The Skeletal System: Structural Support, Protection, Movement, and Metabolic Functions

The skeletal system (206 bones in adults) provides the body's architectural framework. Its functions extend far beyond structural support.

Five Core Functions

1. Support and Shape

• Axial skeleton (skull, vertebral column, thoracic cage) → central axis, organ enclosure
• Appendicular skeleton (limbs, girdles) → leverage for locomotion and manipulation
• Posture maintenance via ligamentous tension and muscular tone

2. Protection

• Cranium → brain
• Vertebral column → spinal cord
• Thoracic cage → heart, lungs, great vessels
• Pelvis → urogenital organs

3. Movement (Leverage System)

• Bones = rigid levers
• Joints = fulcrums
• Muscles = force (via tendons)
• Classes of levers: 1st (neck extension), 2nd (plantarflexion), 3rd (most limb movements — speed/range over force)

4. Hematopoiesis

• Red marrow (flat bones, epiphyses) → all blood lineages
• Daily output: ~2.5×10¹¹ RBCs, 1×10¹¹ platelets, 1×10¹⁰ granulocytes
• Yellow marrow (fat) converts to red under hypoxic stress

5. Mineral Homeostasis

• Calcium reservoir (99% of body calcium): bone ↔ blood via remodeling
• Phosphate, magnesium, carbonate also stored
• PTH, calcitonin, vitamin D regulate flux

Bone Types and Examples

Type	Shape	Examples	Function
Long	Length > width	Femur, humerus, phalanges	Leverage, growth plates
Short	Cube-shaped	Carpals, tarsals	Stability, limited motion
Flat	Curved plates	Skull, ribs, sternum, scapulae	Protection, broad muscle attachment
Irregular	Complex	Vertebrae	Protection, weight-bearing
Sesamoid	Embedded in tendon	Patella, fabella	Reduce friction, alter mechanical advantage

Dynamic Nature

Bone is continuously remodeled (~10%/year). Peak bone mass ~age 30. Lifelong weight-bearing exercise, nutrition, and hormonal health preserve skeletal function.