Chapter 16: Electrolytes

1. Chapter 16: Electrolytes By George A. Harwell

2. Introduction to Electrolytes • Electrolytes: ions capable of carrying an electric charge • Two types • 1.Anions have negative charge & move toward anode. • 2.Cations have positive charge & move toward cathode. • Electrolytes are an essential component in many processes: • Volume & osmotic regulation (Na, Cl, K) • Myocardial rhythm & contractility (K, Mg, Ca) • Cofactors in enzyme activation (Mg, Ca, Zn) • Blood coagulation (Ca, Mg)

3. Water • Water = 40–75% of human body weight • Declines with age & obesity • Less in women than in men, due to higher % of body fat • Water is solvent for all processes in human body: • Transports nutrients to cells • Determines cell volume by its transport into & out of cells • Removes waste products by way of urine • Acts as body’s coolant by way of sweating • Found in intra- (2/3) & extracellular (1/3) compartments

4. Water (cont’d) • Osmolality • A physical property of a solution based on concentration of solutes (millimoles) per kilogram of solvent (w/w) • Related to changes in properties of solution relative to pure water (decreases in freezing point & vapor pressure) • Clinical significance of osmolality • Osmolality is parameter to which hypothalamus responds. • Regulation of osmolality affects plasma sodium concentration. • Regulation of sodium & water controls blood volume. • Determination of osmolality: may be measured in serum or urine

5. Water (cont’d) • Responses to changes in blood osmolality and blood volume

6. The Electrolytes • Sodium • Most abundant cation in ECF (90%) & largely determines osmolality of plasma • Sodium concentration in ECF is much larger than inside cells. • Active transport systems, such as ATPase ion pumps, prevent sodium equilibrium in all cells. • Regulation • Intake of water in response to thirst • Excretion of water • Blood volume status

7. The Electrolytes (cont’d) • Sodium • Clinical applications • Hyponatremia: lower-than-normal (<135 mmol/L) concentration of sodium in serum • Hypernatremia: higher-than-normal concentration of sodium in serum • Determination of sodium • Specimen: serum, plasma, whole blood, urine, sweat • Methods: Ion-selective electrodes method is most common.

8. The Electrolytes (cont’d) • Potassium • The major intracellular cation in body; concentration 20 times greater inside cells than outside • Functions include: • Regulation of neuromuscular excitability • Contraction of heart • ICF volume • Hydrogen ion concentration • Because K+ is intracellular, it is critical for the laboratorian to be aware of a hemolysis in a serum/plasma sample, as this can falsely elevate the K+ results.

9. The Electrolytes (cont’d) • Potassium • Regulation • Kidneys regulate potassium balance. • Potassium uptake from EFG into cells normalizes acute rise in plasma potassium concentration due to increased intake. • Factors that influence distribution of potassium: • 1. Inhibition of NaK ATPase pump by certain conditions • 2. Insulin promotes potassium ions entering muscle & liver. • 3. Catecholamines promote cellular entry of potassium.

10. The Electrolytes (cont’d) • Potassium • Regulation • Exercise increases plasma potassium. • Hyperosmolality depletes potassium. • Cellular breakdown releases potassium into ECF. • Clinical applications • Hypokalemia: lower-than-normal plasma potassium • Hyperkalemia: higher-than-normal plasma potassium • Collection of samples: Proper collection & handling is important. • Determination of potassium: specimen: serum, plasma, urine

11. The Electrolytes (cont’d) • Chloride • The major extracellular anion • Functions: maintains osmolality, blood volume, electric neutrality • Ingested in diet & almost completely absorbed by intestinal tract • Clinical applications • Hyperchloremia • Hypochloremia • Determinations of chloride • Specimen: serum, plasma, urine, sweat; method: ISE

12. The Electrolytes (cont’d) • Bicarbonate • Second most abundant anion in ECF • Major component of buffering system in blood • Regulation • Reabsorbed by proximal (85%) & distal (15%) tubules in kidneys • Clinical applications • Metabolic acidosis may decrease bicarbonate. • Determinations of carbon dioxide • Specimen: serum, plasma; methods: ISE & enzymatic

13. The Electrolytes (cont’d) • Magnesium • Physiology • 4th most abundant cation in body, 2nd intracellularly • Average human body contains 1 mole (24 g) of magnesium. • Widespread role in body • Glycolysis • Transcellular ion transport • Neuromuscular transmission • Synthesis of carbohydrates, proteins, lipids, nucleic acids • Release of & response to certain hormones

14. The Electrolytes (cont’d) • Magnesium • Regulation • Rich sources of magnesium: raw nuts, dry cereal, hard drinking water, vegetables, meats, fish, & fruit • Consumption of processed foods can lead to inadequate intake. • Regulation of body magnesium controlled largely by kidneys, which can reabsorb it in deficiency states or excrete excess • Clinical applications: hypomagnesemia & hypermagnesemia • Specimen: nonhemolyzed serum or lithium heparin plasma • Methods: calmagite, formazan dye, methylthymol blue

15. The Electrolytes (cont’d) • Calcium • Physiology • Essential for myocardial contraction • Blood-ionized calcium is closely regulated & has mean concentration in humans of about 1.18 mmol/L. • Important to maintain normal ionized levels during surgery & in critically ill patients • Regulation • Three hormones regulate calcium: PTH, vitamin D, calcitonin.

16. The Electrolytes (cont’d) • Calcium • Distribution • 99% of body calcium is in bone; 1% is in blood & other ECF. • Of total amount in blood, 45% circulates as free calcium ions, 40% is bound to protein, & 15% is bound to anions. • Clinical applications • Hypocalcemia: calcium depletion • Hypercalcemia: elevated calcium levels • Determination of calcium • Specimen: serum, plasma; methods: dye binding

17. The Electrolytes (cont’d) • Phosphate • Physiology • Found everywhere in living cells; participates in key biochemical processes • Regulation • May be absorbed in intestine from dietary sources, released from cells into blood, & lost from bone • Distribution: 80% in bone, 20% soft tissues, <1% serum/plasma • Clinical applications: hypophosphatemia & hyperphosphatemia • Specimen: serum, lithium heparin plasma; methods: formation of an ammonium phosphomolybdate complex

18. The Electrolytes (cont’d) • Lactate • Biochemistry and physiology • Byproduct of an emergency mechanism that produces a small amount of ATP when oxygen is severely diminished • Regulation • Not specifically regulated; levels rise rapidly when oxygen delivery decreases below a critical level • Clinical applications: metabolic monitoring in critically ill patients • Specimen handling: Avoid using tourniquet. • Methods: Most common today are enzymatic methods.

19. Anion Gap • Anion gap (AG): the difference between unmeasured anions & unmeasured cations • Created by concentration difference between commonly measured cations (Na+K) & anions (Cl+HCO3) • Useful for: • Indicating an increase in 1 or more unmeasured anions in serum • Quality control for analyzer used to measure electrolytes • Causes of elevation: uremia/renal failure, ketoacidosis, glycol poisoning, lactic acidosis, hypernatremia, error

20. Anion Gap (cont’d) • Demonstration of anion gap from concentrations of anions and cations in normal state and in lactate acidosis

21. Electrolytes and Renal Function • Summary of electrolyte excretion & conservation • Glomerulus: filters out large proteins & protein-bound particles • Renal tubules • Phosphate: reabsorption inhibited by PTH • Calcium: reabsorbed under influence of PTH • Magnesium: reabsorption occurs in Henle’s loop • Sodium: reabsorbed through 3 mechanisms • Chloride: reabsorbed by passive transport in proximal tubule • Potassium: reabsorbed by 2 mechanisms • Bicarbonate: recovered from glomerular filtrate