Fluid & Electrolyte Balance — INBDE Review
Body fluid compartments, sodium and potassium imbalances with their classic signs, calcium regulation by PTH and vitamin D, and acid-base buffering. 11 board-style MCQs.
Concept summary & clinical relevance.
Quick-reference structure first, then nerve-by-nerve detail. Mnemonics in amber, clinical pearls in blue.
Electrolyte questions on the INBDE focus on three patterns: where each ion lives (ICF vs ECF), what symptoms appear when the ion is too high or too low, and how diuretics and hormones change the balance. Calcium gets extra attention for dental relevance — it's the foundation of enamel and dentin and a critical clotting cofactor.
| Compartment | % of TBW | Major cation | Notes |
|---|---|---|---|
| Total body water (TBW) | ~60% body weight | — | Lower in elderly, obese, women |
| Intracellular fluid (ICF) | ~2/3 of TBW | K⁺ | Largest compartment |
| Extracellular fluid (ECF) | ~1/3 of TBW | Na⁺ | Plasma + interstitial |
| Plasma | 1/4 of ECF | Na⁺ | Inside vessels |
| Interstitial fluid | 3/4 of ECF | Na⁺ | Between cells |
| Imbalance | Classic findings | Common causes |
|---|---|---|
| Hyponatremia | Seizures, confusion, weakness | SIADH, thiazides |
| Hypernatremia | Thirst, dry mucosa, irritability | Dehydration, diabetes insipidus |
| Hypokalemia | Muscle weakness, arrhythmia, flat T waves, U waves | Loop/thiazide diuretics, vomiting |
| Hyperkalemia | Peaked T waves, fatal arrhythmia risk | K⁺-sparing diuretics, ACE-I, CKD |
| Hypocalcemia | Tetany, Chvostek/Trousseau signs, perioral tingling | CKD (low active vit D), hypoparathyroidism |
| Hypercalcemia | “Stones, bones, groans, psychiatric overtones” | Hyperparathyroidism, malignancy |
| Hormone | Action |
|---|---|
| ADH (vasopressin) | ↑ water reabsorption (collecting duct aquaporins) |
| Aldosterone | ↑ Na⁺ reabsorption, ↑ K⁺ secretion |
| ANP | Opposes RAAS → ↑ Na⁺ and water excretion |
| PTH | ↑ plasma Ca²⁺, ↓ plasma phosphate (renal effects) |
| Vitamin D (calcitriol) | ↑ Ca²⁺ AND phosphate absorption from gut |
| Calcitonin | ↓ plasma Ca²⁺ (minor role in adults) |
Sodium
- Major ECF cation; principal determinant of ECF volume.
- Hyponatremia → seizures, confusion, weakness; classic causes include SIADH and thiazides.
- Hypernatremia → thirst, dry mucosa, irritability; usually from water loss (dehydration, diabetes insipidus).
Potassium
- Major ICF cation; controls cardiac and skeletal muscle excitability.
- Hypokalemia → muscle weakness, arrhythmias, flat T waves, U waves. Caused by thiazides, loop diuretics, vomiting/diarrhea.
- Hyperkalemia → peaked T waves; risk of fatal arrhythmia. Caused by K⁺-sparing diuretics, ACE inhibitors, CKD.
Calcium
- 99% in bone; ~1% in plasma. Regulated by PTH, vitamin D, and (minor in adults) calcitonin.
- PTH ↑ Ca²⁺ and ↓ phosphate (acts on bone, kidney, and indirectly via vitamin D activation).
- Vitamin D (calcitriol): ↑ Ca²⁺ AND phosphate absorption from the gut.
- Hypocalcemia: tetany, Chvostek/Trousseau signs, perioral and finger tingling.
- Hypercalcemia: stones, bones, groans, psychiatric overtones.
- Dental relevance: enamel and dentin mineralization; clotting cofactor (factor IV).
Acid-base & buffers
- Bicarbonate (HCO₃⁻) is the major plasma buffer; works with CO₂ (lungs) and renal HCO₃⁻ reabsorption (kidneys).
- Chloride is the major ECF anion, often shifting in parallel with sodium.
11 board-style MCQs.
Active recall is the highest-yield study method for the INBDE. Pick an answer, check it, and read why every distractor is wrong — that's where the learning compounds.
The MCQs above are Core Recall — testing what you've memorized. The book adds a full Clinical Integration set: board-style patient scenarios where you apply this anatomy to real clinical reasoning. That's the section the INBDE actually weights heaviest.
- Question 1EasyWhich fluid compartment contains the largest proportion of total body water?
- Question 2EasyWhich ion is the major extracellular cation?
- Question 3EasyWhich ion is the major intracellular cation?
- Question 4ModerateWhich clinical feature is most associated with hyponatremia?
- Question 5ModerateWhich ECG finding is typical of hyperkalemia?
- Question 6ModerateWhich symptom is most associated with hypokalemia?
- Question 7EasyWhich ion is most important for enamel and dentin mineralization?
- Question 8ModerateWhich hormone raises plasma calcium while lowering plasma phosphate?
- Question 9ModerateWhich clinical feature is most typical of hypocalcemia?
- Question 10ModerateWhich phrase best summarizes the clinical features of hypercalcemia?
- Question 11EasyWhich buffer system is most important for maintaining blood pH?
900 INBDE-style MCQs with full explanations across 18 chapters — Core Recall plus board-style Clinical Integration scenarios — alongside Learning Summaries, Integration Bridges, and Review Boxes. Built by Dr. Isaac Sun for dental students who want to think like a clinician, not just memorize.
Founder, KYT Dental Services · Author, KYT INBDE series. These MCQs and Learning Summaries are part of a structural-thinking framework Dr. Sun uses with patients in the chair.