4.1 Body Organization, Integumentary & Skeletal Systems
Key Takeaways
- Anatomy (11 items) plus Physiology (11 items) = 22 questions (~36%) of the 60-question NEX Science section — nearly double the old PAX
- Levels of organization run chemical → cellular → tissue → organ → organ system → organism; questions test correct ordering
- Four tissue types: epithelial (covers/lines), connective (supports/binds), muscle (movement), nervous (signaling) — every organ blends these
- Anatomical position (erect, palms forward) is the reference for all directional terms; sagittal/frontal/transverse are the three planes
- Skin has three layers — epidermis (avascular), dermis (vascular, glands, nerves), hypodermis (fat) — and performs five core functions
- The 206-bone adult skeleton splits into 80 axial and 126 appendicular bones and performs five functions including hematopoiesis
- Joints classify as fibrous (immovable), cartilaginous (slightly movable), and synovial (freely movable)
Why This Section Carries So Much Weight
The NEX Science section has 60 items (55 scored, 5 unscored) answered in 60 minutes — about one minute per question. Within it, Anatomy (11 items) and Physiology (11 items) combine to 22 questions, roughly 36% of the section, nearly double the old PAX. Physics was removed entirely, and chemistry shrank to 5 items, so structure-and-function knowledge is now the single biggest scoring opportunity. Treat 4.1–4.3 as the highest-yield chapter in the whole exam.
Levels of Body Organization
A classic NEX item asks you to put these in order or pick the level a structure belongs to. Memorize the chain simplest → most complex.
| Level | What It Is | Example |
|---|---|---|
| Chemical (atom) | Smallest unit of matter | Carbon, oxygen, hydrogen |
| Molecular | Bonded atoms | Water (H₂O), glucose, DNA |
| Organelle | Structure inside a cell | Mitochondria, nucleus, ribosome |
| Cellular | Basic unit of life | Neuron, red blood cell |
| Tissue | Similar cells, shared job | Epithelium, muscle |
| Organ | ≥2 tissues together | Heart, kidney, stomach |
| Organ system | Organs cooperating | Cardiovascular, digestive |
| Organism | All systems together | A human being |
Trap: the cell — not the atom — is the smallest unit of life. Atoms are the smallest unit of matter.
The Four Tissue Types
- Epithelial — covers body surfaces, lines cavities and vessels, forms glands (skin surface, gut lining, endothelium).
- Connective — supports and binds; most diverse type (bone, cartilage, blood, fat, tendons, ligaments).
- Muscle — generates movement (skeletal, cardiac, smooth).
- Nervous — transmits electrical signals (neurons + glial support cells).
Muscle subtypes are a favorite distractor set:
| Type | Control | Location | Features |
|---|---|---|---|
| Skeletal | Voluntary | Attached to bones | Striated, multinucleated |
| Cardiac | Involuntary | Heart only | Striated, intercalated discs |
| Smooth | Involuntary | Organ/vessel walls | Non-striated, single nucleus |
Trap: blood is connective tissue, and tendons are connective (dense regular), not epithelial.
Anatomical Terminology
All directional terms reference anatomical position: standing erect, face forward, arms at sides, palms forward.
| Term | Meaning | Opposite |
|---|---|---|
| Superior | Toward head | Inferior |
| Anterior (ventral) | Front | Posterior (dorsal) |
| Medial | Toward midline | Lateral |
| Proximal | Closer to trunk/origin | Distal |
| Superficial | Near surface | Deep |
Three planes: sagittal (left/right), frontal/coronal (front/back), transverse/horizontal (superior/inferior). Worked example: a cut from ear to ear across the top of the head, dividing front from back, is a frontal cut; slicing at the waist into top and bottom is transverse.
Integumentary System (Skin)
Skin is the body's largest organ by surface area. Learn the three layers and which one is avascular.
| Layer | Vascular? | Key Structures |
|---|---|---|
| Epidermis (outer) | No | Keratinocytes, melanocytes |
| Dermis (middle) | Yes | Blood vessels, nerves, hair follicles, sweat & sebaceous glands |
| Hypodermis (subcutaneous) | Yes | Adipose (fat), connective tissue |
Five functions: protection, temperature regulation (sweat, vasodilation/constriction), sensation, vitamin D synthesis, and limited excretion. Trap: the epidermis has no blood vessels — burns or cuts only bleed once they reach the dermis.
Skeletal System
The adult skeleton has 206 bones (newborns have ~270; many fuse). Functions: support, protection, movement, blood-cell production (hematopoiesis in red marrow), and mineral storage (calcium, phosphorus).
| Division | Bones | Includes |
|---|---|---|
| Axial | 80 | Skull, vertebral column, rib cage, sternum |
| Appendicular | 126 | Arms, legs, pectoral & pelvic girdles |
Joint classes: fibrous (immovable — skull sutures), cartilaginous (slightly movable — vertebral discs, pubic symphysis), synovial (freely movable — knee, shoulder, hip).
Worked Example and Common Traps
Worked example: The NEX asks where the elbow sits relative to the wrist. Using proximal (closer to the trunk) versus distal (farther from the trunk), the elbow is proximal to the wrist and the fingers are distal to it — a textbook directional pairing that recurs in several forms.
Watch these recurring distractors before test day:
- "Largest organ" confusion: the skin is the largest organ by surface area; the liver is the largest internal organ — read whether the stem says "internal."
- Bone-count drift: the answer is 206 for adults, not the ~270 a newborn has. A stem mentioning fusion is steering you toward 206.
- Tissue mislabels: blood and tendons are connective tissue, even though blood is liquid and tendons feel like cords.
- Plane mix-ups: a "midline left/right" cut is sagittal; a "waist-level top/bottom" cut is transverse; an "ear-to-ear front/back" cut is frontal/coronal.
How the Skeletal and Integumentary Systems Cooperate
These two systems are tested together because both are about protection and structure. The skin's dermis synthesizes the precursor for vitamin D, which the body needs to absorb the calcium that bones store and release. When blood calcium drops, parathyroid hormone (PTH) pulls calcium out of bone; when it rises, calcitonin from the thyroid pushes it back in. This calcium loop ties chapter 4.1 forward to the endocrine material in 4.3, so a single NEX item can reasonably mix a skeletal fact (mineral storage) with an endocrine trigger (PTH).
Knowing that red marrow performs hematopoiesis also connects to the blood components in 4.2 — the bones literally manufacture the red and white cells and platelets you study there. Treat these cross-links as exam-writer favorites: questions that span two systems are how the NEX separates strong test-takers from rote memorizers.
What are the four basic tissue types in the human body?
Which body plane divides the body into anterior (front) and posterior (back) portions?
Which layer of the skin contains blood vessels, hair follicles, and sweat glands?
The adult human skeleton contains _____ bones.
Type your answer below
Match each anatomical directional term to its correct meaning.
Match each item on the left with the correct item on the right
Which type of muscle tissue is found ONLY in the heart and contains intercalated discs?
The production of blood cells in red bone marrow is called:
Which of the following are functions of the skeletal system? (Select all that apply)
Select all that apply
The correct hierarchy of body organization from simplest to most complex is: