Electrolytes & Water Balance PDF

ELECTROLYTES 3-4 & WATER BALANCE WATER Average water content of the human body ranges from 40% to 75% of total body weight. Location intracellular (2/3) extracellular (1/3) Concentrations of ions within cells are maintained both by active and passive transport. Osmolality Physical property based on...

ELECTROLYTES 3-4 & WATER BALANCE WATER Average water content of the human body ranges from 40% to 75% of total body weight. Location intracellular (2/3) extracellular (1/3) Concentrations of ions within cells are maintained both by active and passive transport. Osmolality Physical property based on the concentration of solutes (colligative property) The sensation of thirst and arginine vasopressin hormone (AV formerly ADH; posterior pituitary gland) secretion are stimulated by the hypothalamus in response to an increased osmolality of blood Normal plasma osmolality = 275-295 mOsm/kg of plasma H20 Osmolal Gap Indirectly indicates the presence of osmotically active substances other than Na+, urea or glucose, such as: Ethanol Methanol Ethylene glycol Lactate Beta-hydroxybutyrate OSMOLAL GAP = (M) OSMOLALITY - (C) OSMOLALITY 2Na + 1.86 Na + glucose (mg/dl) 20 + BUN (mg/dL) 3 glucose (mg/dl) + 18 BUN (mg/dL) 2.8 + 9 REGULATION OF SODIUM Intake of water in response to thirst, as stimulated or suppressed by plasma osmolality The excretion of water, largely affected by AVP release in responses to changes in either blood volume or osmolality Blood volume status, which affects Na* excretion through aldosterone, angiotensin II and atrial natriuretic peptide FUNCTION ELECTROLYTES INVOLVED FUNCTION ELECTROLYTES INVOLVED Bicarbonate Potassium Chloride Volume and osmotic regulation Sodium Potassium Chloride Acid-Base balance Myocardial rhythm and contractility Potassium Magnesium Calcium Blood coagulation Magnesium Calcium Magnesium Calcium Zinc Neuromuscular excitability Potassium Calcium Magnesium Production and use of ATP Magnesium Phosphate Enzyme activation ATPase pump Magnesium Sodium Most abundant cation in the ECF 90% of all extracellular cations Active transport systems, e.g. Na*-K*-ATPase pumps, prevent equilibrium from occurring Na+ and its associated anions account for approximately 90% of the osmotic activity in plasma. Na ATPase ion pumps 2K+ ions inside the cell in exchange for 3 Na+ ions moving out of the cell Na* prevents osmotic rupture of cells whenever it moves out of that cell, since it brings along water as it moves outside of the cell HYPONATREMIA serum/plasma level ___ H2O ---> ___ NaCl Hypochloremia Salt-losing nephritis Addisonian crisis Metabolic acidosis caused by increased production or diminished excretion of organic acids DKA or renal failure Prolonged vomiting Metabolic alkalosis plasma CI- tends to fall as HCO* increases Hyperchloremia Dehydration Renal tubular acidosis Acute renal failure Metabolic acidosis Salicylate intoxication Respiratory alkalosis Hyperchloremic acidosis Extremely high dietary intake of salt Overtreatment w/ saline solutions Primary hyperparathyroidism DETERMINATION Mercurimetric Titration (Schales & Schales method) Indicator: diphenylcarbazone End product: HgCl2 (blue violet) WHITEHORN TITRATION METHOD Spectrophotometric Reagent: mercuric thiocyanate End product: reddish complex COULOMETRIC AMPEROMETRIC TITRATION Cotlove Chloridometer ION-SELECTIVE ELECTRODE routine; silver polymeric membranes that incorporate quatemary ammonium salt anion-exchangers Bicarbonate Second most abundant anion in the ECF Accounts for more than 90% of total CO2 at physiologic pH Major component of the buffering system in the blood Diffuses out of the cell in exchange for CI- to maintain ionic charge neutrality within the cell (chloride shift) TOTAL CARBON DIOXIDE IN PLASMA HCO3 or CO3 ions H2CO3 CO2 in physical solution CO2 loosely bound to proteins (carbamino compounds) CLINICAL SIGNIFICANCE Alterations of HCO3 & CO2 dissolved in plasma are characteristic of acid-base imbalance Evaluation of blood gases & pH is required to provide a definitive picture of the over-all pattern of imbalances DETERMINATION Specimen Serum or lithium heparin plasma Sample should be capped until serum or plasma is separated CO2 escapes (decreases by 6mmol/L/hour) Methods Ion-Selective Electrode (total CO2) all forms of CO2 are converted to CO2 gas through acidification CO2 gas measured using pCO2 electrode Enzymatic methods phosphoenolpyruvate carboxylase malate dehydrogenase) Magnesium Fourth most abundant cation in the body Second most abundant intracellular ion Only less than 1% of total Mg2* is present in serum and RBCs Essential cofactor of more than 300 enzymes REGULATION Specimen Serum or lithium heparin plasma Sample should be capped until serum or plasma is separated CO2 escapes (decreases by 6mmol/L/hour) REGULATION Overall regulation of body Mg2 is controlled largely by the kidneys Parathyroid Hormone Increases renal reabsorption Enhances reabsorption in the intestine Thyroxine and aldosterone Increase renal excretion DETERMINATION Specimen Nonhemolyzed serum or lithium heparin plasma Oxalate, citrate and EDTA are unacceptable 24-h urine may also be analyzed (acidified with HCI to avoid precipitation) Methods 1. Colorimetric methods Calmagite method - reddish violet complex (532 nm) Formazen dye method Methylthymol blue method 2. Dye-lake method (Titan Yellow Dye) 3. Atomic Absorption Spectrometry reference method Calcium Involvement: blood coagulation enzyme activity excitability of skeletal and cardiac muscle maintenance of blood pressure Only 1% is in the blood and ECF REGULATION Parathyroid Hormone Secretion is stimulated by a decrease in Ca2 Activates bone resorption; increases breakdown of bone by osteoclasts Increases tubular reabsorption of Ca2+ Stimulates renal production of active vitamin D Vitamin D Increases calcium absorption in the intestines Enhances effect of PTH on bone resorption Calcitonin Medullary cells of the thyroid Inhibits action of both PTH and vitamin D DETERMINATION Precipitation and Redox Titration Clark-Collip Precipitation Ferro Ham Chloranilic Acid Precipitation Colorimetric methods Ortho-cresolphthalein complexone (CPC) Arsenazo Ill dye AAS Reference Method ISEs Phosphate Predominant intracellular anion Found everywhere in living cells Found as part of genetic material, coenzymes, reservoir of biochemical energy Indirectly affects affinity of hemoglobin for oxygen REGULATION Parathyroid hormone Increases renal excretion Vitamin D 2 forms: Vit. D = Cholecalciferol Vit. D2 = (Ergocalciferol) Increases absorption in the intestine Increases reabsorption in the kidneys Growth Hormone in cases of excessive secretion or administration Decreases renal excretion CALCIUM PARATHYROID HORMONE VITAMIN D PO4 DETERMINATION Specimen Serum or lithium heparin plasma Oxalate, citrate or EDTA interferes with analytic method Hemolysis should be avoided 24-h urine may be analyzed Fiske-Subbarow method End product: ammonium phosphomolybdate complex (340 nm) molybdenum blue (600-700 nm) Anion Gap Difference between unmeasured anions and unmeasured cations Useful in indicating an increase in one or more of the unmeasured anions in the serum and also as a form of QC for the analyzer used to measure these electrolytes ELEVATED ANION GAP uremia/ renal failure ketoacidosis methanol, ethanol, ethylene glycol, salicylate poisoning lactic acidosis hypernatremia instrument error LOW ANION GAP Hypoalbuminemia Severe hypercalcemia Formulas Specimen AG = Na* - (Cl + HCO3) AG = (Na* - K*) - (Cl + HCO3) NV: 7-16 mmol/L NV: 10-20 mmol/L