WATER AND ELECTROLYTE
BALANCE
DRANITZKI ELHALEL MICHAL, MD
NEPHROLOGY AND HYPERTENSION
SERVICES
Total Body Water (TBW)
• Intracellular space
• Interstitium
}
extracellular
• Vascular Space }
TBW
= 60% of Body Weight
60% of TBW (36% of weight) - intracellular
40% of TBW (24% of weight) - extracellular
Exchange of water between
cellular and extracellular fluids
1. OSMOTIC PRESSURE - generated
by number of particles per unit
volume
2. HYDROSTATIC PRESSURE
Main intracellular osmole  K+
Main extracellular osmole  Na+
Extracellular osmolarity ~2xNa+
Extracellular osmolarity = Intracellular
osmolarity
Normal osmolarity ~280 mOsmol/Kg
~140 mEq/L of Na+
70 kg, male
TBW - 42 liter
Total Body Solute - 42 liter x 280 mOsmol/l
= 11.760 mOsmol
Intracellular volume - 25 liter
Intracellular osmoles - 25 liter x 280 mOsmol/l
= 7000 mOsmol
Extracellular volume - 17 liter
Extracellular osmoles - 17 liter x 280 mOsmol/l
= 4760 mOsmol
Substance Plasma
Plasma Extracellular Intracellular
added
osmolarity sodium volume
volume
NaCl




Water




Isotonic NaCl
0
0

0
Plasma Na+ concentration
is a measure of
concentration
and not of volume, or of
total body sodium
Plasma osmolarity 
2 x plasma [Na+] + Glucose + Urea
---------------------------------------Normal values:
PNa - 137 - 145 mEq/L
Glucose - 3.5-6.5 mmol/L
Urea - 3.5-6.5 mmol/L
Posm - 275-290 mOsmol/kg
Effective Posm - 270-285 mOsm/kg
Exchange of Water between
Plasma and Interstitial Fluid
- Oncotic pressure
- Hydrostatic pressure
EFFECTIVE BLOOD VOLUME (EBV)
1. Volume in arterial system
2. Pressure perfusing the arterial
baroreceptors (carotid, glomerular)
REGULATION OF EFFECTIVE BLOOD VOLUME
AFFECTS SODIUM STORES BY AFFECTING
URINARY SODIUM EXCRETION
REGULATION OF EBV
1. SENSORS - volume/pressure
receptors:
- afferent arteriole
- cardiopulmonary - atria
- carotid
REGULATION OF EBV
2. EFFECTORS
a. Sympathetic nervous system:
- sympathetic nervous tone
- secretion of catecholamines from adrenal medulla

Venous constriction
Myocardial contractibility and heart rate
Arteriolar constriction
Renin secretion
Renal tubular Na+ reabsorption
REGULATION OF EBV
b. Renin - Angiotensin - Aldosterone system:
arteriolar vasoconstriction
renal Na+ retention (Angiotensin II,
aldosterone)
c. Atrial Natriuretic Peptide (ANP)
d. Regulation of renal Na+ excretion:
varies directly with effective blood volume
controlled by - GFR
- Tubular reabsorption of Na+
VOLUME REGULATION
Reduced EBV Elevated EBV
Sympathetic tone
Renin, Angiotensin,
Aldosterone
ANP
Renal sodium excretion








60-70%
4%
5%
20-30%
OSMOREGULATION
SENSING - Osmoreceptors in hypothalamus
EFFECTORS - Thirst  Drinking
- Antidiuretic Hormone (ADH)
 water excretion
Water Balance
Obligatory water output:
Urine
- 500 ml
Skin
- 500 ml
Respiratory tract
- 400 ml
Stool
- 200 ml
Obligatory Water Intake
• Drinking
400 ml
• Water content of food
850 ml
• Water pruduced by oxidation
350 ml
Excersice on a Hot Day
A. Water loss
 Osmolarity 
B. Water + Sodium loss
Volume

Osmolarity   Thirst  } water retention 
ADH 
Volume 
Urinary sodium excretion 
ADH water retention 
Urine - Osmolarity 
Na+, Cl- 
Half Isotonic Saline Infusion
A. Osmolarity   ADH 
B. Volume

Sodium excretion 
ADH 

Urine - Osmolarity 
Na+, Cl- 
Isotonic Saline Infusion
A. Osmolarity - no change
B. Volume
 
Sodium excretion
ADH 

Urine - Isosmotic urine
Congestive Heart Failure
A. EBV   Urinary Sodium excretion 
ADH 

Total Body Sodium  (Edema)
Total Body water 
Plasma Osmolarity 
Plasma Sodium 
Urine Osmolarity 
Urine Na+, Cl- 
Primary Renal Sodium
Retention
Total Body Sodium 
Total Body Water

Secretory Diarrheas = Isoosmotic Fluid
containing Na+ and K+ as in the Plasma
EBV - decreased
POsm - no change
PNa - no change
ADH - increased
Renin + Aldosterone - increased
ANP - decreased

Urinary Sodium Excretion - decreased
Urine Osmolarity - increased
Hyponatremia and hypoosmolality
= Impaired renal water excretion
Hypernatremia and hyperosmolarity
= Impaired thirst mechanism or
no access to water
Hypoosmolarity and
Hyponatremia - SYMPTOMS
Nausea
Malaise
Headache
Lethargy
Seizures
Coma
Cause - Brain Edema
Hyponatremia – Etiology
Disorders of impaired water excretion
A. Effective blood volume depletion
GI losses
Renal losses: diuretics, hypoaldostronism,
Na+-wasting nephropathy
Skin losses: exercise, burns
Edematous states: heart failure,
hepatic cirrhosis, nephrotic syndrome, protein
loosing enteropathy
B. Diuretics: Thiazides, loop diuretics
C. Renal failure
Hyponatremia – Etiology
D. Non-hypovolemic states ofADH excess
Syndrom of inappropriate ADH secretion
Cortisol deficiency
Hypothiroidism
E. Decreased solute intake
F. Cerebral salt wasting
Disordders with normal water excretion
A. Primary polydipsia
B. Reset osmostat: pregnancy, psychosis,
quadriplegia, malnutrition
Diuretics
1. Volume depletion
2. Inhibition of urinary dilution
3. K+ depletion
Most common - THIAZIDES
60-70%
4%
5%
20-30%
Syndrome of Inappropriate
ADH Secretion
= Impaired water excretion
1. Hypoosmolarity and Hyponatremia
2. Increased urine osmolarity
NO DECREASED EBV
Na+ EXCRETION IS NORMAL
SIADH - ETIOLOGY
1. Neuropsychiatric disorders
2. Drugs
3. Pulmonary disease
4. Post-operative
5. Severe nausea
6. Ectopic production
7. Exogenous administration of ADH
PSEUDOHYPONATREMIA
Low plasma Na+ with normal Posm
A. Severe hyperlipidemia
B. Severe hyperproteinemia
Low plasma Na+ with elevated Posm
A. Hyperglicemia
B. Administration of hypertonic Manitol
Hyponatremia - DIAGNOSIS
1. Patient history
2. Estimate Volume status
3. Plasma Osmolarity
4. Urine Osmolarity
5. Urine Na+ concentration
Hyponatremia – Treatment
1. Estimate neurological symptoms
2. Estimate volume status
3. Decide – water restriction
or sodium load
or both
4. Correct slowly!!!! ~ ½ meq/l/h
TREAMENT COMPLICATION:
CENTRAL PONTINE MYELINOLYSIS
Estimation of Sodium deficit
Na+ deficit = 0.5 x weight (125-plasma Na+)
Example: 70kg female, plasma Na+ 113meq/l
Na+ deficit = 35(125-113)=420 meq
Time of correction: 12meq/l
=24h
0.5meq/l/h
Fluids: Hypertonic (3%) saline = 513meq/l
420 meq = 818 ml of hypertonic saline
If hypovolemia coexist:
Treat with normal (0.9%) saline.
First, Sodium will rise slowly in
plasma.When hypovolemia will
be corrected ADH levels will
drop, and water excretion will
correct plasma Na+.
Treatment of SIADH
1.Water restriction
2. Hypertonic saline or NaCl tablets
3. Loop diuretics
4. Demeclocycline
Hyperosmolarity and
Hypernatremia - SYMPTOMS
Lethergy
Weakness
Irritability
Twitching
Seizures
Coma
Cause - Brain Dehydration
Hypernatremia – Etiology
water loss
A. Insensible loss – Increased sweating, evaporation
Burns
Respiratory infection
B. Renal loss – Central Diabetes Insipidus
Nephrogenic Diabetes Insipidus
Osmotic diuresis
C. Gastrointestinal loss – Osmotic diarrhea
D. Hypothalamic disoreders
E. Water loss into cells – Seizures
Rabdomyolysis
Hypernatremia – Etiology cont.
Sodium retention
A. Administration of hypertonic NaCl or
NaHCO3
B. Ingestion of sodium
Hypernatremia – DIAGNOSIS
1. Patient history
2. Estimate volume status
3. Plasma osmolarity
4. Urine volume
5. Urine osmolarity
6. Urine Na+ concentration
DIABETES INSIPIDUS- CENTRAL-ETIOLOGY
1.Idiopathic – familial
2. Post surgery to hyopthalamus
3. Head trauma
4. Hypoxic or ischemic encephalopathy (shock, arrest,
Sheehan’s syndrom)
5. Neoplastic
6. Histiocytosis X
7. Sarcoidosis
8. Anorexia nervosa
9. Cerebral aneurysm
10. Encephalitis or meningitis
DIABETES INSIPIDUS –
NEPHROGENIC- ETIOLOGY
1.Decreased water permeability of the collecting
tubule
a. Congenital
b. Hypercalcemia
c. Hypokalemia
d. Drugs (Lithium, Demeclocycline,
Streptozocin)
e. Sjogren’s syndrom
f. Amyloidosis
Diabetes Insipidus – Nephrogenic Etiology –
con.
2. Interference with countercurrent mechanism
a. Osmotic diuresis
b. Loop diuretics
c. Renal failure
d. Hypercalcemia
e. Hypokalemia
f. Sickle cell anemia
3. Increased periferal degradation of ADH
a. Pregnancy
4. Unknown mechanism
a. Isofamid
b. Methoxyflurane
Hypernatremia – Treatment
1. Estimate neurological symptoms
2. Estimate volume status
3. Treat slowly ~ 0.5 meq/l/h
Estimate water deficit
Water deficit = CBW[plasma Na+/140 – 1]
CBW= Current Body Water
For males 0.5 X weight
For females 0.4 X weight
Example: 50kg female has plasma Na+ of
183meq/l
Example: 50kg female has plasma Na+ of
183meq/l
Water deficit = 0.4 X 50[183/140-1]=6 liters
Time of treatment = (183-140)meq/l =86h
0.5 meq/l/h
If no need for quick fluid replacment, treat
with 5% glucose.
Don’t forget to add insensible loss, ~700 ml/d
Estimate continous loss of water and
electrolites.
Hyponatremia and hypoosmolality
= Impaired renal water excretion
Hypernatremia and hyperosmolarity
= Impaired thirst mechanism or
no access to water
POLYURIA – D.D.
1.Water diuresis (urine osmolarity < 250)
Primary polydipsia
I.V. infusion of dilute solutions
Diabetes insipidus (central and nephrogenic)
2. Solute diuresis (urine osmolarity > 300)
Saline loading
Postobstructive diuresis
Hyperglycemia
Very high protein diet
Na+ wasting nephropathy
POLYURIA – DIAGNOSIS
1. Patient history
2. Estimate volume status
3. Measure Glucose level in plasma
4. Plasma osmolarity
5. Urine osmolarity
6. If DI is suspected but no hyperosmolarity is
present, perform water restriction test
Water restriction test
Purpose – to induce hyperosmolarity to stimulate
maximal ADH secretion
Follow: 1. Plasma osmolarity
2. Urine volume
3. Urine osmolarity
When plasma osmolarity > 295 mosmol/l
or Urine osmolarity is stable , give exogenous ADH
THE END