BackBone Remodeling and Homeostasis: Chapter 6 Study Notes
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Bone Remodeling
Overview of Bone Remodeling
Bone remodeling is a continuous process that maintains bone integrity and adapts bone structure to changing mechanical demands. It involves the coordinated actions of specialized bone cells and is essential for skeletal health throughout life.
Definition: Bone remodeling refers to the ongoing replacement of old bone tissue with new bone tissue.
Key Functions:
Maintains bone strength and mineral homeostasis
Repairs micro-damage from daily stress
Adapts bone structure to mechanical loads
Cell Types Involved:
Osteocytes: Mature bone cells that sense mechanical stress and signal for remodeling.
Osteoblasts: Cells responsible for bone formation and deposition of new bone matrix.
Osteoclasts: Cells that resorb (break down) bone tissue.
Balance of Activities:
Normally, bone resorption and formation are balanced.
If removal (resorption) exceeds replacement, bones weaken.
If deposition predominates, bones become stronger.
Exercise, Nutrition, and Hormones
Effects of Exercise on Bone
Physical activity, especially weight-bearing exercise, plays a crucial role in bone health by stimulating bone formation and increasing bone strength.
Mineral Recycling: Allows bones to adapt to mechanical stress.
Bone Strength: Heavily stressed bones become thicker and stronger.
Osteoblast Stimulation: Exercise stimulates osteoblasts, promoting bone deposition.
Bone Degeneration: Inactivity leads to rapid bone loss; up to one-third of bone mass can be lost in a few weeks of inactivity.
Nutritional and Hormonal Effects on Bone
Proper nutrition and hormonal regulation are essential for bone development, maintenance, and repair.
Minerals:
Calcium and phosphorus are required in the diet for bone mineralization.
Small amounts of magnesium, fluoride, iron, and manganese are also necessary.
Calcitriol and Vitamin D3:
Calcitriol is synthesized in the kidneys from vitamin D3 (cholecalciferol).
Essential for normal calcium and phosphate absorption in the digestive tract.
Vitamin C: Required for collagen synthesis and stimulates osteoblast differentiation.
Vitamin A: Stimulates osteoblast activity.
Vitamins K and B12: Required for synthesis of bone proteins.
Growth Hormone and Thyroxine: Stimulate bone growth.
Sex Hormones (Estrogen and Testosterone): Stimulate osteoblasts and help maintain bone mass.
Parathyroid Hormone (PTH) and Calcitonin: Regulate calcium ion homeostasis.
Calcium Homeostasis
Role of Calcium in the Skeletal System
Calcium is the most abundant mineral in the body and is vital for many physiological processes. The skeleton serves as the primary calcium reserve.
Bone Storage: Bones store 99% of the body's calcium.
Physiological Importance: Calcium ions are essential for muscle contraction, nerve transmission, blood clotting, and other functions.
Hormonal Regulation of Calcium
Calcium ion concentrations in body fluids must be tightly regulated. Two main hormones, parathyroid hormone (PTH) and calcitonin, control calcium storage, absorption, and excretion.
Parathyroid Hormone (PTH):
Produced by parathyroid glands in the neck.
Increases blood calcium levels by:
Stimulating osteoclast activity (indirectly)
Increasing intestinal absorption of calcium
Enhancing calcitriol secretion by kidneys
Reducing calcium excretion by kidneys
Calcitonin:
Secreted by C cells in the thyroid gland.
Decreases blood calcium levels by:
Inhibiting osteoclast activity
Increasing calcium excretion and reducing calcitriol secretion by kidneys
Decreasing intestinal absorption of calcium
Fractures and Bone Repair
Types of Fractures
Fractures are cracks or breaks in bones due to physical stress. They are classified based on their characteristics and location.
Open (Compound) Fracture: Bone breaks through the skin.
Closed (Simple) Fracture: Bone does not penetrate the skin.
Major Types: Transverse, displaced, compression, spiral, epiphyseal, comminuted, greenstick, Colles, and Pott's fractures.
Steps in Fracture Repair
Bone repair after a fracture occurs in four main steps:
Fracture Hematoma Formation:
A large blood clot forms at the fracture site.
Establishes a fibrous network and bone cells in the area die.
Callus Formation:
Cells of endosteum and periosteum divide and migrate into the fracture zone.
Internal callus develops in the medullary cavity; external callus of cartilage and bone surrounds the break.
Calluses stabilize the break.
Spongy Bone Formation:
Osteoblasts replace central cartilage of external callus with spongy bone.
Compact Bone Formation:
Remodeling produces compact bone; repaired bone may be slightly thicker and stronger than normal.
Effects of Aging on the Skeletal System
Bone Loss and Osteopenia
With age, bones become thinner and weaker due to reduced ossification, a condition known as osteopenia.
Onset: Begins between ages 30 and 40.
Rate of Loss: Women lose about 8% of bone mass per decade; men lose about 3%.
Most Affected Areas: Epiphyses, vertebrae, and jaws.
Consequences: Fragile limbs, reduced height, and tooth loss.
Osteoporosis
Osteoporosis is a severe loss of bone mass that compromises normal function and increases fracture risk.
Prevalence: Over age 45, occurs in 29% of women and 18% of men.
Role of Sex Hormones: Help maintain bone mass; osteoporosis accelerates in women after menopause.
Cancer and Bone Loss
Cancerous tissues can release osteoclast-activating factors, leading to increased bone resorption and severe osteoporosis.
Osteoclast-Activating Factor: Stimulates osteoclasts, causing rapid bone loss.
Table: Hormonal Regulation of Blood Calcium Levels
This table summarizes the effects of parathyroid hormone (PTH) and calcitonin on blood calcium levels and their mechanisms of action.
Hormone | Source | Effect on Blood Calcium | Mechanism |
|---|---|---|---|
Parathyroid Hormone (PTH) | Parathyroid glands | Increases |
|
Calcitonin | Thyroid gland (C cells) | Decreases |
|
