Ch 7 pt 3

Epiphyseal Plate and Growth

  • Maintains thickness during childhood.

  • At maturity:

    • Cartilage production slows.

    • Osteoblastic activity increases.

    • Plate narrows until disappearance, stopping interstitial growth.

    • Leaves an epiphyseal line of compact bone.

  • Female epiphyseal plates fuse 1-2 years earlier than males.

Appositional Bone Growth

  • Occurs within periosteum.

  • Bone matrix deposited parallel to surface.

  • Osteoclasts resorb bone along medullary cavity.

Bone Remodeling

  • Continuous in adulthood, relies on osteoblasts, osteocytes, and osteoclasts.

  • Influenced by:

    • Blood calcium levels.

    • Hormones (e.g., growth hormone, thyroid hormones, calcitonin, parathyroid hormone).

    • Mechanical stress (exercise and gravity).

Blood Calcium Regulation

  • Essential concentration: 8.9−10.1extmg/dL8.9 - 10.1 ext{ mg/dL}.

  • Required for:

    • Muscle contraction initiation.

    • Exocytosis in neurons.

    • Heart stimulation by pacemaker cells.

    • Blood clotting.

  • Primary hormones:

    • Calcitriol.

    • Parathyroid hormone (PTH).

Activation of Vitamin D to Calcitriol

  1. UV light converts 7-dehydrocholesterol to vitamin D3 (Cholecalciferol).

  2. Vitamin D3 converted to calcidiol in the liver.

  3. Calcidiol converted to calcitriol (active form) by kidney.

    • PTH increases calcitriol formation.

    • Calcitriol stimulates calcium absorption in the small intestine.

PTH and Calcitriol Effects

  • PTH secreted in response to low blood calcium, increases calcitriol.

  • Acts on:

    • Bones: Increases calcium release via osteoclast activity.

    • Kidneys: Reduces calcium excretion & increases reabsorption.

    • Small Intestine: Calcitriol enhances calcium absorption.

Calcitonin

  • Regulates blood calcium levels, less significant than PTH.

  • Released from thyroid in response to high calcium.

  • Inhibits osteoclast activity and increases calcium excretion via kidneys.

Aging Effects on Bone

  1. Decreased tensile strength due to reduced osteoblast activity.

    • Increased inorganic material, leading to brittleness.

  2. Bone loss of calcium leads to thinning and insufficiencies (osteopenia).

Osteoporosis

  • Significant bone mass reduction compromising function.

  • Related to reduced hormones (Vitamin D, growth hormone, estrogen, testosterone).

Fractures

  • Types:

    • Stress (from activity).

    • Pathologic (weakened bone).

    • Simple (non-skin penetrating).

    • Compound (pierces skin).

Fracture Healing Steps

  1. Hematoma forms from torn blood vessels.

  2. Fibrocartilaginous callus forms:

    • Collected tissue from fibroblasts and chondroblasts.

  3. Hard callus forms through osteoblast activity.

  4. Bone remodeling occurs:

    • Osteoclasts remove excess material, compact bone replaces primary bone.