Skeletal System: Structure, Function, and Anatomy (Chapters 6–8 Study Guide)

Skeletal System Overview

Major Functions of the Skeletal System

• Support: Provides a structural framework for the body, maintaining posture and supporting soft tissues.

• Protection: Shields vital organs (e.g., skull protects brain, ribs protect thoracic organs).

• Leverage: Acts as levers for muscles, enabling movement.

• Mineral & Lipid Storage: Stores calcium (as hydroxyapatite) and lipids (in yellow marrow).

• Blood Cell Production: Red marrow produces blood cells (hematopoiesis).

Bone Classification

Types of Bones

• Long Bones: Longer than wide; diaphysis and epiphyses (e.g., femur, humerus).

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

• Flat Bones: Thin, protective (e.g., skull, ribs, sternum).

• Irregular Bones: Complex shapes (e.g., vertebrae, sphenoid).

• Sesamoid Bones: Develop within tendons (e.g., patella).

• Sutural (Wormian) Bones: Small bones within skull sutures.

Long Bone Anatomy

Structure	Description	Contains
Diaphysis	Shaft; compact bone walls	Medullary cavity
Epiphyses	Ends; spongy bone interior	Red or yellow marrow
Metaphyses	Between diaphysis & epiphyses; growth zone	Epiphyseal plate/line
Medullary cavity	Hollow center of diaphysis	Yellow marrow (adults)
Periosteum	Dense connective tissue covering outer bone	Blood vessels, nerves, osteogenic cells
Endosteum	Thin layer lining medullary cavity	Osteogenic cells, osteoblasts

Bone Cells

Cell Type	Mnemonic	Action	Location	Key Detail
Osteogenic	O = Origin/stem cell	Divides; becomes osteoblasts	Periosteum, endosteum	Stem cells
Osteoblast	B = Builder	Secretes osteoid; triggers calcification	Bone surface	Bone formation (ossification)
Osteocyte	C = Caretaker	Maintains matrix	Lacunae in matrix	Monitors & maintains bone
Osteoclast	CL = CLear away	Bone resorption (osteolysis)	Bone surface (resorption pits)	Large, multinucleated

Compact vs. Spongy Bone

Feature	Compact Bone	Spongy Bone
Basic Unit	Osteon (Haversian system)	Trabeculae
Location	Diaphysis (shaft)	Epiphyses (ends)
Stress Handling	Directional stress	Multidirectional stress
Marrow	Yellow (adults)	Red (hematopoiesis)
Osteocyte Nourishment	Central canal blood supply	Diffusion through canaliculi

Ossification & Bone Growth

Types of Ossification

• Intramembranous Ossification: Bone forms directly within connective tissue; produces flat bones (e.g., skull, mandible, clavicle).

• Endochondral Ossification: Bone replaces a hyaline cartilage model; responsible for lengthening of long bones.

Type	Starting Tissue	What Grows?	Example Bones
Endochondral	Hyaline cartilage	Bone length (via epiphyseal plate)	Femur, humerus, tibia
Intramembranous	Fibrous connective tissue	Dermal/flat bones	Skull, mandible, clavicle
Appositional	Periosteum surface	Bone diameter/width	All bones

Hormones & Calcium Regulation

Hormone/Nutrient	Source	Effect on Bone/Calcium
Growth Hormone	Anterior pituitary	Stimulates bone growth (length & width)
Thyroxine	Thyroid gland	Stimulates bone growth; required for normal ossification
PTH (Parathyroid Hormone)	Parathyroid glands	Increases blood Ca2+; activates osteoclasts
Calcitonin	Thyroid gland (C cells)	Lowers blood Ca2+; preserves bone density
Vitamin D	Diet/skin (UV)	Required for Ca2+ absorption in intestines

• PTH: Main calcium-regulating hormone; increases blood Ca2+ by stimulating osteoclasts and kidney reabsorption.

• Calcitonin: Lowers blood Ca2+; preserves bone density, especially during pregnancy/nursing.

Fracture Repair

1. Fracture Hematoma: Blood clot forms at fracture site.

2. External Callus: Fibrocartilage bridge stabilizes bone.

3. Internal Callus: Spongy bone replaces cartilage.

4. Remodeling: Compact bone restored; bone returns to original shape.

Aging Effects on Bone

• Osteopenia: Reduced bone mass; normal aging process.

• Osteoporosis: Severe bone density loss; increased fracture risk.

• Remodeling: Balance of formation (osteoblasts) and resorption (osteoclasts) is crucial; exercise helps maintain bone density.

Axial Skeleton

Skull

• 8 Cranial Bones: Occipital (1), Parietal (2), Frontal (1), Temporal (2), Sphenoid (1), Ethmoid (1).

• 14 Facial Bones: Maxillae (2), Palatine (2), Nasal (2), Vomer (1), Zygomatic (2), Lacrimal (2), Inferior nasal conchae (2), Mandible (1).

• Major Sutures: Lambdoid, Coronal, Sagittal, Squamous.

• Fontanelles: Fibrous gaps in infant skull; allow brain growth.

• Hyoid Bone: U-shaped; supports larynx; not part of skull.

• Paranasal Sinuses: Air-filled spaces; lighten skull, resonate voice.

• Auditory Ossicles: Malleus, incus, stapes; transmit sound in middle ear.

Vertebral Column

Region	Count	Notable Features	Curve Type
Cervical	7 (C1–C7)	Transverse foramina; C1=atlas, C2=axis, C7=vertebra prominens	Secondary
Thoracic	12 (T1–T12)	Articulate with ribs; heart-shaped bodies	Primary
Lumbar	5 (L1–L5)	Massive bodies; bear greatest loads	Secondary
Sacrum	5 fused	Articulates with pelvic girdle	Primary
Coccyx	3–5 fused	Vestigial tailbone	—

• Primary Curves: Thoracic & sacral (present at birth; kyphotic).

• Secondary Curves: Cervical & lumbar (develop after birth; lordotic).

• Intervertebral Discs: Fibrocartilage pads; absorb shock.

• Vertebral Foramen: Forms spinal canal.

Thoracic Cage

• Sternum: Manubrium (superior), body (middle), xiphoid process (inferior).

• Ribs (12 pairs):

  • True ribs (1–7): Attach directly to sternum.

  • False ribs (8–10): Attach indirectly via cartilage.

  • Floating ribs (11–12): No anterior attachment.

• Costal Groove: Houses nerves and blood vessels.

Appendicular Skeleton

Pectoral (Shoulder) Girdle

• Clavicle: Positions shoulder joint; muscle attachment.

• Scapula: Triangular; features include acromion, coracoid process, glenoid cavity (shoulder joint socket).

Pelvic Girdle

• Hip (Coxal) Bone: Fusion of ilium (superior), ischium (posteroinferior), and pubis (anterior).

• Acetabulum: Socket for femoral head (hip joint).

• Pelvis: 2 hip bones + sacrum + coccyx; divided into false (greater) and true (lesser) pelvis.

• Sex Differences: Female pelvis is wider and shallower; male pelvis is narrower and deeper.

Upper Limb Bones

• Humerus: Upper arm; articulates with scapula and forearm bones.

• Radius: Lateral forearm (thumb side).

• Ulna: Medial forearm; olecranon forms elbow point.

• Carpals: 8 wrist bones.

• Metacarpals: 5 palm bones.

• Phalanges: Fingers; 3 per finger, 2 for thumb (pollex).

Lower Limb Bones

• Femur: Thigh; longest and heaviest bone.

• Patella: Kneecap; largest sesamoid bone.

• Tibia: Medial leg; main weight-bearing bone.

• Fibula: Lateral leg; not weight-bearing.

• Tarsals: 7 ankle bones; talus (articulates with tibia), calcaneus (heel).

• Metatarsals: 5 foot bones.

• Phalanges: Toes; 3 per toe, 2 for great toe (hallux).

Foot Arches

• Longitudinal Arch: Heel to toes; absorbs shock.

• Transverse Arch: Runs across foot; distributes weight.

• Flat Feet: Fallen longitudinal arch.

Pectoral vs. Pelvic Girdle Comparison

Feature	Pectoral Girdle	Pelvic Girdle
Bones	Clavicle + Scapula (each side)	Ilium + Ischium + Pubis (fused) each side
Primary Function	Attach upper limbs; enable free movement	Attach lower limbs; weight bearing & locomotion
Bone Mass	Lighter, more delicate	More massive & robust
Mobility	High mobility (wide ROM)	Less mobile, more stable
Joint w/ Axial Skeleton	Sternoclavicular joint	Sacroiliac joint

Key Vocabulary

• Ossification (Osteogenesis): Bone formation process.

• Calcification: Deposition of calcium salts in tissue.

• Hydroxyapatite: Calcium phosphate crystals in bone matrix.

• Osteolysis: Bone resorption by osteoclasts.

• Bone Remodeling: Continuous bone matrix recycling.

• Epiphyseal Plate: Growth plate; site of bone lengthening.

• Red Marrow: Hematopoietic tissue; produces blood cells.

• Yellow Marrow: Fat storage; can revert to red marrow if needed.

Quick Reference Tables

Spinal Curves

Curve Type	Regions	When Present	Direction	Also Called
Primary (accommodation)	Thoracic + Sacral	At birth	Concave anteriorly	Kyphotic
Secondary (compensation)	Cervical + Lumbar	After birth	Concave posteriorly	Lordotic

Hand vs. Foot Structure

Structure	Hand	Foot
Proximal bones	Carpals (8)	Tarsals (7)
Mid bones	Metacarpals (5)	Metatarsals (5)
Distal bones	Phalanges: 3 per finger, 2 for pollex	Phalanges: 3 per toe, 2 for hallux
Special digit name	Pollex = thumb	Hallux = great toe

Mnemonic Devices

• Vertebral Count: Breakfast at 7am (cervical), Lunch at 12pm (thoracic), Dinner at 5pm (lumbar).

• Red Marrow: Red blood cells (hematopoiesis); in flat bones & epiphyses.

• Yellow Marrow: Yellow fat (lipid storage); in medullary cavity.

Example Application

• Clinical: Osteoporosis increases fracture risk in elderly; exercise and calcium intake help maintain bone density.

• Forensics: Pelvic shape can help determine biological sex; bone markings reveal muscle use and age.

Additional info: This guide expands on brief notes with definitions, examples, and logical groupings for clarity and exam preparation.