Most fluid and electrolyte questions that feel unfair are questions about two electrolytes disguised as questions about one. The potassium that will not come up. The tetany in a patient whose calcium is normal. The sodium of 128 that nobody treats. Each has the same shape: one value is coupled to another, and the exam is checking whether you know the coupling. Here are the six that generate most of those questions, with the mechanism and the nursing move for each.
Why will potassium not correct until magnesium is replaced?
Magnesium sits on the ROMK channel in the kidney’s distal tubule and blocks it. When magnesium is low, the block comes off, the channel dumps potassium into the urine, and every dose you infuse leaves almost as fast as it goes in. That is refractory hypokalemia, and the fix is to replace magnesium first — the potassium often comes up on its own afterwards. The same deficiency suppresses parathyroid hormone, so calcium refuses to correct too. On a question, “low potassium and low magnesium” means magnesium is the priority replacement.
Try it · hang potassium, then magnesium
Mg²⁺ and K⁺
See the whole patient respond.
Click or hover any value to see why it changed.
Built for pattern recognition
Solid chips are direct physiology. Dashed chips are common clinical pairings — not automatic cause and effect.
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Bedside monitor · pattern
Hypomagnesemia
Low magnesium removes a membrane brake and unblocks renal potassium wasting, so potassium and calcium fall with it.
QTc 489 msFlattening T waves and an emerging U wave — the earliest ECG evidence that potassium is low.
Extracellular volume at baseline. The dashed line marks euvolemia.
What the nurse would notice
Hypomagnesemia across body systems
Prolonged QT, torsades de pointes, digoxin sensitivity
Tremor, cramps, tetany, hyperreflexia, Chvostek and Trousseau
Agitation, confusion; seizures when severe
Nausea, anorexia, decreased motility
What the nurse does
IV magnesium sulfate on a pump. Replace magnesium before or with potassium and calcium — neither holds until it is back.
Continuous ECG for QT and torsades, reflexes during replacement, potassium and calcium afterwards.
Magnesium-rich foods; in alcohol-use disorder the deficit is expected. Report palpitations and tremor.
Teaching model, not a clinical calculator. Patterns show high-yield directional relationships under the stated assumption. Real symptoms, ranges and treatment depend on cause, acuity, comorbidities and local protocols.
Signs & symptoms
Present on this panel. Tap any sign to see which values produce it.
How to think through fluid and electrolytes
Do not memorize eight disconnected lists. Move one value, follow what it changes, and connect the lab pattern to the symptoms you see at the bedside.
Excitable or suppressed?
Low calcium or magnesium removes the brakes: tingling, cramps, hyperreflexia, tetany, seizures. High levels add too much brake: weakness, hyporeflexia, lethargy, respiratory depression.
What can stop the heart?
Potassium and magnesium deserve rhythm attention. In dangerous hyperkalemia, stabilize the myocardium first, then shift potassium into cells, then remove it from the body.
Is circulation failing?
A dramatic lab can distract from volume loss. In DKA and major GI loss, restoring circulating volume comes before the correction students notice first.
Why values move together
- Mg²⁺ → K⁺ ↓ 0.4 — Magnesium blocks the ROMK channel in the kidney. When magnesium falls the block comes off, the channel dumps potassium into the urine, and every dose of potassium you give follows it out.
- Mg²⁺ → Ca²⁺ ↓ 0.7 — Magnesium is required to release parathyroid hormone and for bone to answer it. Low magnesium therefore drags calcium down and keeps it there — calcium will not correct until the magnesium does. The effect is one-directional: a high magnesium does not push calcium up.
- Ca²⁺ → PO₄³⁻ ↑ 0.3 — The same seesaw from the other end: parathyroid hormone raises calcium and dumps phosphate into the urine, so the two move opposite each other.
- Ionized Ca²⁺ 4.34 — Total calcium reads normal but alkalosis has bound the ionized fraction to albumin — the patient tingles anyway.
Normal values
The reference bands this tool uses. Switch to Set and mark them from memory.
Reference ranges vary slightly between laboratories and textbooks. These match the ranges used across NurseSavvy.
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Why do calcium and phosphate move in opposite directions?
They bind each other. When phosphate rises — classically in kidney failure — it complexes with calcium and pulls the ionized calcium down, so the symptoms of hyperphosphatemia are really hypocalcemia: tingling, cramps, tetany, a long QT. When their product (calcium times phosphate, both in mg/dL) climbs past about 55, the salts precipitate into vessels and soft tissue. The nursing logic follows: phosphate binders with meals, and caution about pushing calcium while the phosphate is still high.
How does pH move potassium?
Hydrogen ions and potassium trade places across the cell membrane. In acidosis, hydrogen moves into cells and potassium comes out, so the serum potassium rises — roughly half a point for every 0.1 fall in pH in a mineral acidosis, less predictably in DKA. In alkalosis the trade runs the other way and the potassium falls. Two exam consequences: the high potassium in DKA is a shifted potassium in a patient who is actually depleted, and it will crash the moment insulin starts; and bicarbonate, by raising the pH, lowers the potassium — which is why it is one of the shift treatments for hyperkalemia.
Why does alkalosis cause tetany when the calcium is normal?
About half of serum calcium rides on albumin, biologically inactive. Alkalosis increases that binding, so the ionized calcium — the only fraction the membrane feels — falls while the total calcium reads normal. The patient gets perioral tingling, a positive Trousseau sign and carpopedal spasm at a calcium of 9.5. Treat the alkalosis and order an ionized calcium; do not chase the total. The reverse also matters: acidosis frees calcium from albumin, so a low total calcium in an acidotic patient may be less dangerous than it looks.
Why is sodium a water problem?
Serum sodium is a concentration, so it reads low when there is too much water and high when there is too little, regardless of how much sodium the body holds. That single idea decides treatment. Hyponatremia with normal or high volume — SIADH, heart failure, thiazides — is treated by restricting water; hyponatremia with volume depletion is treated with isotonic saline. Hypernatremia is nearly always a water deficit and is treated with free water, slowly. And a high glucose dilutes the sodium about 1.6 mEq/L per 100 mg/dL above normal, so the sodium of 128 in a DKA patient with a glucose of 620 corrects to roughly 136 and needs no treatment at all.
Why does low potassium make digoxin dangerous?
Digoxin and potassium compete for the same binding site on the sodium–potassium pump. When potassium is low, digoxin binds more avidly, and toxicity — nausea, visual halos, bradycardia, new arrhythmias — appears at a therapeutic digoxin level. The nursing rule is mechanical: check the potassium before the dose, and treat a potassium under 3.5 in a digoxin patient as a hold-and-call finding.
How to make these permanent
Do not read them again. Break each one on the simulator: set magnesium to 1.0 and hang potassium; drag phosphate to 7 and watch calcium fall; drag the pH and watch potassium move the other way; drag bicarbonate up and watch the ionized calcium drop while the total sits still. Five minutes of that beats an hour of rereading — the evidence for why is in the best way to study for nursing school, and the whole unit’s method is in how to learn fluid and electrolytes.
Practise the two-electrolyte questions
Free account, no card. Questions built on exactly these couplings, with a rationale on every option — including the ones that look right.