loss calcium in urine so more stays in the blood
      • Stimulates formation of calcitriol
        • Inhibits activity of osteoclasts
        • Speeds blood calcium uptake by bone
        • Accelerates calcium deposits

    Exercise and Bone Tissue

  • When placed under stress bone tissue becomes stronger through increased deposition of mineral salts
  • Without mechanical stress bone does not remodel normally
    • Resorption occurs more quickly than formation
  • Walking and weight lifting (weight bearing activities) build and retain bone mass

    Aging and Bone Tissue

  • More bone tissue is produced than lost
  • As levels of sex hormones decrease there is a decrease in bone mass
    • Because bone resorption by osteoblasts outpaces deposition of osteoblasts
  • Two aging effects
    • Loss of bone mass:
      • Results from demineralization
    • Brittleness
      • Results from decreased rate of protein synthesis

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