Regulation of Fluid and Electrolyte Balance
Regulation of Fluid and Electrolyte Balance
Regulation of Fluid and Electrolyte Balance
ELECTROLYTE BALANCE
• Pathological losses
vascular bleeding (H20, Na+)
vomiting (H20, H+)
diarrhea (H20, HCO3-).
Electrolyte (Na+, K+, Ca++)
Steady State
• Amount Ingested = Amount Excreted.
• Normal entry: Mainly ingestion in food.
• Clinical entry: Can include parenteral
administration.
Electrolyte losses
• Renal excretion
• Stool losses
• Sweating
• Abnormal routes: e.g.. vomit and
diarrhea
Body Fluids and Fluid
Compartments
• The percentage of total body water: 45-75%
• Intracellular compartment
– 2/3 of body water (40% body weight)
• Extracellular compartment
– 1/3 of body water (20% body weight)
– the blood plasma (water=4.5% body weight)
– interstitial fluid and lymph (water=15% body weight)
– transcellular fluids: e.g. cerebrospinal fluid, aqueous
humor (1.5% BW)
• Distribution of substances within the body is NOT
HOMOGENEOUS.
Body Water Distribution
•Individual variability (lean body mass)
–55 - 60% of body weight in adult males Input
–50 - 55% of body weight in adult female
–~42 L For a 70 Kg man.
TRANSCELLULAR WATER
1% 1L
Electrochemical Equivalence
0
Plasma Interstitial Cell
H2O H2O H2O
Net Osmotic Force
Development
• Semipermeable membrane
• Movement some solute obstructed
• H2O (solvent) crosses freely
• End point:
– Water moves until solute concentration on
both sides of the membrane is equal
– OR, an opposing force prevents further
movement
Osmotic Pressure ()
S S S
S
S S S
S S S
S S S
Glucose Example
Initial Gl Gl Gl Gl
10 L 10 L
Final Gl Gl Gl Gl
15 L 5L
Osmotic Concentration
• Proportional to the number of osmotic
particles formed: Osm/L = moles x n (n, # of
particles in solution) e.g. 1 M NaCl = 2 M Glu in Osm/L
• Assuming complete dissociation:
– 1mole of NaCl forms a 2 osmolar solution in 1L
– 1mole of CaCl2 forms a 3 osmolar solution in 1L
• Physiological concentrations:
– milliOsmolar units most appropriate
– 1 mOSM = 10-3 osmoles/L
Principles of Body Water
Distribution
• Body control systems regulate ingestion
and excretion:
– constant total body water
– constant total body osmolarity
• Osmolarity is identical in all body fluid
compartments (steady state conditions)
– Body water will redistribute itself as
necessary to accomplish this
Intra-ECF Water
Redistribution
Plasma vs. Interstitium
Effects of aldosterone