Water & Electrolyte Balance

The Body as an Open SystemThe Body as an Open System “Open System”. The body exchanges

material and energy with its surroundings.

Water Balance

Water is the solvent of life. Undoubtedly, water is more important than any other single compound to life. It is involved in several body function.

Function of water1. Water provides the aqueous medium to the organism which is essential for the various biochemical reactions to occur.

2. Water directly participates as a reactant in several metabolic reaction.

3. It serves as a vehicle for transport of solutes.

4. Water is closely associated with the regulation of body temperature.

Distribution of water

Major Compartments for Fluids

• INTRACELLULAR FLUID (ICF)

• Inside cell

• Most of body fluid here - 63% weight

• Decreased in elderly

• EXTRACELLULAR FLUID (ECF)

• Outside cell: 37% – Interstitial fluid -

between cells & blood vessels (25%)

– Intravascular fluid - within blood vessels (5~8%)

– Transcellular fluid - cerebrospinal, pericardial , synovial, gastrointestinal tract fluid (1~2%)

Water Steady State• Amount Ingested = Amount Eliminated

Solutes – dissolved particles

• Electrolytes – charged particles– the compounds that when dissolved can

carry an electrical current– Cations – positively charged ions

• Na+, K+ , Ca2+, H+

– Anions – negatively charged ions• Cl-, HCO3

- , PO43-

– # Cations must = # Anions for homeostatsis to exist in each fluid compartment

• Non-electrolytes - Uncharged •Proteins, urea, glucose, O2, CO2

Electrochemical Equivalence

• The concentration of electrolytes are expressed as milliequivalents / liter (mEq/l) rather than milligrams.

• Equivalent (Eq/L) = moles x valence

• Monovalent Ions (Na+, K+, Cl-):– 1 milliequivalent (mEq/L) = 1 millimole

• Divalent Ions (Ca2+, Mg2+, and HPO42-)

– 1 milliequivalent = 0.5 millimole

K+

Mg2+

PO43-

Na+

Cl-

HCO3-

ICF ECF

Table Table ELECTROLYTE COMPOSITION OF BOOD FLUIDSELECTROLYTE COMPOSITION OF BOOD FLUIDS

  Compartmental concentration (mEq/L)Compartmental concentration (mEq/L)

PlasmaPlasma Interstitial fluidInterstitial fluid Intracellular fluidIntracellular fluid

Positive ion Positive ion  NaNa++ 142 142 140140 1010

K K ++ 5 5 5 5 150150

CaCa2+2+ 5 5 5 5 0.0001 0.0001

    MgMg2+2+ 3 3 403 3 40

Total 155 153 200Total 155 153 200

Negative ionNegative ion ClCl 103 103 112112 3 3

    HCOHCO33 27 27 2828 10 10

HPOHPO4422 2 4 2 4 142142

SOSO4422 1 2 51 2 5

Organic acid 6 6 —Organic acid 6 6 —

Protein(Pr Protein(Pr ) 16 1 ) 16 1 4040

TotalTotal 155 153 200155 153 200

Electrolyte composition of body fluids

• Electrolytes are well distributed in the body fluids in order to maintain the osmotic equilibrium and water balance. • There is marked difference in the concentration of electrolytes between the ECF and ICF.

• Na+ is the principal extracellular cation while K+ is the intraxcellular cation. • This difference in the concentration is essential for the cell survival which is maintained by Na+-K+ ATPase.

Na+-K+ ATPase (or Na+-K+ pump)

Body Fluid Exchange

Movement of body fluids• Diffusion

– From higher concentration →lower concentration.

• Osmotic pressure– Drawing power of water (dependent on

the number or molecules in solution)• Isotonic • Hypotonic • Hypertonic

• Water: move from hypotonic → hypertonic solution.

Regulation of electrolyte balance

• Electrolytes and water balance are regulated together and the kidney play a critical role.

• The regulation is mostly achieved through the hormones aldosterone, ADH (Antidiuretic hormone) and renin-angiotensin.

Aldosterone

• mineralocorticoid,

steroid hormone.• produced by adrenal

cortex, increase Na+ reabsorption at the expense of K+ and H+.• retain Na+, lose K+

Antidiuretic hormone (ADH)

• or vasopressin

• a 9-amino acid peptide hormone

• produced by specialized nerve cells in the hypothalamus and transported in the bloodstream to the posterior pituitary gland.

• mainly released when the body is low on water, increases water reabsorption by renal tubules.

Renin-angiotensin system(RAS)

• Renin: peptide hormone (340 aa), secreted by the kidney.

• Angiotensinogen: α2-globulin, produced constitutively and released into the circulation mainly by the liver.

• Angiotensinogen →AngiotensinⅠ →AngiotensinⅡ

angiotensin-converting enzyme

PAI-1: Plasminogen activator inhibitor-1, inhibits fibrinolysis

• AngiotensinⅡ can

stimulate the release of

aldosterone.• Renin-angiotensin

system regulates blood pressure and water (fluid) balance.

adrenal glands

retain Na+

Renin-angiotensin system (RAS)

Dehydration • Dehydration is condition characterized by water depletion in body. It may be due to insufficient intake or excessive water loss or both.

• Two types:1. due to loss of water alone.2. Due to deprivation of water and electrolytes.

• Causes of dehydration: 1. Diarrhea2. Vomiting3. Excessive sweating4. Adrenocortical dysfunction5. Kidney disease6. Deficiency of ADH

Characteristic feature of dehydration

•Features of dehydration

a) The volume of ECF decrease, electrolytes concentration and osmotic pressure increase.

b) Water is drawn from the ICF, shrunken cells and disturbed metabolism.

c) ADH secretion is increased.

d) Plasma protein and blood urea concentration increased.

e) Loss of electrolytes from the body (Na+, K+, etc.).

Clinical symptoms of sever dehydration

•Increased pulse rate, Low blood pressure, Sunken eyeballs,

Decreased skin turgor, Lethargy, Confusion and coma

•Treatment: intake plenty of water, 5% glucose solution.

Overhydration

Overhydration or water intoxication is caused by excessive

retention of water in the body. It may be due to excess intake or large

volumes of salt free fluids, renal failure, overproduction of ADH.

Clinical syndromes: headache, lethargy and convulsions.

Treatment: stop water intake, administration of hypertonic saline.

Points

• Function of water• Distribution of water: ICF and ECF• Electrolyte composition of body

fluids• Regulation of electrolyte balance

– aldosterone, ADH (Antidiuretic hormone) and renin-angiotensin

• Dehydration and Overhydration