What this page covers, and what it doesn't.NG-054 (Fluid Balance I) gave you the three compartments and the
hormones that run them. This page is the mechanism page: the four pressures that decide which compartment
water sits in, every route water leaves the body, the three big “drains” (DI, DKA, diuretics), third spacing, and
how the body compensates while it happens.
NG-059 (Fluid Balance III) is next: how you measure and monitor all
of it at the bedside. For the finished clinical pictures see
NG-191 · FVD,
NG-169 · FVO and
NG-207 · FVO vs FVD side by side.
💧 3 roomsICF 2/3 · ECF 1/3 (interstitial ~15% + plasma ~5% of body weight).
⚖️ 2 forcesHydrostatic PUSHES water out. Albumin PULLS water in. Everything else is detail.
🕳️ 3 D'sDiabetes Insipidus · DKA · Diuretics — all three drain the tank through the kidney.
🛡️ Compensation liesPulse rises FIRST. A normal BP does not mean a normal volume.
💧
WHERE WATER MOVES, AND WHY
STEP 1 · MECHANISM
Water never moves “because”. It moves because something pushed it or something pulled it.
🗺️ The three rooms, drawn to scale
Total body water is roughly 60% of adult body weight —
less in older adults and in people with more body fat, more in infants.
🧠 “Two thirds in, one third out.” Two thirds of your water is INSIDE cells.
Only about 1/12 of it is in the bloodstream — which is why small losses hit the BP hard.
⚖️ The tug-of-war — the four Starling forces
At the arterial end, pressure wins and fluid filters out to feed the tissue.
At the venous end, albumin wins and most of it is pulled back. Whatever is left over is collected by the
lymphatics. Break any part of that and you get edema.
Hydrostatic pressureThe push of fluid against a wall. In a capillary it is basically the blood pressure. High hydrostatic pressure = fluid squeezed OUT into the tissue.
Oncotic (colloid) pressureThe pull of large proteins — mainly albumin — that cannot leave the vessel. It drags water back in behind it.
Heart failure and venous obstruction raise hydrostatic pressure. Liver failure,
nephrotic syndrome and malnutrition drop albumin. Both end in edema, for opposite reasons.
🧠 “Hydro = hose. Onco = sponge.” A hose sprays water out; a sponge sucks it back.
🚪 Four ways things cross a membrane
PROCESS
WHAT MOVES
DRIVEN BY
Osmosis
WATER only
Concentration of particles — water goes to the saltier side
Diffusion
Solutes
High concentration → low concentration
Filtration
Water + solutes together
Hydrostatic PRESSURE (higher → lower)
Active transport
Solutes, uphill
ATP — e.g. the sodium–potassium pump
Only osmosis moves water on its own. That is why sodium controls where water sits: water chases sodium.
🧠 “Where sodium goes, water follows.” Sodium is the ECF's water magnet.
🧠 “HypO = O for swOllen. HypER = shrivEled.” Hypotonic swells the cell; hypertonic shrivels it.
🅰️ Albumin is the whole story of oncotic pressure
Typical adult albumin is 3.5–5.0 g/dL (varies by lab). It is made in the
liver, and it is too big to slip out of a healthy capillary — so it stays in the plasma and holds water
there like a magnet.
Liver failure → cannot make it.
Nephrotic syndrome → loses it into the urine.
Severe burns → leaks it out of damaged capillaries.
Malnutrition → no protein to build it from.
All four end with the same picture: edema everywhere and an empty circulation at the same time.
🧠 “No albumin, no magnet, no water in the pipes.”
🧪 Osmolality — the one number for “thick or thin?”
Serum osmolality measures how crowded the blood is with particles. Typical adult reference is about
275–295 mOsm/kg, and it varies by lab.
HIGH osmolalityBlood is concentrated → the patient is dry, or the sodium/glucose is high.
LOW osmolalityBlood is dilute → too much water, or the sodium is low.
Sodium is the biggest single driver, which is why a quick mental estimate of
osmolality is roughly double the sodium plus a small contribution from glucose and urea.
🧠 “Osmolality follows sodium.” Sodium up → osmolality up → thirst and ADH switch on.
🕳️
EVERY WAY FLUID IS LOST
STEP 2 · CAUSE
Four big buckets: out of the gut, out of the kidney, out of the skin, out of the vessels.
🗺️ The map of losses — sensible and insensible
🧠 “Sensible = you can measure it. Insensible = you can't.” Sweat you can see is sensible;
the water vapor leaving your lungs and skin all day is not.
Every one of these is isotonic loss to start with — whole fluid goes out, water
and electrolytes together — so the sodium can look completely normal in a patient who is badly dry.
🧠 “Gut · kidney · skin · heat.” Four boxes. If you can list them you have the exam question.
🚽 THE 3 D's — where each one drains the tank
🧠 DI = “Dry Inside.” DKA = “D for Dry.” Diuretics end in “-ide” = the body is dr-ied.
💧 D #1 — Diabetes insipidus
Not diabetes mellitus. There is no sugar problem here at all — the problem is no ADH
(or kidneys that cannot respond to it).
The collecting duct cannot pull water back → liters of very dilute urine.
Urine specific gravity is LOW (dilute); serum sodium and osmolality climb.
The patient is desperately thirsty and cannot keep up.
🧠 “DI = Dilute In the toilet, Dry Inside.” Opposite of SIADH, which holds water in.
🍬 D #2 — DKA and osmotic diuresis
Blood glucose climbs so high that sugar spills into the filtrate. Sugar is an osmotic particle, so it
drags water out with it into the urine.
Kussmaul respirations (deep and fast) blow off acid — and add to the insensible water loss.
Often triggered by an infection, which adds fever and more losses on top.
The patient loses water, sodium and potassium at the same time, so the potassium picture is a trap:
total body potassium is low even when the first lab looks normal or high.
🧠 “Sugar is a sponge in the urine.” It carries water out of the body with it.
💊 D #3 — Diuretics
These are the losses we cause on purpose — and the ones we most often overshoot.
Loop diuretics (e.g. furosemide) act on the loop of Henle. Strongest effect; potassium is wasted.
Thiazides (e.g. hydrochlorothiazide) act on the distal tubule. Gentler; potassium also wasted.
Potassium-sparing agents lose water without losing potassium — the risk flips to hyperkalemia.
Never give a scheduled diuretic without checking the potassium, the blood pressure and yesterday's weight.
Bowel lumen → ileus and obstruction (liters can sit in a paralyzed gut)
Injured tissue → burns, crush injury, big surgical fields, pancreatitis
🧠 “First cell, second vessel, third nowhere useful.”
🚨 Why it is dangerous
Third-spaced fluid does not circulate. So the patient can be:
Fluid-overloaded on the outside — swollen, heavy, tight skin, a big belly.
Fluid-depleted on the inside — tachycardic, hypotensive, oliguric, poorly perfused.
Give a diuretic to “treat the swelling” and you pull the little volume they have left out of the pipes.
Never assume visible swelling means the circulation is full.
🧠 “Swollen ≠ full.” Check the pulse, the BP and the urine output before you judge volume.
⏳ It happens in two phases — and the second one is the trap
1
LOSS phase. Fluid shifts out of the vessels into the third space. The patient looks dry inside: fast pulse, low BP, low urine output — while gaining weight.
2
MOBILIZATION phase. Days later the fluid shifts back into the circulation. Now the risk flips to fluid OVERLOAD — crackles, bounding pulses, rising BP, big diuresis.
The IV fluids that were exactly right in phase 1 can drown the patient in phase 2.
Watch daily weights and lung sounds across both phases.
🧠 “Out, then back.” Fluid that left the pipes always tries to come home.
🧪 What the labs do in third spacing
Hematocrit rises — plasma left the vessel but the red cells didn't, so the blood is concentrated.
BUN rises out of proportion to creatinine — a classic pre-renal pattern.
Albumin is often low — and low albumin is frequently what caused the shift in the first place.
Sodium is unreliable — it depends on what was lost with the water.
🧠 “Concentrated blood, empty patient.” High Hct + low BP = the fluid is somewhere else.
✅ What the nurse actually does
Daily weight, same scale, same time, same clothing — the single best measure.
Abdominal girth daily, marked at the same spot, if ascites is present.
Strict I&O, including drains and ostomy output.
Vital signs and urine output trends — these tell you about the circulation, not the swelling.
Skin care — edematous skin tears and breaks down fast.