electrolytes 2 (Na+, K+, Cl-,HCO3-)

Na+ ( 90%) • maintains water distribution & osmotic pressure • intake 130-260 mmol NaCl • body uses 1-2 mmol/day the rest is excreted

Na+ & K+ move between ICF & ECF by Na+/K+ ATPase ion pump • 3 Na+ ions move out in exchange for 2 K+ ions as ATP is converted to ADP • water follows Na+ out of cell which prevents osmotic rupture*

• intake of H2O in response to thirst - stimulated/supppressed by plasma osmolality • excretion of H2O - in response to AVP ( causes water to be reabsorbed) • excretion of Na+ - involves aldosterone, angiotensin ll & atrial natiuretic peptide (ANP)

• serum • heparinized plasma ( lithium heparin ) • urine ( random or 24hr) • not significantly affected by hemolysis ( but if grossly hemolyzed Na+ decreased bc of dilution) • refrigerate or freeze for delayed analysis

chemical methods- outdated flame emission spectrophometry atomic absorption spectrophotometry (AAS) ion- selective electrodes (ISEs) - most common method in clinical labs

• a glass ion-exchange membrane is used for Na+ measurement • Na+ interacts with the tip ( measuring electrode) producing a potential that is dependent on the activity/concentration of Na+ • reference electrode maintains a constant potential • difference in potential between the reference & measuring electrodes = the activity of Na+

Direct • undiluted • elevated lipid/proteins don’t interfere • more accurate Indirect • diluted • increased lipids & proteins cause electrolyte exclusion effect electrolyte exclusion effect • excess lipids or proteins displace plasma water • less aqueous phase gets added to the diluent • causes Pseudohyponatremia***- falsely decreased Na+ concentration

protein build up on electrode membranes through continuous use - causes poor selectively & poor reducibility for results solution - routine maintenance to remove protein build up

use disposable ISE slides • drop of sample & reference fluid are applied at same time • dry slide technology

Na+ level < 135 mmol/L caused by - increased Na+ loss ; hypoaldsosteronism, K+ deficiency, diuretics, ketonuria • Increased H2O retention; renal failure, nephrotic syndrome, CHF • water imbalance; excessive water intake, pseudohyponatremia

Na+ >145mmol/L caused by - increased intake or retention; hyperaldosteronism, excess salt ingestion • excess H2O loss; diabetes insipidus, profuse sweating • decreased H2O intake; older ppl or infants, mental impairment

serum/plasma: 135-145 mm/L ( CSF is around the same) critical <120 or >160 mmol/L

potassium (K+) • dietary requirement : 50-150mmol/day • small amount used, most excreted by kidneys ( not stored well) • K+ conc regulates neuromuscular excitability, heart contractions • too low or too high can lead to cardiac arrest

• nearly all K+ is reabsorbed in the proximal tubules of the kidney • additional K+ is secreted into urine in exchange for Na+ • any excess K+ consumed in the diet is excreted in the urine; can accumulate to toxic levels in renal failure

K+ loss due to Na+, K+ ATPase pump inhibition - hypoxia, hypomagnesemia, digoxin overdose Insulin promotes acute entry of K+ into skeletal muscle & liver by increase Na+,K+ATPase activity Catecholamines like epinephrine ( beta- stimulator) promote entry of K+ into cells Propanolol ( beta-blocker) impairs entry of K+ into cells excercise • K+ released from muscle cells during excercise • fist pumping during venipuncture can cause falsely high K+ levels cellular breakdown - damage to RBCs releases K+ into ECF

serum plasma ( heparin preferred) don’t use EDTA ( K2 EDTA contains potassium ) urine ( random or 24 hr) Minimize hemolysis ( hemolysis falsely increases K+ results ) separate plasma/ serum from cells ASAP or may give false increased K+

flame photometry -older method q ion-selective electrodes (ISEs) -most routinely used

ion-exchange electrode solid plastic membrane contianing valinomycin - an antibiotic that will selectively bind with K+ KCL is the inner electrolyte solution Can be direct or indirect mode - difference between the two method is only slight compared to Na+

increased or decreased concentration of K+ will affect muscle irritability = cardiac arrest

serum : 3.5-5.1 mmol/L critical: < 2.8 or >6.0 - always check for hemolysis first

Chloride - major contributor to osmolality ( with Na+ )

Cl- usually shifts due to changes in Na+ or HCO3- Cl- in the diet • absorbed by intestinal tract • then filtered by glomerulus • then passively reabsorbed ( with Na+) by proximal tubules excess Cl- is ecreted in urine & sweat excessive sweating stimulates aldosterone to act on the sweat glands to conserve Na+ & Cl-

• Na+ is reabsorbed along with Cl- in proximal tubules - Na+ reabsorption is limited by amount of Cl- available • Chloride shift ( hamburger phenomenon) - CO2 from cellular metabolism in tissues moves into the plasma & the red cells - in RBCs, CO2 form carbonic acid which splits into H+ & HCO3- - Deoxyhemoglobin (HHb) buffers H+ - HCO3- moves diffuses into plasma - Cl- dissuses into RBC & electric balance is maintained

serum/plasma ( separate from cells) lithium heparin ( best anticoagulant) urine ( 24hr is best ) CSF ( decreased CL in bacterial meningitis ; glucose also decreased in bacterial meningitis) Sweat ( screen for cystic fibrosis ) - >60 mmol/L is positive for CF