Musculoskeletal System Overview
Bones, muscles, and joints working together for support and movement.
The musculoskeletal system is made of the skeleton (bones, joints, and cartilage) and the skeletal muscles attached to it. Together they give the body shape, hold it upright, and allow movement.
Five Main Functions
- Support & shape
- The skeleton is the body's framework
- Movement
- Muscles pull on bones across joints
- Protection
- Skull protects the brain; ribs protect heart & lungs
- Mineral storage
- Bones store ~99% of the body's calcium and phosphorus
- Blood cell production
- Red bone marrow makes red & white blood cells and platelets
This system connects to many others: bones store the calcium regulated by the endocrine system, marrow makes the blood cells of the hematologic system, and the nervous system drives muscle contraction. Understanding normal structure lets you reason out fractures, osteoporosis, and the dangers of immobility.
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Bones & the Skeleton
206 bones, organized into two divisions.
The adult skeleton has 206 bones, divided into two parts:
- Axial skeleton — the central axis: skull, vertebral column, and rib cage (protects vital organs).
- Appendicular skeleton — the limbs and the girdles (shoulders and pelvis) that attach them.
Bone Structure
- Diaphysis
- The shaft of a long bone
- Epiphysis
- The end of a long bone (growth plate in children)
- Periosteum
- Tough membrane covering the bone; rich in nerves & vessels
- Compact bone
- Dense outer layer
- Spongy bone
- Inner lattice; contains red marrow
- Marrow
- Red marrow makes blood cells; yellow marrow stores fat
In children, bones lengthen at the epiphyseal (growth) plate. An injury to this plate can affect a child's future bone growth — which is why pediatric fractures near a joint are taken seriously.
Bone Growth & Remodeling
Living tissue that constantly rebuilds itself.
Bone is living tissue that is always being broken down and rebuilt. Two cell types do the work:
- Osteoblasts
- BUILD bone (osteoBlasts Build)
- Osteoclasts
- Break down bone & release calcium (osteoClasts Chew)
- Osteocytes
- Mature bone cells that maintain the tissue
What Controls Remodeling
- PTH (parathyroid hormone) — raises blood calcium by activating osteoclasts to pull calcium from bone.
- Calcitonin (thyroid) — lowers blood calcium by depositing it into bone.
- Vitamin D — needed to absorb calcium from the gut.
- Weight-bearing activity — stress stimulates bone building (Wolff's law); immobility weakens bone.
When calcium intake is low or estrogen falls (after menopause), osteoclasts outpace osteoblasts and bones become porous and weak — osteoporosis. This is why weight-bearing exercise, calcium, and vitamin D protect bone, and why immobile or post-menopausal clients are at higher fracture risk.
? Which bone cell builds new bone and which breaks it down?
Joints & Cartilage
Where bones meet — and what holds them together.
Three Types of Joints (by movement)
- Fibrous
- Immovable — e.g., skull sutures
- Cartilaginous
- Slightly movable — e.g., between vertebrae
- Synovial
- Freely movable — e.g., knee, elbow, shoulder, hip
Connective Tissues
- Cartilage — smooth, cushioning tissue that covers the ends of bones in synovial joints and reduces friction.
- Ligaments — connect bone to bone and stabilize joints (Ligaments Link bone to bone).
- Tendons — connect muscle to bone (Tendons Tie muscle to bone).
- Synovial fluid — lubricates freely movable joints.
A sprain is an injury to a ligament (bone-to-bone), often from twisting a joint. A strain is an injury to a muscle or tendon from overstretching. Both are treated with RICE: Rest, Ice, Compression, Elevation.
Skeletal Muscle
The motors that move the skeleton.
Three Types of Muscle
- Skeletal
- Voluntary, striated — attached to bones
- Smooth
- Involuntary, non-striated — organs, blood vessels, GI tract
- Cardiac
- Involuntary, striated — heart only
How Muscles Contract
- A nerve signal triggers the release of calcium inside the muscle cell.
- Calcium lets the protein filaments actin and myosin slide past each other (the "sliding filament" model), shortening the muscle.
- This requires ATP (energy). Without enough ATP or calcium, muscles can't contract or relax normally.
Calcium isn't just for bones — it's required for muscle contraction itself. That's why severe calcium imbalances cause muscle problems: low calcium leads to twitching and tetany, while abnormal potassium and calcium levels can affect both skeletal and cardiac muscle.
Movement & Body Mechanics
How muscles produce coordinated motion.
Muscles can only pull, not push, so they work in antagonistic pairs — when one contracts, its partner relaxes. For example, the biceps flexes the elbow while the triceps extends it.
Common Movement Terms
- Flexion / Extension
- Decreasing / increasing the angle of a joint
- Abduction / Adduction
- Moving away from / toward the midline
- Rotation
- Turning around an axis
- Supination / Pronation
- Palm up / palm down
Because muscles move bones using leverage, lifting with the large leg muscles (not the back) and keeping loads close to the body protects the spine. This A&P explains the nursing rule: bend the knees, keep the back straight, and use your legs.
Fractures & Bone Healing
What happens when a bone breaks.
How a Fracture Heals (in order)
Healing needs good blood supply, calcium, vitamin D, and the bone ends held still (immobilization). It is slower in older adults and with poor circulation, smoking, or poor nutrition.
After a fracture or cast, swelling can cut off circulation (compartment syndrome). Watch for the 6 P's: Pain (severe, unrelieved), Pallor, Pulselessness, Paresthesia (numbness/tingling), Paralysis, and Poikilothermia (coolness). This is an emergency — circulation must be restored quickly to save the limb.
Clinical Applications & Key Labs
Connecting the anatomy and physiology to the bedside.
A&P at the Bedside
- Neurovascular checks — assess color, warmth, movement, sensation, pulses, and capillary refill distal to an injury or cast.
- Immobility complications — bed rest causes muscle atrophy, bone loss, contractures, pressure injuries, and clot (DVT) risk.
- Weight-bearing & calcium — protect bone density; important after menopause and in older adults.
Key Musculoskeletal Labs
Ranges vary by lab, facility, and textbook — for exams, use the range given in the question. See the full NCLEX-PN lab-value reference & sources →
- Serum calcium
- 9.0–11.0 mg/dL (ranges vary by lab) — Calcium balance (bone & muscle)
- Serum phosphorus
- 2.5–4.5 mg/dL — Moves inversely with calcium
- Vitamin D
- ~30–100 ng/mL — Needed for calcium absorption
- Alkaline phosphatase (ALP)
- ~44–147 U/L — Rises with bone formation/healing
- Creatine kinase (CK)
- ~30–200 U/L — Rises with muscle injury
You don't need to memorize every orthopedic condition in A&P — you need to know that bone stores calcium and makes blood cells, muscles need calcium and ATP to contract, and movement happens at joints held by ligaments and tendons. Once you know that, fractures, osteoporosis, and immobility problems make sense. The Med-Surg musculoskeletal guide builds on this with disorders and nursing care.
Frequently Asked Questions
Common student questions on this topic.
What is the difference between osteoblasts and osteoclasts?
What is the difference between a ligament and a tendon?
What are the three types of muscle tissue?
Why is calcium important for the musculoskeletal system?
How does a bone fracture heal?
What are the main types of joints?
Quick-Reference Cheatsheet
Every key point from this guide on a single printable page.
Musculoskeletal System Cheatsheet
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