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Module 20 · Anatomy & Physiology II
One leaking filter behind every number.
Swollen ankles, a calcium flagged low, a bicarbonate(HCO₃⁻) – A crucial buffer in blood that helps maintain pH balance; formed when carbon dioxide react drifting down — three findings, two pieces of advice, one visit in 2025. This module builds fluid, electrolyte and acid–baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻). physiologyThe study of how the body functions. from the body’s three waterThe universal solvent essential for life. tanks to the arterial blood gas, reads the kidney’s own slides and ledger, and shows that all three findings are the same leaking, stiffening filter read from three angles.
The module’s question
Three numbers off, each with a sensible fix of its own. What would have to be true for one organ to have written all three — and what would treating the organ, instead of the numbers, have looked like?
Module trailer · about 2 minutes
One Leaking Filter Behind Every Number — narrated, with captions and a full text description.
The module trailer. Watch it first.
Animation · in production · 2 min
One Leaking Filter Behind Every Number
Two minutes: a sock that leaves a ring, two pressures at every capillary, a protein that carries nearly half the calcium, a reaction with two ends, and a visit note with two errands.
How every chapter works
- 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
Read the chart and make a private prediction → the story → the scenes → sort your evidence → build your verdict → the finding. Nothing is graded, nothing is recorded, nothing is locked.
Where this module sits in Stina’s life
2013 · 38
DEXA low · “perimenopause”
2015 · 40
Immunologist: MGUS
2024 · 49
“A little protein, recheck sometime”
2025 · 50
Edema · albuminA plasma protein that helps maintain osmotic pressure and transport substances. 2.4 · “low” Ca · HCO₃⁻ 20 · Clue #20
Jan 2026 · 50
Sterile pyuria
11 May 2026 · 51
FMF diagnosed · colchicine
Summer 2026 · 51
Renal biopsy: AA amyloid
Ochre = chart clue · Purple = diary entry.
The case this module reads
Fifty years old, a year after “a little protein, recheck it sometime”: socks leaving rings in both ankles by evening, a puffy face on waking, three kilograms in two months. Albumin 2.4; total calcium 7.9, flagged low; bicarbonate 20; 3.8 g of albumin-rich protein in the urineThe liquid waste excreted by the kidneys. a day; creatinine 1.3, “nearly normal”, with an estimated GFR of 48. Sodium 138 and a normal osmolality; no thirst; cellsThe basic structural and functional units of life. neither shrunken nor swollen. Renin and aldosteroneA hormone that increases sodium and water reabsorption in the kidneys, helping regulate blood pressu high, urine sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. low. At the next visit: ionized calcium 1.21, normal; PTH normal; an arterial gas of pHA measure of hydrogen ion concentration in a solution. 7.33, PCO₂ 33, bicarbonate 19, breathing a little deeper than she noticed. The visit closed with two errands: drink less salt, take some calcium.
Patient chart · Stina · Chart Clue #20
Exam
Pitting ankle and periorbital edemaExcess fluid in interstitial spaces.; +3 kg
Labs
Albumin 2.4; total Ca 7.9 (ionized 1.21); HCO₃⁻ 20
Urine
Protein 3.8 g/day; sodium low
Gas
pH 7.33 · PCO₂ 33 · HCO₃⁻ 19
Kidney
Creatinine 1.3 · eGFR 48 · amyloid on biopsy (2026)
Seven chapters, seven decisions
Each chapter opens on the chart and ends with a verdict you build from evidence. Work them in order — later chapters assume the cards you collected earlier.
1
20.1 The Swollen Ankles That Point Back to the Kidney
Three numbers off, each with a sensible fix of its own. What would have to be true for one organ to have written all three — and how would you tell?
You decide: two errands, or one organ?
2 animations · 3 slides · 5 things to explore · 10 quick checks · 1 verdict
2
20.2 The Body’s Fluid Compartments & Water Movement
Water goes where the solute is. Which compartment did her water leave, which did it flood — and does that alone tell you whether salt is to blame?
You decide: which tank did the water leave — and does that blame the salt?
3 animations · 2 slides · 5 things to explore · 10 quick checks · 1 verdict
3
20.3 Hydrostatic vs Oncotic Pressure
Hydrostatic pushes out, oncotic pulls in, lymph drains the difference. When the pull weakens, why does the kidney make it worse — and what restores it?
You decide: the push, the pull, or the wall?
2 animations · 3 slides · 5 things to explore · 10 quick checks · 1 verdict
4
20.4 The Major Electrolytes & the Albumin-Corrected Calcium
A total calcium of 7.9 with an albumin of 2.4. Is she short of calcium, or short of the protein that carries it — and how do you tell without a second test?
You decide: short of calcium, or short of its carrier?
3 animations · 2 slides · 5 things to explore · 10 quick checks · 1 verdict
5
20.5 Acid–Base Balance & the Bicarbonate Buffer System
The bicarbonate reaction runs both ways, the lungs on one end and the kidneys on the other. If one organ fails, whose job cannot be covered — and how would the numbers say so?
You decide: which end of the reaction moved first?
3 animations · 2 slides · 5 things to explore · 10 quick checks · 1 verdict
6
20.6 Metabolic Acidosis from a Failing Kidney
When the kidney cannot fix an acidosisA condition where blood pH falls below 7.35., the lungs try. Why can they only get partway — and what can a healthy kidney do that they cannot?
You decide: why can the lungs only get partway?
3 animations · 2 slides · 5 things to explore · 10 quick checks · 1 verdict
7
20.7 One Filter, Not Three Problems
Edema, a “low” calcium and an acidA substance that releases hydrogen ions (H⁺) in solution. drift — three problems, or one? Which single failure could write all three, and what would treating it instead of them have looked like?
You decide: three problems, or one filter?
2 animations · 3 slides · 5 things to explore · 10 quick checks · 1 verdict
20.8 · Conclusion and Assessment
The module verdict, then everything to practice
20.9 · The Evidence Behind the Case
Check our reasoning against the literature
Stina’s diary
The entries behind this module
“Perimenopause” (38) · “Undetermined significance” (40) · “Fever of unknown origin” (41) · “A little protein” (49) · “Colonized” (50).
The original lectures and histology
Eight mini-lectures, one simulator, one game, one IV page
Every original fluids mini-lecture on bettybroadbent.com (Introduction through Renal Clearance and Transport Max) is linked from the scene it belongs to, with the Osmolarity/Tonicity/IV Fluids mini-lecture, the Fluid & Electrolyte Imbalance Simulator and its six patients, the 3-level ABG game, the Essays on Osmosis page, the Calcium Homeostasis lecture, the glossary and flashcard maker, and the Urinary Histology pages the resources page links to (there is no fluids histology on the site — the kidney’s slides stand in).
By the end you can
- Describe the body’s fluid compartmentsDivisions of body water: intracellular, extracellular, and transcellular. by volume and composition, distinguish electrolytes from non-electrolytes and osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments. from osmolality and tonicityThe ability of a solution to affect the water balance in a cell., and predict fluid shifts between compartments.
- Explain water balance — intake, output, obligatory loss, thirst, ADH and aldosterone — and recognise dehydrationA condition in which fluid loss exceeds intake, leading to a decrease in total body water., hypotonic hydrationExcess water intake dilutes ECF, causing water to move into cells (ICF), leading to swelling. and edema as three different disorders.
- Explain hydrostatic and oncotic pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. at a capillary, the lymphatic return and the four mechanisms of edema, and predict the effect of hypoalbuminemia and the kidney’s response to it.
- Identify the six major electrolytes, their hormonal controlsVariables that remain constant to ensure a fair test. and the signsObjective clinical findings observable by a provider (e.g., edema, fever). of their excess and deficit, and calculate and interpret an albumin-corrected calcium.
- Explain the sources of acid, the three tiers of pH defense and the bicarbonate buffer system’s two ends, and identify the proximalCloser to the point of attachment or origin. tubule and collecting-duct cells that reclaim and manufacture bicarbonate on a slide.
- Read an arterial blood gas with the three-question method, distinguish metabolic from respiratory disturbances and partial from complete compensation, and integrate edema, hypoalbuminemia and acidosis as consequences of one renal lesion.
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.
Hop to:
← Familial Mediterranean Fever — case home
All pages for Module 20 — Fluids, Electrolytes & Acid–Base Balance
20.0 Module Overview
20.1 The Swollen Ankles That Point Back to the Kidney
20.2 The Body’s Fluid Compartments & Water Movement
20.3 Hydrostatic vs Oncotic Pressure
20.4 The Major Electrolytes & the Albumin-Corrected Calcium
20.5 Acid-Base Balance & the Bicarbonate Buffer System
20.6 Metabolic Acidosis from a Failing Kidney
20.7 One Filter, Not Three Problems
20.8 Conclusion and Assessment
20.9 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
- bicarbonate
- base
- physiology
- water
- albumin
- urine
- cells
- aldosterone
- sodium
- pH
- edema
- acidosis
- acid
- fluid compartments
- osmolarity
- tonicity
- dehydration
- hypotonic hydration
- pressure
- controls
- signs
- proximal
- development