14. Water and electrolyte balance dehydration and hyperdration

Total body water and its distribution

~60% of body weight

2/3 intracellular, 1/3 extracellular

Daily water intake sources

Drinks ~60%, food ~30%, metabolic water ~10%

Daily water loss routes

Urine ~60%, lungs + skin (insensible) ~28%, sweat ~8%, faeces ~4%

⚠ Your notes said: '8% precipitation' — it is perspiration (sweat).

Main intracellular ions

K⁺, phosphate, Mg²⁺ (+ proteins)

Main extracellular ions

Na⁺, Cl⁻, HCO₃⁻

Which ion determines ECF volume?

Na⁺ — Na⁺ retention expands ECF, Na⁺ loss shrinks it

RAAS: hypovolaemia → kidney releases _____ → angiotensinogen → _____ (vasoconstriction) → adrenal _____ → Na⁺ + water retention

renin | angiotensin II | aldosterone

ADH (vasopressin) — trigger and action

Trigger: ↑ plasma osmolality (osmoreceptors) or ↓ volume

Action: inserts aquaporin-2 in collecting duct → water reabsorption

⚠ Your notes said: 'ADH stimulates Na⁺ and water retention' — Na⁺ retention is aldosterone's job.

ANP — source and action

Atrial myocytes (stretch) → ↑ GFR, ↓ Na⁺/water reabsorption → natriuresis + diuresis, ↓ BP

Two stimuli for true thirst

1. Cellular dehydration (↑ ECF osmolality)

2. ↓ blood volume

Where are the osmoreceptors for thirst?

In/near the thirst centre of the hypothalamus

Define hypodipsia

Decreased ability to sense thirst (common in elderly)

Polydipsia types: _____ (water loss: DM, DI, diarrhoea), _____ (renal failure, CHF), _____ (schizophrenia)

symptomatic/true | inappropriate/false | psychogenic

Define diabetes insipidus

Deficiency of or resistance to ADH → inability to concentrate urine → 3–20 L/day dilute urine + polydipsia

Central vs nephrogenic DI

Central: ↓ ADH synthesis/release

Nephrogenic: kidneys don't respond to ADH

When does DI become dangerous?

When the patient can't drink (unconscious, infant, no water) → hypertonic dehydration

Isotonic / hypertonic / hypotonic saline examples

Isotonic: 0.9% NaCl (240 – 349 mOsm, department lecture)

Hypertonic: 3% NaCl

Hypotonic: 0.45% NaCl

Normal plasma osmolality: 275 – 295 mOsm/kg

⚠ Your notes listed hyperosmolar diabetic coma and DKA as HYPOtonic states — both are HYPERtonic.

Darrow–Yannet principle

Any non-isotonic change in ECF causes a net water shift between ECF and ICF until osmolality equalises

Define dehydration

↓ ECF volume from inadequate intake, excessive loss, or both

Dehydration severity by % body weight loss

Mild ~2%, moderate ~5%, severe ≥ 8%

Clinical signs of dehydration

Thirst, weight loss, ↓ urine output, tachycardia, ↑ haematocrit, dry mucosa, ↓ skin turgor

Isotonic dehydration: definition + example

Na⁺ and water lost in proportion

No shift between compartments

E.g. haemorrhage, burns

Hypotonic dehydration: definition, shift, example

Loss of Na⁺ > water → ECF hypotonic → water moves into cells (cell swelling, brain oedema)

E.g. vomiting/diarrhoea or sweating replaced with plain water, diuretics, Addison's

⚠ Your notes said: 'profuse sweating' as the example — sweating alone gives hypertonic dehydration.

Hypertonic dehydration: definition, shift, example

Loss of water > Na⁺ → ECF hypertonic → water leaves cells (cellular dehydration, intense thirst)

E.g. sweating, DI, osmotic diuresis in DM, no access to water

Isotonic hyperhydration — example

Gain of water + Na⁺ in proportion: excess isotonic saline infusion, heart failure, nephrotic syndrome → ECF oedema

Hypotonic hyperhydration (water intoxication): shift + examples

Water gain > Na⁺ → water moves into cells → cerebral oedema

E.g. psychogenic polydipsia, SIADH, acute renal failure + fluids

Hypertonic hyperhydration: shift + examples

Na⁺ gain > water → water leaves cells

E.g. Conn's syndrome, hypertonic saline, drinking sea water

Causes of fluid overload (hypervolaemia)

Excess fluid/Na⁺ intake, CHF, cirrhosis, renal failure, SIADH, normal post-operative response

Clinical signs of fluid overload

Weight gain, oedema, ↑ BP, jugular venous distension, dyspnoea, crackles, pleural effusion → pulmonary oedema

SIADH

Excess ADH → water retention → hyponatraemia with concentrated urine (hypotonic hyperhydration)

Hyponatraemia: range, causes, danger

Na⁺ < 135 mmol/L

SIADH, diuretics, HF, cirrhosis, polydipsia, Addison's

Water into brain cells → cerebral oedema, seizures

Correct slowly

Hypernatraemia: range, causes, effects

Na⁺ > 145 mmol/L

Water loss (DI, sweating, osmotic diuresis), poor intake

Cells shrink → thirst, confusion

Hypokalaemia: range, causes, ECG

K⁺ < 3.5 mmol/L

Vomiting, diarrhoea, diuretics, hyperaldosteronism, insulin, alkalosis

Weakness, ileus, flat T, U waves, arrhythmias

Hyperkalaemia: range, causes, ECG

K⁺ > 5.0 mmol/L

Renal failure, K⁺-sparing drugs/ACEi, Addison's, acidosis, cell lysis

Peaked T, wide QRS → VF/asystole

Acid–base and K⁺ shifts

Acidosis → K⁺ leaves cells (hyperkalaemia)

Alkalosis → K⁺ enters cells (hypokalaemia)

Body water by age

Infant ~70–75%, adult ~60%, elderly ~50–55% of body weight

ECF as % of body weight and its split

ECF ≈ 20% of body weight: interstitial ~15%, plasma ~5%

Normal plasma osmolality and its formula

275–295 mOsm/kg

≈ 2×Na⁺ + glucose + urea (mmol/L)

Why are infants and the elderly most at risk of dehydration?

Infants: high water turnover, large surface area, immature kidneys, can't ask for water

Elderly: ↓ total water, hypodipsia, ↓ renal concentrating ability, diuretics

Psychogenic polydipsia

Compulsive drinking (often > 5 L/day), e.g. in schizophrenia → hypotonic hyperhydration (water intoxication, hyponatraemia)

How do you assess a patient's fluid status?

History, daily weight (best single measure), intake/output chart, vital signs (pulse, BP, orthostasis), skin turgor, mucous membranes, JVP/oedema

Labs: Na⁺, osmolality, urea:creatinine, haematocrit, urine specific gravity

EXAMINER ASKS: A patient has lost fluid through diarrhoea and drunk only plain water. What type of dehydration and what happens to his cells?

Hypotonic dehydration: salt-rich loss, water replaced → ECF hypotonic → water moves into cells → cell swelling (cerebral oedema risk), no thirst

EXAMINER ASKS: Why is a patient with hypertonic dehydration intensely thirsty but one with hypotonic dehydration is not?

Thirst follows ECF osmolality: hypertonic ECF pulls water out of osmoreceptor cells → thirst. Hypotonic ECF swells them → thirst is suppressed despite volume loss.

EXAMINER ASKS: Why must hypernatraemia be corrected slowly?

Brain cells adapt by making idiogenic osmoles

Rapid dilution makes water rush in → cerebral oedema (and the reverse — rapid correction of chronic hyponatraemia → osmotic demyelination)

15-MINUTE NOTES / Write your note sheet for essay 14 Water-electrolyte balance – dehydration, hyperhydration

1. Body water by age + compartments (ICF 40%, ECF 20%), intake/loss

2. Electrolytes ICF vs ECF; Na⁺ sets ECF volume; osmolality 275–295

3. Regulation: RAAS, ADH, ANP, thirst; DI vs SIADH

4. Tonicity + Darrow–Yannet principle

5. 3×2 table: iso/hypo/hypertonic de- and hyperhydration with water shift + example

6. Clinical signs + assessment

7. Na⁺ and K⁺ disorders

Draw: the 3×2 table with ICF and ECF boxes and arrows showing which way water moves in each of the 6 states