Bone Physiology

1. See Also
   1. Bone Densitometry
   2. Bone Turnover Biochemical Marker
   3. Osteoporosis
   4. Paget's Disease of Bone
2. Bone Types
   1. Cortical Bone (80%)
      1. Outer supporting structure (Dense)
      2. Sites
         1. Radius
         2. Skull
         3. Long bones
      3. Peak density: age 30 to 40 years
   2. Trabecular Bone (20%)
      1. Inner supporting structure (Spongy)
      2. Sites
         1. Hip
         2. Spine
         3. Femur
3. Physiology
   1. Microfractures occur via everyday wear and tear
      1. Healed by ongoing bone remodeling
   2. Bone Remodeling
      1. Bone remodeling occurs in 120 day cycles
         1. Resorption by Osteoclasts over first 20 days
         2. Bone formation by Osteoblasts over last 100 days
      2. Osteoblasts
         1. Fill in bony cavity with bone matrix
         2. Triggers for activity
            1. Parathyroid Hormone
            2. Thyroxine
            3. Growth factors
               1. Interleukins (1, 6, 11)
               2. Insulin-Like Growth Factors
               3. Transforming Growth factor-B
         3. Releases cytokines to attract Osteoclasts
      3. Osteoclasts
         1. Release proteases
            1. Dissolves bone mineral matrix
            2. Dissolves collagen
         2. Clears away damaged bone
         3. Releases matrix-bound growth factors
            1. Chemoattractant for Osteoblasts
   3. Chemicals involved in bone formation
      1. Calcitonin
         1. Inhibits bone resorption
         2. Inhibits intestinal calcium, phosphorus absorption
         3. Inhibits renal calcium excretion
         4. Inhibits calcitriol production in Kidney
      2. Calcitriol
         1. Increases intestinal calcium absorption
         2. Increases intestinal phosphorus absorption
         3. Increases intestinal Magnesium absorption
         4. Decreases Parathyroid Hormone secretion
      3. Parathyroid Hormone
         1. Mobilizes calcium and phosphorus from bone
         2. Increases Calcitriol
4. References
   1. Rosen (1998) Postgrad Med 104(4):101-14