WATER AND ELECTROLYTE
BALANCE
Lecture for MBBS by:
Dr. ANITHA
Water distribution in the body
Total body water is 60% of body weight
Intracellular fluid
(40% of body weight)
Intravascular fluid
(4% of body weight)
Extracellular fluid
(20% of body weight)
Extravascular fluid
(16% body weight)
Water balance:
Water intake/day
about 2000 to
2900 ml/ day
Water output/day
about 2000 to
2900 ml/ day
 It refers to maintenance of water content of the
body by balancing the water intake and water
output.
02/49
Regulation of water balance:
Regulator
Thirst
center
Antidiuretic
hormone
(ADH)
Location/
secreted by
Mechanism of regulation
Hypothalamus Regulates water intake;
of brain
water loss causes dehydration,
which stimulates thirst centre
causing us to drink water
Posterior
pituitary
gland
Promotes water reabsorption from
kidneys;
reduces water loss from body
02/49
Dehydration
It is a condition characterized by excessive loss of
body water.
Causes of dehydration
 reduced water intake for
prolonged period
 diarrhea, vomiting, excessive
sweating, burns
 kidney diseases
 ADH deficiency
(diabetes insipidus)
Complications of dehydration
 Decreased ECF volume,
increased electrolyte
concentration
 Decreased plasma volume,
reduced cardiac output,
circulatory failure
 Decrease of more than 15-25%
of body water is invariably fatal.
Electrolytes
 Electrolytes are inorganic substances that can readily dissociate
and exist as charged particles/ ions in solution.
Types:
1. Cations (Positively charged ions)
2. Anions (Negatively charged ions)
Major Electrolytes of ECF and ICF
Electrolyte
Sodium (Na+)
Chloride (Cl-)
Potassium (K+)
Functions
• Maintenance of osmotic
pressure in ECF, ECF volume
and water balance
• Maintenance of osmotic
pressure in ICF
Osmolarity
 It is the concentration of osmotically active solutes expressed as
osmoles of solutes per litre solution (osmol/L or Osm/L).
 It is a measure of the body's electrolyte-water balance.
 The normal plasma osmolarity is
280-296 mOsmol/L
(Note: Osmolality is an expression of solute concentration as
osmol / kg solution.)
Regulation of electrolyte balance:
Regulator
1. Aldosterone
(mineralocorticoid)
2. Atrial
natriuretic
factor (ANF)
a polypeptide
hormone
Location/
Mechanism
secreted by
Adrenal
Increases Na+ and Clcortex
reabsorption by renal
tubules ; K+ is excreted
Right atrium
of heart
Increases urinary Na+
excretion
02/49
3. Renin-angiotensin system
Angiotensinogen (Liver)
ECF, BP Renin (Kidney)
Angiotensin I
Angiotensin converting enzyme
(ACE) (Lung)
Angiotensin II
+
+
Aldosterone secretion
(Adrenal cortex)
Na+ and
water retention,
K+ excretion
Normal plasma electrolyte levels
Na+ :
136 - 145 mEq/L
K+ :
3.5 - 5 mEq/L
HCO3- : 22 - 26 mEq/L
Cl- :
96 - 106 mEq/L
Clinical significance of electrolytes
Electrolyte
Na+
K+
Decrease in serum
level
Hyponatremia
Cause: Addison’s
disease
Hypokalemia
Causes: Vomiting,
diarrhea, renal disease
Increase in serum
level
Hypernatremia
Cause: Cushing’s
syndrome
Hyperkalemia
Causes: Renal failure,
metabolic acidosis
You have successfully completed learning
the topic:
Water, Electrolyte Balance
13