Key Takeaways
- The adult skeleton has 206 bones, divided into the axial skeleton (80 bones) and appendicular skeleton (126 bones)
- Bones provide support, protection, movement (with muscles), mineral storage (calcium/phosphorus), and blood cell production (hematopoiesis)
- Joints (articulations) are classified by movement: synarthrosis (immovable), amphiarthrosis (slightly movable), diarthrosis (freely movable)
- The three types of muscle tissue are skeletal (voluntary, striated), cardiac (involuntary, striated), and smooth (involuntary, non-striated)
- Skeletal muscles produce movement through contraction; they work in antagonistic pairs (biceps/triceps)
- The integumentary system includes skin, hair, nails, and glands; skin is the largest organ of the body
- The skin has three layers: epidermis (outermost, protection), dermis (middle, blood vessels and nerves), hypodermis (deepest, fat storage)
- Burns are classified by depth: first-degree (epidermis), second-degree (epidermis + dermis), third-degree (all layers)
Last updated: February 2026
Skeletal, Muscular & Integumentary Systems
These three systems work together to provide structure, protection, movement, and defense for the body.
Skeletal System
The adult human skeleton contains 206 bones divided into two divisions:
| Division | Bones | Includes |
|---|---|---|
| Axial skeleton | 80 | Skull (22), vertebral column (26), thoracic cage (25), hyoid (1), auditory ossicles (6) |
| Appendicular skeleton | 126 | Upper limbs (64), lower limbs (62), pelvic girdle, pectoral girdle |
Functions of Bones
- Support — framework for the body
- Protection — skull protects the brain, ribs protect the heart and lungs
- Movement — serve as levers for muscle attachment
- Mineral storage — calcium and phosphorus
- Blood cell production — hematopoiesis occurs in red bone marrow
- Energy storage — yellow bone marrow stores fat
Types of Bones
| Type | Shape | Examples |
|---|---|---|
| Long bones | Longer than wide | Femur, humerus, tibia |
| Short bones | Cube-shaped | Carpals (wrist), tarsals (ankle) |
| Flat bones | Thin, flat | Skull, sternum, ribs, scapula |
| Irregular bones | Complex shapes | Vertebrae, hip bones |
| Sesamoid bones | Within tendons | Patella (kneecap) |
Joints (Articulations)
| Classification | Movement | Example |
|---|---|---|
| Synarthrosis | Immovable | Skull sutures |
| Amphiarthrosis | Slightly movable | Pubic symphysis, intervertebral discs |
| Diarthrosis (synovial) | Freely movable | Knee, shoulder, hip, elbow |
Types of synovial joints: hinge (elbow, knee), ball-and-socket (hip, shoulder), pivot (atlas-axis), saddle (thumb), gliding (wrist), condyloid (wrist-forearm)
Muscular System
The body has over 600 skeletal muscles. There are three types of muscle tissue:
| Type | Control | Appearance | Location |
|---|---|---|---|
| Skeletal | Voluntary | Striated (striped) | Attached to bones |
| Cardiac | Involuntary | Striated with intercalated discs | Heart only |
| Smooth | Involuntary | Non-striated (spindle-shaped) | Organs, blood vessels, GI tract |
How Muscles Work
- Muscles produce movement by contracting (shortening)
- Muscles work in antagonistic pairs: when one contracts, the other relaxes
- Example: Biceps (flexor) contracts → forearm bends; Triceps (extensor) contracts → forearm extends
- Origin: The fixed attachment point (does not move)
- Insertion: The movable attachment point (moves during contraction)
Types of Muscle Contractions
- Isotonic: Muscle length changes (lifting a weight)
- Isometric: Muscle length stays the same but tension increases (pushing against a wall)
Integumentary System
The skin is the largest organ of the body, covering approximately 20 square feet in adults.
Three Layers of Skin
| Layer | Description | Contains |
|---|---|---|
| Epidermis | Outermost layer; avascular (no blood vessels) | Keratinocytes, melanocytes, Langerhans cells |
| Dermis | Middle layer; vascular | Blood vessels, nerves, hair follicles, sweat glands, sebaceous glands |
| Hypodermis (subcutaneous) | Deepest layer | Adipose tissue (fat), larger blood vessels |
Functions of Skin
- Protection — barrier against pathogens, UV radiation, and chemicals
- Temperature regulation — sweating cools; vasoconstriction conserves heat
- Sensation — receptors for touch, pain, temperature, pressure
- Vitamin D synthesis — UV light converts cholesterol to vitamin D
- Excretion — sweat removes small amounts of waste
Burn Classification
| Degree | Depth | Appearance | Pain |
|---|---|---|---|
| First-degree (superficial) | Epidermis only | Red, dry, no blisters | Painful |
| Second-degree (partial-thickness) | Epidermis + dermis | Red, blistered, moist | Very painful |
| Third-degree (full-thickness) | All layers + possibly deeper | White, charred, leathery | Painless (nerve damage) |
Test Your Knowledge
How many bones are in the adult human skeleton?
A
B
C
D
Test Your Knowledge
Which type of muscle tissue is involuntary AND striated?
A
B
C
D
Test Your Knowledge
A third-degree burn is characterized by:
A
B
C
D
Test Your KnowledgeFill in the Blank
The process of blood cell production in red bone marrow is called _____.
Type your answer below
Test Your KnowledgeMatching
Match each muscle type to its key characteristics.
Match each item on the left with the correct item on the right
1
Skeletal muscle
2
Cardiac muscle
3
Smooth muscle
Test Your Knowledge
Which layer of skin contains blood vessels, nerves, and hair follicles?
A
B
C
D