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
        hormones:
        
        • 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
        callus
        
        • 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