Nursing Field Notes / Renal + Fluid · Fundamentals of Nursing

Fluid Balance 💧

Where body water lives · how it moves · who controls it

NG-054 RENAL + FLUID ADHD-friendly visual edition

Water is not stored in one bucket — it lives in three. Two-thirds sits inside your cells; one-third sits outside them, and that outside third is split between the space around cells and the blood vessels. Every fluid disorder you will ever be tested on is just water moving between those three tanks.

📄 Simple Nursing original — opens in Drive →

💧 60 · 40 · 20Total body water ≈ 60% of body weight · 40% inside cells · 20% outside.

🪣 ⅔ in · ⅓ outICF = ⅔ of body water. ECF = ⅓ — and that ⅓ is ¾ interstitial, ¼ intravascular.

⚖️ 1 kg = 1 L = 2.2 lbDaily weight is the single best fluid-status measure. Same scale, same time, same clothes.

🎛️ Hold vs. dumpADH + aldosterone HOLD water. ANP/BNP DUMPS it. Thirst tops you up.

🫗

COMPARTMENTS

STEP 1 · THE THREE TANKS

Learn the three tanks and the doors between them — then every fluid problem is just “which way is the water going?”

🗺️ The map: total body water, split three ways

ICF ⅔ · ECF ⅓ · and ECF splits ¾ interstitial / ¼ intravascular. In a 70 kg adult that is roughly 42 L of water total.

Water crosses the capillary wall by pressure (filtration & reabsorption) and crosses the cell membrane by osmosis. Sodium stays outside, potassium stays inside — water chases sodium.

🧠 “⅔ – ¾ – ¼.” Two-thirds of your water is in the cells. Of what's left, three-quarters bathes the cells and only one-quarter is in the pipes. That tiny quarter is the one that keeps your blood pressure up — which is why losing “just a little” volume hits so hard.

🧊 ICF — intracellular fluid ⅔

≈ 28 L in a 70 kg adult · 40% of body weight.
Main cation: potassium K⁺ (also Mg²⁺, phosphate).
This is where cells do their chemistry — swelling or shrinking it changes how the brain works first.

🧠 “K stays in the Kell.” Potassium = inside the cell. That's why cell death (crush injury, burns, tumor lysis) dumps K⁺ into the blood and causes hyperKalemia.

🌊 ECF — extracellular fluid ⅓

Interstitial ≈ 10.5 L — the puddle around cells. This is the space that puffs up as edema.
Intravascular (plasma) ≈ 3.5 L — inside vessels. Drives BP, pulse and perfusion.
Transcellular ≈ 1 L — CSF, synovial, pleural, peritoneal, pericardial, GI secretions. Where “third spacing” hides.

Main cation: sodium Na⁺; main anion Cl⁻.

🧠 “Na⁺ is the OUTsider.” Sodium waits outside the cell holding the water hose. Wherever sodium goes, water follows.

🚪 Four ways things move — and which one moves WATER

Diffusion — solutes move from greater to lesser concentration until equal.
O₂ & CO₂ swapping in the lungs

Osmosis — water moves from lower to higher solute concentration.
large intestine reabsorbing water

Filtration — both fluid and solutes, from high hydrostatic pressure to low.
the glomerulus filtering blood

🔋 Active transport is the fourth: it burns ATP to push ions against their gradient. The sodium–potassium pump shoves 3 Na⁺ out for every 2 K⁺ in — that pump is the whole reason Na⁺ is an outsider and K⁺ an insider.

🧠 “Solutes Diffuse, Water Osmoses, Pressure Filters, ATP Pumps.” If the question says water moved — it was osmosis. If it says water and particles moved because of pressure — it was filtration.

🧪 Osmolality — the one number that says “thick or thin?”

Osmolality = the concentration of a solution. High osmolality = more solute, heavier, thicker. Low osmolality = fewer solutes, lighter, more dilute.

Normal serum osmolality: 275–295 mOsm/kg (adult reference range). Sodium is the biggest driver — a quick bedside estimate is 2 × Na⁺.

Read those scales like a see-saw: too much water pushes every one of them LOW (diluted). Too little water pushes every one of them HIGH (concentrated).

🧠 “Low when Liquidy, High when Dry.” One sentence covers osmolality, sodium, hematocrit, BUN and urine specific gravity. Memorize it now — it is the backbone of the whole fluid unit.

🧫 Tonicity — what a solution DOES to a cell

🧠 “HypO = O for swellO. HypER = shrivER.” Or picture a raisin: put a raisin in plain water (hypotonic) and it plumps; put a grape in syrup (hypertonic) and it shrivels into a raisin.

🔁

INTAKE & OUTPUT

STEP 2 · THE LEDGER

Roughly 2,500 mL in, roughly 2,500 mL out. When the two columns stop matching, the patient's weight tells you first.

⚖️ The daily balance sheet ≈ 2,500 mL EACH WAY

Skin and lung losses are insensible — you cannot measure them, but they climb fast with fever, tachypnea, diaphoresis, burns and open wounds.

🧠 “2.5 in, 2.5 out.” If output drops below intake for two shifts running, the water is going somewhere — look for edema, crackles and a rising weight.

⭐ Daily weight is the BEST fluid indicator

1 kg = 1 L = 2.2 lb of fluid. A scale catches fluid changes that I&O charting misses.

⏰ Same time — early morning, after voiding, before breakfast.
⚖️ Same scale, same amount of clothing, every day.
📈 Report a gain or loss of 2–3 lb in 1 day or 5 lb in 1 week.

🧠 “Weight gain = water gain. Weight loss = water loss.” Overnight, a body cannot build or burn 3 lb of tissue — it can only hold or lose water.

🚨 The number that means “kidneys still working”

Minimum acceptable urine output: 30 mL/hr — about 0.5 mL/kg/hr in an adult.

Under 30 mL/hr = oliguria. Think: not enough volume reaching the kidney, or the kidney itself is failing. Never “just keep watching” two hours of oliguria — report it.

🧠 “30 or Tell.” Under 30 mL an hour, you tell somebody.

📋 What actually counts on the I&O sheet

INTAKE: oral liquidsice chips (count ½ the volume)soup, gelatin, ice cream, popsiclesIV fluids & IV medstube feeds + flushesirrigants that stay in

OUTPUT: urineemesisdiarrhea / liquid stoolNG & other suctionsurgical drainswound & fistula drainageblood loss

Anything liquid at room temperature is intake. A cup of gelatin is fluid, not food.

🧠 “If it melts, it counts.” Ice chips, popsicles, ice cream, gelatin — melt it and it's a fluid.

⚠️ Third spacing — the fluid you can't find

Fluid leaves the vessels and gets trapped in a space that does no work: the abdomen (ascites), the gut lumen, the pleural space, or burned tissue.

😵 The scale goes UP and the abdomen swells, but the patient is intravascularly DRY — tachycardic, hypotensive, oliguric.
🩸 Causes: burns, sepsis, peritonitis, bowel obstruction, cirrhosis, pancreatitis, major surgery, low albumin.
🔄 Phase 2 is the trap: a few days later the fluid pours back into the vessels → sudden fluid overload, ↑BP, crackles. Watch for it.

🧠 “Third space = wrong space.” The water is in the body (weight up) but not in the pipes (BP down). Trust the vital signs over the weight.

📈 Who dries out fastest — age & body build

👶 Infants ≈ 70–80% water, high metabolic rate, large body surface area, immature kidneys → dehydrate in hours.
👵 Older adults ≈ 45–50% — less muscle, blunted thirst, decreased kidney concentrating ability → high risk, quiet symptoms.
💪 Muscle holds water; fat does not. More muscle = more total body water.
🚻 Men average more total body water than women because they average more muscle and less fat.

🧠 “Muscle is a sponge, fat is a raincoat.” The more muscle, the more water on board — so babies (mostly water) and frail elders (mostly not) are the two ends of the danger scale.

🎛️

THE CONTROLS

STEP 3 · WHO TURNS THE TAPS

Four hormones and one thirst center. Two of them HOLD water, one DUMPS it — that is the whole control panel.

🔗 RAAS — the “blood pressure is dropping!” cascade

🧠 “Renin Angers the Lungs' ACE, and ACE calls Aldo to Add Salt.” Run it in order every time: Renin → Angiotensin I → ACE → Angiotensin II → Aldosterone. Five steps, four A's, one job: raise the pressure.

💧 ADH (vasopressin) — the water-saving hormone

📍 Made in the hypothalamus, stored & released by the posterior pituitary.
🔔 Trigger: osmoreceptors sense the blood is too concentrated (high osmolality), or volume/BP drops.
🚰 Action: opens water channels in the collecting duct → water back into the blood.
🧪 Result: small amount of dark, concentrated urine; serum osmolality falls back to normal.

Too much ADH = SIADH → holds water, dilute blood, low Na⁺. Too little ADH = diabetes insipidus → dumps water, huge pale urine output, high Na⁺.

🧠 ADH = “Adds Da H₂O back.” And for the two diseases: SIADH = Soaked Inside · DI = Dry Inside.

🧂 Aldosterone — the salt-saving hormone

📍 From the adrenal cortex (outer layer of the adrenal gland).
🔔 Trigger: angiotensin II, low blood volume, or a high serum K⁺.
🔁 Action at the distal tubule: Na⁺ and water IN, K⁺ OUT.
📈 Result: ECF volume and BP rise; serum K⁺ falls.

Too much = Conn's / hyperaldosteronism → hypertension + hypokalemia. Too little = Addison's → hypotension, hyperkalemia, salt craving.

🧠 “Aldo SAVES salt, WASTES potassium.” Aldo is a bouncer: sodium gets in the club, potassium gets thrown out.

🫀 ANP & BNP — the “too full” release valve

📍 ANP from stretched atria; BNP from stretched ventricles.
🔔 Trigger: the heart chambers are stretched by too much volume.
🚽 Action: natriuresis (dump Na⁺) + diuresis + vasodilation, and it blocks renin, aldosterone and ADH.
🧪 BNP is a lab you will actually see: it rises in heart failure and fluid overload — <100 pg/mL makes HF unlikely.

🧠 “ANP is the ANti-Pressure hormone.” RAAS and ADH are the hold team; ANP/BNP is the one-player dump team. Stretch the heart → open the valve.

🥤 Thirst — the backup you cannot count on

Osmoreceptors in the hypothalamus fire when serum osmolality rises by only a small amount — you feel thirsty, you drink, osmolality falls.

👵 Blunted in older adults — they get dry before they ever feel thirsty.
👶 Useless in infants, the sedated, the confused, the intubated, anyone NPO or restrained.
🩺 So: offer fluids on a schedule, don't wait for a request.

🧠 “Thirst is the last alarm, not the first.” By the time a patient says they're thirsty, they have already lost fluid — and many patients can't say it at all.

🧾 The control panel on one page

Hormone	From	Fires when…	Does what
ADH 💧	Posterior pituitary	Serum osmolality HIGH (too salty) or volume low	Reabsorbs water → concentrated urine, more volume
Aldosterone 🧂	Adrenal cortex	Angiotensin II, low volume, high K⁺	Na⁺ + water in, K⁺ out → more volume, lower K⁺
Renin / Ang II 🔗	Kidney / lungs (ACE)	Low renal perfusion or BP	Vasoconstricts and triggers aldosterone
ANP / BNP 🫀	Atria / ventricles	Heart chambers overstretched	Dumps Na⁺ & water, vasodilates, blocks RAAS + ADH
Thirst 🥤	Hypothalamus	Osmolality rises / volume falls	Makes you drink — unreliable in the very old & very young

🧠 “TWO HOLD, ONE DUMPS.” ADH holds water · Aldosterone holds salt (and water follows) · ANP/BNP dumps both. Any fluid question is asking which team is winning.

⚡

QUICK RECALL

SAY IT OUT LOUD

🪣 ⅔ ICF · ⅓ ECFand ECF = ¾ interstitial + ¼ intravascular

💧 Water follows saltNa⁺ outside · K⁺ inside · osmosis moves only water

⚖️ 1 kg = 1 L = 2.2 lbDaily weight beats I&O · report 2–3 lb/day or 5 lb/week

🚨 30 mL/hrminimum urine output · under it, report

🧪 275–295 mOsm/kgnormal serum osmolality · Na⁺ 135–145 mEq/L

🎛️ ADH + Aldo HOLDANP/BNP DUMPS · thirst is the last alarm

🔗 R–AI–ACE–AII–Aldofive steps, one goal: raise the pressure

⚠️ Third spacingweight UP, but the pipes are DRY

🎯 Cover & check — 6 rapid-fire questions

Q1: A 70 kg adult has about how much total body water, and how is it split?

≈ 42 L (60% of body weight). ⅔ intracellular (~28 L); ⅓ extracellular (~14 L), which is ¾ interstitial (~10.5 L) and ¼ intravascular/plasma (~3.5 L).

Q2: Osmosis vs. diffusion vs. filtration — which moves what?

Diffusion moves SOLUTES from greater → lesser concentration. Osmosis moves WATER from lower → higher solute concentration. Filtration moves BOTH fluid and solutes from high → low hydrostatic pressure (the glomerulus).

Q3: What is the single most reliable indicator of fluid status, and how do you do it right?

Daily weight. Same time each morning, after voiding, before breakfast, same scale, same clothing. 1 kg = 1 L = 2.2 lb. Report 2–3 lb in a day or 5 lb in a week.

Q4: Serum osmolality is 305 mOsm/kg. What will the body do?

That is above the 275–295 normal range — the blood is too concentrated. Osmoreceptors trigger THIRST and ADH release; the kidney reabsorbs water and makes a small amount of dark concentrated urine until osmolality falls.

Q5: Aldosterone does what to sodium and potassium?

Holds sodium (and water follows) and wastes potassium in the urine. So too much aldosterone = high BP + LOW K⁺; too little (Addison's) = low BP + HIGH K⁺.

Q6: A post-op patient's weight is up 4 lb but the BP is 88/50, HR 118 and urine output 20 mL/hr. What is happening?

Third spacing — fluid has shifted out of the vessels into a non-functional space (abdomen, gut, burned or surgical tissue). Weight is up but the intravascular compartment is DRY. Treat the vital signs, and anticipate a later reabsorption phase with risk of fluid overload.