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The Skeletal System · Chapter 6 of 7
1986–1990, ages 11–15: a twenty-pound backpack on a spineProminent ridge on the posterior scapula dividing it into supraspinous and infraspinous fossae. that hurt, four years. Bone remodels to its loads — Wolff’s law — recycling five to seven percent of itself a week under PTH, calcitoninA hormone from the thyroid that lowers blood calcium levels by inhibiting osteoclasts. and vitamin D. In 2013, at thirty-eight, a DEXA read her bones as thin for her age: “perimenopause”, calcium. This chapter runs the remodeling machine, the thermostat and the loading law, then reads the scan the way a Z-score should be read.
The question you will answer
OsteoclastsBone-resorbing cells that break down bone matrix. dig, osteoblasts fill, and the balance decides density. What sets the balance — hormones, load, or cytokines? Why is a low Z-score at thirty-eight a question and not an answer? And what does a backpack have to do with it?
Opener · The Backpack · about 1¾ minutes
The opener is the same narrated animation you will watch in Scene 1.
Watch the opener first. It sets the question; the chapter answers it.
1
Read the chart
Patient chart · Stina
1986–1990 · ages 11–15 · the backpack years → 2013 · 38 · the scan
1986–90 · 11–15
Twenty-pound backpack; concrete floors; feet, shins, back and legs hurt; gym leaves her soaked and shaking (Module 7).
Hindsight · 2013 · 38
A wrist film after a fall leads to a DEXA: spine and hip low for age. “Perimenopause.” Calcium and vitamin D. Cycles and estrogen normal; cortisolA glucocorticoid involved in stress response, metabolism, and immune regulation. flat; calcium wandering; vitamin D low (Module 13).
Not done
A Z-score read as a Z-score; a secondary-cause workup; the question “why is a thirty-eight-year-old resorbing bone?”
Chart clues this module: 6 · this chapter · 7 · The backpack · 13 · The scan
Before you go on · your prediction
Low bone density at thirty-eight with normal cycles and normal estrogen, handed calcium and told “perimenopause”. The best first reading is…
Open the one you lean toward. Nothing is recorded and nothing is revealed — you come back to it at the end.
Early menopause — estrogen must be falling, whatever the labs say
Ask what estrogen does to osteoclasts, and whether “perimenopause” fits a woman whose hormones are normal.
Not enough calcium in the diet — the supplement will fix it
Ask whether calcium intake sets density on its own, and what PTH does when blood calcium falls.
A secondary cause — something driving resorption that a Z-score at thirty-eight demands you find
Ask what the secondary-cause list is, which items her chart held, and what IL-1β and IL-6 do to RANKL and OPG.
From Stina’s diary
2
The story
Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.
I · The backpack
From eleven to fifteen she carried a twenty-pound backpack — every textbook, every day — on a spine that already hurt. Everyone carries a backpack, she was told. Wolff’s law says bone grows and remodels in response to stress, and the packet lists the proofs — a right-hander’s thicker arm, ridges where muscles pull, trabeculae as trusses along the lines of force, a curved bone thickest where it would buckle. A loaded adolescent spine should have been laying down bone. In 2013, at thirty-eight, after a fall onto a wrist, a DEXA found her spine and hip low for her age. The chart said perimenopause and prescribed calcium. Her cycles were regular and her estrogen normal. Nobody asked why a thirty-eight-year-old was resorbing bone.
See it move: Scene 1 · The Backpack · Scene 4 · Wolff’s Four Implications
II · The remodeling machine
Remodeling runs for life: five to seven percent of bone mass is recycled each week, spongy boneA porous bone tissue found inside bones, providing lightweight support. is replaced every three to five years, compact boneDense, strong bone tissue forming the outer layer of bones. about every ten. It happens in remodeling units — osteoclasts tunneling or scooping, osteoblasts following to line the space with new lamellae — thousands active at once; deposit under the periosteum and along trabeculae, resorption at the endosteumA thin connective tissue lining the marrow cavity. and in the pits. Density is the balance: matched, mass holds; when resorption outruns deposit, bone thins, spongy first. Two things set the balance. Hormones: estrogen restrains osteoclasts (the site’s Bone page says the reverse — a slip), so the balance tips at menopauseThe natural cessation of menstruation and ovarian function.; glucocorticoidsSteroid hormones regulating metabolism and immune response. kill osteoblasts. And load: osteocytes feel strain through their canaliculiTiny channels that connect osteocytes, allowing communication and nutrient exchange. and tell the surface to build where bone is used and resorb where it is not. Unload a bone — a cast, a bed, a leg spared because it hurt — and the osteoclasts win.
See it move: Scene 2 · Remodeling Units · Scene 4 · Wolff’s Four Implications
III · The calcium thermostat, and the scan read properly
Blood calcium is held near 8.5–10.5 mg/dL because nerves, muscle and the heart fail outside it; bone is the reserve the body spends to hold the line. When calcium falls, the four parathyroids release PTH, with three targets: osteoclasts release calcium from bone, the kidney keeps calcium and activates vitamin D, and calcitriolThe active form of vitamin D, promoting calcium absorption in the intestines. raises absorption from the gut. When calcium rises, the thyroid’s C cellsThe basic structural and functional units of life. release calcitonin, which restrains osteoclasts — minor in adults. Blood over bone, every time. A T-score compares you to a young adult and is for postmenopausal women; at thirty-eight with normal cycles you get a Z-score, against women your age, and Z at or below −2.0 is ‘below the expected range for age’ — not ‘perimenopause’ but a secondary cause to be found, present in half to four-fifths of such women. The list is standard — cortisol, thyroid, parathyroid, celiac disease, vitamin D, a plasma-cell disorder, drugs, immobility — and chronic inflammation, in which IL-1β and IL-6 push osteoblasts to make RANKL, the signal that drives osteoclasts, and suppress OPG, the decoy. FMF cohorts have lower density, and the longer the delay, the lower it goes. Her 2013 chart held wandering calcium, low vitamin D, a flat cortisol, raised markers — and a prescription for calcium.
See it move: Scene 3 · The Calcium Thermostat · Scene 5 · The Scan at Thirty-Eight
3
Watch · explore · think · check
5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.
How to read the scenes
- Animation (dark teal) — a short animated explainer, one idea each
- Slide (dark teal) — a still to study: a micrograph, a diagram or a map
- Explore (purple) — something to push on while you watch
- Game (purple) — a quick challenge
- Think (gold) — reason it out, then spot-check yourself with a few true/false questions; not graded
- Check (gold) — a quick H5P that scores itself
Scene 1 of 5
The Backpack
Twenty pounds, four years, a spine that hurt — and a law that says loaded bone should get stronger.
Animation · in production · 1 min 45 s
The Backpack
A girl with a twenty-pound pack crossing a concrete hallway; the spine drawn with load arrows. Inside a vertebra: trabeculae re-aligning along the arrows, osteoblasts thickening them — Wolff’s law at work. Then the leg she spared: the arrows fade, osteoclasts win, the trabeculae thin. 2013: a DEXA printout with two Z-scores. Caption: load builds bone; sparing loses it; neither explains a fever.
Prefer the full lecture? Remodeling and Osteoporosis (original video on the site)
Explore
Load it, lose it
A weight room for bone. Load two bones, predict which cells win, then call four people’s loaded and unloaded sites and test a classmate’s backpack theory.
Text description of this explore
A weight room. Step 1: two bones side by side: a lumbar vertebra cut from the side (a cortical shell around pink marrow, crossed by vertical struts and thinner horizontal cross-struts) and the upper femur cut from the front (trabecular arches running from the head into the shaft). Red arrows press down on both; heavier loads get thicker arrows. Choose the load (bed rest, a desk all day, everyday walking, a 20-lb backpack, or running and jumping) and the age (12 and still growing, or 38), with buttons for bed rest, the backpack at 12 and the backpack at 38. Predict what happens in the vertebra over six months (osteoblasts win and the vertical struts thicken; osteoclasts win and the cross-struts thin first; or a draw), then run it: the struts thicken or thin, yellow osteoblasts or purple osteoclasts appear, and a readout gives the six-month change at the vertebra and the femoral neck from a teaching model (about 1 % a month lost at full bed rest; gains slower, and about three times larger while growing). Step 2: four people, each a simple standing figure with two numbered sites: a right-handed tennis player (1 racquet arm, 2 other arm); an astronaut after six months on the space station (1 hips and spine, 2 arms); a child six weeks in a cast for a broken shin (1 casted leg, 2 other leg); and Stina at 11 to 15, sparing a sore leg with a 20-lb pack on her back (1 spared leg, 2 carrying leg). For each, choose whether site 1 gains, holds or loses, and whether site 2 gains, holds or loses; after you commit the sites are colored and labeled. Step 3: a classmate claims the backpack wore her vertebrae thin; choose whether Wolff’s law supports or refutes that. Step 4: write what Wolff’s law predicts for a 20-lb pack on an adolescent spine and what it cannot say about a Z-score at thirty-eight, then compare with a model answer.
Think · not graded
Wolff’s law predicts that four years of a heavy backpack should have built bone in her spine, and that sparing a painful leg should have lost it. Which does the 2013 scan agree with, and what does the law say nothing about?
Spot-check your thinking: true or false?
Check · Multiple Choice
Select the outcomes Wolff’s law predicts.
Evidence card 1
Add it to your evidence board
Loaded for years, normal estrogen, and still thin — Wolff’s law and estrogen both argue against the chart’s reading; something else was driving the osteoclasts.
Scene 2 of 5
Remodeling Units
Five to seven percent a week, three to five years for spongy, ten for compact — dig, fill, repeat.
Animation · in production · 1 min 50 s
Remodeling Units
A trabeculaThe lattice-like structures in spongy bone that support stress distribution.. Osteoclasts scoop a pit; they die off and macrophages tidy; osteoblasts arrive and fill the pit with new lamellae; the surface quiets. A counter runs: 5–7% a week; spongy bone every 3–5 years; compact every 10; “five skeletons by fifty”. Then the balance beam — deposit and resorption level; estrogen loss tips it; cytokines tip it harder; the spongy bone thins first. Caption: a skeleton is a process, not an object.
Prefer the full lecture? Remodeling and Osteoporosis (original video on the site)
Explore
Run the cycle
One pit, dug and refilled. Put the cycle in order, set how deep the osteoclasts dig and how much the osteoblasts refill, run ten years, then decide where a scan sees it first.
Text description of this explore
A remodeling cascade. Step 1: put the four phases of one remodeling unit in order, starting from the quiet surface: quiescence (flat lining cells cover a quiet surface); resorption (osteoclasts seal on and dig a pit, a few weeks); reversal (the osteoclasts die off and macrophages tidy the floor); formation (osteoblasts fill the pit with osteoid, which then calcifies, a few months). Step 2: a drawing of one remodeling unit in spongy bone, in section: a purple osteoclast beside a pit outlined with a dashed line, refilled from the bottom with new lamellae, yellow osteoblasts beside it, and a caption giving the dent or bump each unit leaves. Below it, a plot of density from 0 to 10 years, 60 to 120 % of start. Set how deep the osteoclasts dig (80 to 130 % of a matched pit), how much the osteoblasts refill (70 to 110 %), and whether the tilt is steady for ten years or lasts six weeks in a cast; buttons load a matched pair, menopause (dig 115, refill 100), an IL-1β and IL-6 surge (dig 120, refill 90) and a cast (dig 125, refill 80, six weeks). Predict the density after ten years (about where it started; down 2 to 10 %; down more than 10 %; or up), then run it: the curve is drawn and a readout gives the change after one and ten years (teaching model). Step 3: a plot for the same steady tilt (dig 120, refill 90) with the curves hidden; for four scan sites (lumbar spine L1–L4, femoral neck, total hip, one-third radius) choose whether each thins first, in between or last, and whether it is mostly spongy bone, a mix or mostly compact bone. After you commit, two curves appear: spongy bone falls to about 78 % and compact bone to about 90 % of start. Step 4: a classmate claims a few months of a sore leg, a cast-sized tilt, explains a spine and hip both low for age at 38; choose whether the ten-year runs support or refute that.
Think · not graded
If five to seven percent of bone is recycled every week, a small steady tilt toward resorption compounds. Say what it does over a decade, why spongy bone shows it first, and name two things on her chart that could tilt it without touching estrogen.
Spot-check your thinking: true or false?
Check · Multiple Choice
How much bone is recycled, and how often is each kind replaced?
Evidence card 2
Add it to your evidence board
A process that can be tipped: by estrogen (normal), by load (present), or by the cytokines (raised on every draw) — only one of the three fits her 2013 numbers.
Scene 3 of 5
The Calcium Thermostat
Four glands, one hormone, three targets, a vitamin — and blood always wins over bone.
Animation · in production · 2 min
The Calcium Thermostat
A balance: blood calcium dips. Four parathyroids fire PTH. Three arrows — to bone (osteoclasts release Ca), to the kidney (keep Ca; activate vitamin D), to the gut (calcitriol raises absorption). The balance levels. It overshoots; the thyroid’s C cells send calcitonin; osteoclasts stand down. Then the vitamin: skinThe body’s largest organ, providing protection and regulation. makes D in sunlight; liverA large organ that produces bile, detoxifies blood, and stores nutrients., then kidney, activate it; without it, the gut arrow fails and PTH leans on bone. Caption: blood over bone, every time. Terms: hypocalcemiaLow calcium levels in the blood, causing muscle spasms and weakness. (spasms, tetany) and hypercalcemiaExcess calcium in the blood, leading to nervous and muscular dysfunction. (weakness, stones) — the two failures the thermostat prevents.
Prefer the full lecture? Calcium Homeostasis (original video on the site)
Explore
Fix the calcium
A boiler gauge for blood calcium. Drive the dial low, high and low without vitamin D, predict the loop each time, then follow three patients and one normal-looking number for a decade.
Text description of this explore
A boiler room. Step 1: a brass gauge for blood calcium from 7 to 12 mg/dL, with a low zone, a green band from 8.5 to 10.5 and a high zone; beside it the thyroid seen from behind with four dark parathyroid glands; along the bottom three targets, bone, kidney and gut, with a vitamin D sun on the gut. Set the starting calcium (7.0 to 12.0 mg/dL) and vitamin D (enough or deficient), or use the buttons set it low, set it high, and low without vitamin D. Predict what fires (PTH from the parathyroids, with bone, kidney and gut all pushing calcium up; PTH switching off while the thyroid’s C cells add calcitonin, so calcium falls; nothing new, because it is in the band; or PTH while the gut route fails, so bone pays most), then run the loop: the hormone is named, arrows of different widths reach the targets, each target is labeled with what it does, a red needle shows where calcium settles, and a readout lists each target’s job and the cost to bone (teaching model). Step 2: three bars for bone after ten years, hidden until you commit, for three patients: after a thyroidectomy that also took all four parathyroids; a vitamin-D-deficient adult; and a woman with PTH high on two draws a year apart. For each, choose whether bone thins, holds or gets denser, and what blood calcium does over the decade (low unless treated; held in the band at bone’s cost; high; or wanders in and out of the band). After you commit the bars show about 6 % denser, 8 % thinner and 10 % thinner (teaching model). Step 3: a classmate claims that because her 2013 calcium was inside the band on most draws, her bones were paying for nothing; choose whether to support or refute that.
Think · not graded
PTH raises blood calcium by three routes, and only one of them spends bone. Explain why the body is willing to thin a skeleton to hold blood calcium, what vitamin D deficiency does to that bargain, and why the site’s “8.5 mg/dL target” is a lower limit, not an average.
Spot-check your thinking: true or false?
Check · Drag the Words
PTH’s three targets and what each does.
Evidence card 3
Add it to your evidence board
Wandering calcium and low vitamin D in 2013 — a thermostat under strain, written down and not read.
Scene 4 of 5
Wolff’s Four Implications
Handedness, ridges, trusses, curves — the packet’s four proofs, and the femoral neckNarrow region just below the head; common fracture site. that shows all of them.
Slide · in production
Wolff’s Law
Bone is an argument with its loads: it adds where it is used and subtracts where it is not. Everything else needs a different explanation.
Prefer the full lecture? Spongy and Compact Bone (original video on the site)
Explore
Find the four proofs
A field guide to Wolff’s law. Pin the four proofs on a skeleton, read three femoral necks for bending, then predict a scan from load alone and hold it against her 2013 report.
Text description of this explore
A field guide. Step 1: the site’s skeleton drawing, front view, cropped from the shoulders to the knees, belonging to a right-hander, with six numbered pins: 1 on the middle of her right humerus; 2 on the outer side of her left humerus, halfway down; 3 on the neck of her right femur; 4 on the middle of her left femoral shaft; 5 on a lumbar vertebral body; 6 on the top rim of her right hip bone. Name the implication each pin shows: handedness (thicker on the dominant side), a projection where a muscle pulls, trusses along the lines of force, or a curved bone thick where it would buckle. Two implications appear twice. Step 2: the site’s picture of three upper femurs with their neck-shaft angles drawn: left, more than 140°; middle, 125°; right, less than 120°. For each, choose the bending stress on the neck (most; in between; least, the neck takes it like a column) and where its trabeculae thicken (arches, tension on top and compression below; columns straight down the neck; or the usual mix of both). Step 3: suppose load were the whole story; for the hip of the leg she spared, the hip of the leg that carried her, and the lumbar spine after four years of the pack, predict a DEXA reading (low for age, about as expected, or higher than expected). Step 4: a table of what load alone predicts beside her 2013 scan: lumbar spine, predicted as expected or higher, scanned low for age; hip, predicted low only on the spared side, scanned low for age. Choose what the comparison says; after you commit a Match row marks the spine as no match and the hip as a match only if the spared side was scanned.
Think · not graded
A curved bone thickens on the side that would buckle; a trabecular arch forms along the line of force. Explain both with the same cell — the osteocyteMature bone cells that maintain bone tissue. in its canaliculi — and then say which of her findings Wolff’s law can and cannot explain.
Spot-check your thinking: true or false?
Check · Multiple Choice
Select what Wolff’s law explains; leave what it does not.
Evidence card 4
Add it to your evidence board
A loaded adolescent spine should have gained; the 2013 lumbar Z-score says it lost — the law points away from the backpack and toward the blood.
Scene 5 of 5
The Scan at Thirty-Eight
Z, not T; a question, not a diagnosis — and a secondary-cause list her chart had already half answered.
Slide · in production
Reading the DEXA
A Z-score at thirty-eight is a referral, not a label. The list of reasons is standard; three of them were already on her chart.
Prefer the full lecture? Remodeling and Osteoporosis (original video on the site)
Explore
Read the report
Her 2013 DEXA report, scores hidden. Choose the score and the cut that apply at thirty-eight, run the secondary-cause checklist, tip the RANKL and OPG balance, then rewrite the report’s last line.
Text description of this explore
A report desk. Step 1: her 2013 bone densitometry report: female, 38, premenopausal, regular cycles, referred after a fall onto the wrist; a table of T-scores and Z-scores for the lumbar spine L1–L4, the femoral neck and the total hip, hidden; and the impression on file, “Perimenopausal bone loss. Calcium and vitamin D.” Choose which score and which cut apply (the Z-score, with Z at or below −2.0 below the expected range for age; the T-score, with T at or below −2.5 osteoporosis; the T-score, with −1.0 to −2.5 osteopenia expected around perimenopause; or the Z-score, with Z at or below −2.5 osteoporosis in a young woman). After you commit the illustrative scores appear: lumbar spine T −2.4, Z −2.3; femoral neck T −2.2, Z −2.1; total hip T −2.1, Z −2.0; her chart records only “spine and hip low for age”. Step 2: the standard workup for a low Z-score (ESR and CRP, calcium, 25-OH vitamin D, cortisol, protein electrophoresis, PTH, celiac screen, TSH, urine calcium, complete blood count), with a column for what her records show and a line for what was ordered in 2013 to explain the scan, both hidden until you commit. For seven items (low vitamin D, wandering calcium, a flat cortisol, a leg she spared at 11 to 15, raised ESR and CRP, MGUS, falling estrogen) choose whether it was on her chart by 2013, found later, or never there, and which lever it pulls (the calcium thermostat, a hormone, load, cytokines, or a plasma-cell clone). After you commit the table fills: ESR and CRP raised on every draw, calcium wandering, vitamin D low, cortisol flat, protein electrophoresis showing MGUS, found in 2015; ordered in 2013 to explain the scan, none of the ten. Step 3: a balance with an osteoblast at the top making RANKL (left pan) and OPG, the decoy (right pan), and controls for IL-1β (normal, raised, high), IL-6 (normal, raised), estrogen (normal or falling) and calcium and vitamin D tablets (none or taken); buttons load her 2013 picture and menopause. Predict which way it tips (toward RANKL, osteoclasts win; toward OPG; or level), then tip it: the pans fill with dots, the beam tilts, osteoclasts are drawn, and a readout gives RANKL, OPG, their ratio and the net bone a year (teaching model). Step 4: write the sentence the 2013 report should have ended with, then compare with a model answer.
Think · not graded
“Perimenopause” explains bone loss through falling estrogen; her estrogen was normal. List what that leaves, say which items were already on her chart, and explain how IL-1β and IL-6 thin bone through RANKL and OPG without any hormone changing.
Spot-check your thinking: true or false?
Check · Multiple Choice
Low bone density at thirty-eight with normal cycles and normal estrogen points to…
Evidence card 5
Add it to your evidence board
Low for age, estrogen intact, a thermostat under strain, markers raised, a leg spared, the cytokines that drive RANKL running since childhood — read as a Z-score, the 2013 scan was the fourth skeletal finding on one chart.
4
Your evidence board
Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: Inflammation-driven resorption — a secondary cause the Z-score demanded, Neither on its own, or Perimenopause, or a diet short of calcium. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.
5
Your verdict
Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.
Look back at your prediction
Thin bones at thirty-eight with normal hormones, wandering calcium and low vitamin D — the reading that fits best is…
Find the one you chose at the top. Does the evidence support it, refute it, or revise it?
- Early menopause — estrogen must be falling, whatever the labs say
- Not enough calcium in the diet — the supplement will fix it
- A secondary cause — something driving resorption that a Z-score at thirty-eight demands you find
Build your verdict · no writing
Choose the statement that holds in each set. The ones you keep become your argument.
6
The finding
Finding · the scan
Low for Age, and Nobody Asked Why
1986–90, ages 11–15: a loaded spine that should have gained; 2013, age 38: a DEXA low for age with normal cycles and normal estrogen, “perimenopause”, calcium and vitamin D. Chart entry: remodeling is a balance that load builds, estrogen guards and cytokines tip — IL-1β and IL-6 raise RANKL and lower OPG and FMF cohorts lose density with every year of delay; a Z-score at thirty-eight is a secondary-cause workup, and her chart already held wandering calcium, low vitamin D, raised markers and a spared leg. Held open: how a bone repairs a break — and what a bone does when the same cytokine inflames it instead.
Next · Chapter 7
Broken, Mended, Read Whole
She never broke a bone. This chapter mends one anyway — seven fractureBreaks in bones due to stress or injury. types, four steps, cartilageA flexible connective tissue found in joints, the ear, nose, and rib cage. Cartilage can be of three model to spongy to compact — and then lays the skeletal column of her chart end to end, from the mile at ten to the scan at thirty-eight, and reads it the way a rheumatologist finally did in 2026: Chart Clue #6.
Reliable information on rare diseases
NORD · NIH GARD · Orphanet · MedlinePlus Genetics: FMF · FMF & AID Global Association · Autoinflammatory Alliance · Global Genes · Amyloidosis Foundation
Stina has one rare disease. In the United States, about 1 in 10 people live with one of the more than 7,000 known rare diseases. These organizations are where patients, families and clinicians go for trustworthy information.
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← Familial Mediterranean Fever — case home
All pages for Module 6 — The Skeletal System
6.0 Module Overview
6.1 Growing Pains
6.2 The Fitness Test
6.3 Pillars and Trusses
6.4 She’ll Grow Out of It
6.5 The Joint That Swelled With the Fever
6.6 Five Percent a Week
6.7 Broken, Mended, Read Whole
6.8 Conclusion and Assessment
6.9 The Evidence Behind the Case
All Modules
- Anatomical Language, Membranes & Homeostasis
- Just Enough Chemistry
- The Cell & Its Transport
- Making Cells & Proteins
- The Integumentary System
- The Skeletal System
- The Muscular System
- Nervous Tissue & the Senses
- The Spinal Cord
- The Brain & the Blood–Brain Barrier
- The Autonomic Nervous System
- Special Senses (in developmentThe process of growth and differentiation.)
- The Endocrine System
- Blood
- The Heart
- Blood Vessels
- The Digestive System
- The Respiratory System
- The Urinary System
- Fluids, Electrolytes & Acid–Base Balance
- The Reproductive System
- The Immune System
List of terms
- spine
- calcitonin
- Osteoclasts
- cortisol
- anatomy
- physiology
- spongy bone
- compact bone
- endosteum
- menopause
- glucocorticoids
- canaliculi
- calcitriol
- cells
- trabecula
- skin
- liver
- hypocalcemia
- hypercalcemia
- neck
- osteocyte
- fracture
- cartilage
- development