Chapter 16 Electrolytes By George A Harwell Copyright

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) Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

Water (cont’d) • Responses to changes in blood osmolality and blood volume Copyright ©

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Na. K ATPase pump by certain conditions • 2. Insulin promotes potassium ions entering muscle & liver. • 3. Catecholamines promote cellular entry of potassium. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

The Electrolytes (cont’d) • Magnesium – Physiology • 4 th most abundant cation in body, 2 nd 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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

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. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

Anion Gap • Anion gap (AG): the difference between unmeasured anions & unmeasured cations • Created by concentration difference between commonly measured cations (Na+K) & anions (Cl+HCO 3) • 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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

Anion Gap (cont’d) • Demonstration of anion gap from concentrations of anions and cations

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 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins