Water & Electrolyte Balance

The Body as an Open System l “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 water 1. 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 4 4

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 20 20

K+ Mg2+ PO43- Na+ Cl- HCO3- ICF ECF

Compartmental concentration (mEq/L) Table ELECTROLYTE COMPOSITION OF BOOD FLUIDS  Compartmental concentration (mEq/L) Plasma Interstitial fluid Intracellular fluid Positive ion  Na+ 142 140 10 K + 5 5 150 Ca2+ 5 5 0.0001   Mg2+ 3 3 40 Total 155 153 200 Negative ion Cl 103 112 3   HCO3 27 28 10 HPO42 2 4 142 SO42 1 2 5 Organic acid 6 6 — Protein(Pr ) 16 1 40 Total 155 153 200

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

Renin-angiotensin system (RAS) adrenal glands retain Na+ AngiotensinⅡ can stimulate the release of aldosterone. Renin-angiotensin system regulates blood pressure and water (fluid) balance.

Dehydration Two types: 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. Diarrhea 2. Vomiting 3. Excessive sweating 4. Adrenocortical dysfunction 5. Kidney disease 6. 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.).

Overhydration 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