Skeletal System Functions

Primary Functions

• Provides structural support for the body
• Framework for attachment of soft organs and tissues.
• Stores calcium and phosphate ions, which serves as a source for calcium
• It also serves as a storage site for energy reserves (fats) in yellow marrow cavities of long bones.
• Red marrow within the internal cavities of many bones produces red blood cells (RBCs), white blood cells (WBCs), and other blood elements.
• Encases delicate tissues and organs
• Ribs protect the heart and lungs
• The skull encloses the brain
• Vertebrae shield the spinal cord
• The pelvis cradles digestive and reproductive organs.
• Bones function as levers to change the magnitude and direction of forces generated by skeletal muscles
• A lever is a rigid structure that moves on a fixed point called a fulcrum
• Each bone is a lever, and each joint is a fulcrum
• There are three classes of levers.
  • 1st class: Resistance-Fulcrum-Applied force with the example of a seesaw and muscle arrangement in the neck.
  • 2nd class: Fulcrum-Resistance-Applied force with the example of a wheelbarrow and calf muscles undergoing plantar flexion.
  • 3rd class: Fulcrum-Applied force-Resistance with the example of forceps and biceps brachii flexing the elbow.

Classification of Bones

• The body has 206 bones divided into 6 broad categories based on shape
• Long bones: Relatively long and slender, and are found in the arm, forearm, thigh, leg, palms, soles, fingers, and toes.
  • The femur (thigh bone) is the largest and heaviest bone
• Short bones: Boxlike in appearance, as wide as they are long, and are found in the wrists (carpal bones) and ankles (tarsal bones).
• Flat bones: Thin, roughly parallel surfaces of compact bone sandwiching a thin layer of spongy bone, and are found the roof of the skull, sternum, ribs, and scapula.
• Irregular bones: Have complex shapes with short, flat, notched, or ridged surfaces, and are found in the vertebra and the ethmoid and sphenoid bones of the skull.
• Sesamoid bones: Small and nodular, develop within tendons, and are located near joints at the knees, the hands, and the feet.
• Sutural (or Wormian) bones: Are small, flat, irregular shaped bones found only between flat bones in the skull, filling gaps in sutures
• Each bone contains two types of osseous tissue:
  • Compact Bone: Relatively solid and found on the surface of bones with the osteon which is the functioning unit.
    • Provides a sturdy, protective layer on the bone and withstands extreme forces applied to either end
  • Spongy Bone: Brittle, with an open network of struts and plates (trabeculae) found in the interior of bone without the presence of osteons.
    • Nutrients diffuse into osteocytes within each trabeculae via canaliculi as it reduces weight and houses red marrow for blood cell production.

Parts of a Long Bone

• Diaphysis: Long tubular shaft that houses the medullary cavity
• Marrow Cavity: Hollow chamber in the innermost part of the long bone that holds yellow marrow.
• Epiphysis: Expanded area at each end of the bone, usually covered with hyaline cartilage.
• Articular cartilage: Hyaline cartilage that covers the articulate ends of the epiphysis.
• Periosteum: Fibrous, vascular covering of the entire bone except joint areas.
  • Isolates bone from surrounding tissues.
  • Provides a route for blood and nervous supply.
  • Participates in bone repair (remodeling) and growth.
• Endosteum: Thin layer of squamous epithelial cells that lines the inner portion of the bone.
  • Functions in bone remodeling.

Bone Development and Growth

• Skeletal growth determines body size and proportions
• The bony skeleton begins to develop 6 weeks after fertilization, with the embryo only ½ inch long, and stops growing around 25 years old.
• Ossification replaces other tissues with bone
• Intramembranous ossification: Bone develops within fibrous connective tissue.
• Sheets of primitive connective tissue appear at future bone sites (flat bones-skull) - Primitive cells arrange around blood vessels in connective tissue layers
• These cells differentiate into osteoblasts when bony matrix surrounds them
• Periosteum develops from connective tissue on the surface
• Osteoblasts deposit compact bone over spongy bone and become osteocytes when completely surrounded by bony matrix
• Types: flat bones of the skull.
• Endochondral ossification: Bone replaces existing cartilage.
• Masses of hyaline cartilage serve as models for future bones.
• Cartilage tissue breaks down as periosteum develops from connective tissue around the structure
• Blood vessels and differentiating osteoblasts (spongy bone) invade disintegrating cartilaginous tissue from periosteum
• Types: Nearly all bones.
• Osteoblasts become osteocytes as bony matrix surrounds them
• Osteoblasts deposit compact bone around spongy bone between periosteum.
• Growth of endochondral bones
  • Primary ossification center: Hyaline cartilage in the diaphysis center is replaced with bone
  • Osteoblasts from periosteum deposit a thin compact bone layer around the primary ossification center
  • Epiphysis remains cartilaginous and continues to grow.
  • Secondary ossification center: Located at the center of each epiphysis, spongy bone is laid down in all directions
  • Ossification continues until spongy bone from diaphysis and epiphysis nearly meet
  • An epiphyseal disk (thin band of cartilage) is formed at this point
  • As long as cartilaginous cells are active in the disk, long bone will continue to grow
  • Growth stops when the disk becomes fully calcified

Fracture Types and Repair

• Greenstick
• Fissured
• Comminuted
• Transverse
• Oblique
• Spiral

How a bone heals

• Formation of a Hematoma: Blood-filled swelling.
  • Blood vessels rupture during the break
  • Osteocytes die due to lack of oxygen
• Fibrocartilage formation
  • Fibrocartilage fills gaps in distant regions
  • Osteocytes clean debris and osteoblast deposit new spongy bone
  • Blood vessels reform
• Bony Callus
  • More osteoblasts enter area, more spongy bone replaces fibrocartilage splint
• Bone remodeling
  • Osteoblasts build new bone (Spongy and Compact) to reinforce the bone
  • Osteoclasts shave to reshape bone

Factors Affecting Bone Development, Growth & Repair

• Nutrition:
  • Vitamin D3: Needed for normal absorption of calcium and phosphate ions in the intestines
    • Without vitamin D3, calcium will not be absorbed and bones will weaken and deform over time known as Rickets in children and Osteomalacia in adults
  • Vitamin A: Stimulates Resorption activities.
    • Lack slows growth
  • Vitamin C: Required for collagen synthesis
    • Lack results in fragile-slender bones
• Hormonal Secretions:
  • Growth Hormone: Produced by the pituitary gland
  • Stimulates bone growth at epiphyseal disks
  • Over-excretion: Acromegaly
  • Sex hormones: Stimulates cellular activity increase at epiphyseal disks during puberty
  • Calcitonin: Secreted when blood calcium levels are high.
  • Increases osteoblast activity
  • Increases kidney excretion of calcium ions
  • Parathyroid Glands: Secreted when blood calcium levels are low.
  • Increases osteoclast activity.
  • Decreases kidney excretion of calcium ions.
• Physical Stress: Bones adapt shape to forces applied
  • Heavily stressed bones thicken and enlarge
  • Unstressed bones thin and become brittle
  • If muscle enhances, the bumps and ridges broaden
  • (point of attachment for the muscle) expands

Skeletal Organization

- 2 divisions of the human skeleton
- Axial skeleton (80 bones)
   - Skull: cranium (8) and facial bones (14)
   - Hyoid bone (1) / 6 auditory ossicles
    -Vertebral column (24 vertebrae/1 sacrum/1coccyx)
   - Thorax (24 ribs/1 sternum)    
- Appendicular skeleton (126 bones)
   - Pectoral girdle (scapula-2, clavicle-2)
   - Upper limbs (humerus-2, ulna-2, radius 2, carpals-16, metacarpals-10, phalanges
     28)
  - Pelvic girdle (2 coxal bones) fused ilium, ischium and pubis
  - Lower limbs (femur-2, tibia-2, fibula-2, patella-2, tarsals-14, metatarsals-10,
    phalanges-28)