electrolytes 1

serum : 275-295 mOsm/kg *urine has a wider range than serum osmolar gap : 5-10 mOsm/kg critical serum values : <250 & > 325 mOsm/kg

Kidneys

Arginine Vasopressin Hormone (AVP ) • antidiuretic hormone(ADH) • increases water reabsorption • secreted by the hypothalamus Aldosterone • increases Na+ reabsorption • produced by adrenal cortex • increased aldosterone= more Na+( and Cl-) reabsorbed in exchange for H= (or K+) secretion into urine

40-75% water 2/3 is intracellular -40-50% 1/3 is extracellular • intravascular (plasma) 5% • interstitial (lymph & tissue) 15%

anions (-) move toward the anode (+) cations (+) move towards the cathode (-) electrolytes are substances that split into ions total anions =total cations - an increase in one anion means another anion must decrease or a cation must increase to keep balance

ICF • main cation is K+ • main anion is HPO4(2-) aka phosphate ECF • main cation is Na+ • main anion is Cl-

first check for hemolysis ( hemolysis falsely increase K+) then rerun then report

it has replaced specific gravity as the test to assess renal concentration SG includes number & size of molecules osmolality is a measure of concentration based only on the number of small molecules present in solution ( not weight or size )

solution properties related to the number if molecules ( particles present in the solvent increasing the number of particles in a solution will • lower freezing point • higher boiling point • increased osmotic pressure • lower vapor pressure if 1 Osmol of any solute is added to 1 kg of water the freezing point is decreased by 1.86 degrees Celsius

• The concentration of Na+ in plasma is affected by: • The regulation of osmolality • The regulation of blood volume • Osmoreceptors in the hypothalamus respond to small changes in osmolality. - An increase in osmolality causes an increase in AVP concentration. - A decrease in osmolality shuts off AVP production.

• Thirst is important in preventing water deficit. • Deficit of H2O will increase plasma osmolality. • AVP and thirst will be activated • AVP will cause H2O to be reabsorbed • Thirst is the major defense against hyperosmolality and hypernatremia • Hyperosmolality and hypernatremia is a concern for: - Infants - Unconscious patients - Older patients - Those with diminished mental status • For these patients, dehydration is a concern

• Excess intake of H2O (polydipsia) will lower plasma osmolality. • AVP and thirst will be suppressed • H2O will not be reabsorbed • Large volume of dilute urine excreted (10 - 20 L) • Hypoosmolality and hyponatremia usually only occur if there is an impairment with the renal excretion of water

• No AVP production or no ability to respond to circulating AVP • Excessive thirst • Increased urine output (up to 10 L/day) • Water intake = water output • Plasma osmolality remains normal • Because thirst response is normal in these patients, dehydration is prevented.

Renin converts angiotensinogen to angiotensin l angiotensisn converting enzyme ( ACE) converts Angiotensin l to angiotensin ll angiotensin ll causes vasoconstriction - blood pressure is increased Aldosterone is excreted which increases retention of Na+ ( and the H2O that accompanies it )

• Atrial natriuretic peptide (ANP) is released from myocardial atria • Promotes Na+ excretion in kidney

• Measured on serum or urine (not plasma) • Plasma may have osmotically active particles from the anticoagulant • Sample must be free of particles or centrifuged prior to testing • Clinical unit of measure is the milliosmole (mOsm/kg)

• Freezing point depression • Vapor Pressure decrease -Freezing point osmometers use sodium chloride as reference solutions (calibrators).

• Sample is supercooled to -7 degrees C • Supercooling - cooling below the freezing point but remaining in a liquid state • Mechanical agitation ‘seeds’ crystal formation • As crystals form, the heat of fusion is released and the solution warms to its freezing temperature and freezes • Probe measures freezing point • Final reading is taken reflecting the freezing point of the solution • The more particles present in a sample, the more the freezing point is depressed (lowered)

• Based on the major contributors to serum osmolality: glucose, urea and sodium • Sodium is always with chloride (therefore 2 ions upon dissociation) 1.86*(Na+ mmol/L) + glucose (mmol/L) + urea (mmol/L)

Difference between the measured and the calculated osmolality • An increased osmolal gap may indicate the presence of other osmotically active substances: • Ethanol, methanol, ethylene glycol, isopropanol, lactate or β-hydroxybutyrate - ex. an increase in lactate may occur due to increased anaerobic metabolism of glucose (pyruvate → lactate) during an AMI • an increase in β-hydroxybutyrate ( a ketone body ) is increased if someone is diabetic • The osmolal gap should be 5 - 10 mOsm/kg

• Maintain osmotic pressure and water distribution (Na+) • Maintain pH (HCO3-/H+) • Regulate the function of heart and other muscles (K+, Ca++) • Involved in oxidation-reduction reactions (H+) • Act as cofactors for enzymes

Oxidation - loss of electrons at the anode Reduction - gain of electrons at the cathode Anode - positive electrode; where oxidation takes place Cathode - negative electrode; where reduction takes place Electrochemical cell - combination of 2 half cells (oxidation and reduction), connected by a salt bridge

measures voltage (mV) generated by an electrochemical cell at zero current Difference between potential from the reference electrode and the indicator (ISE) electrode

• Electrochemical cell - 2 half cells or electrodes connected by a salt bridge • Indicator (ISE) Electrode - detects the ion of interest • Reference Electrode - provides constant potential Note: ISE’s used in instruments have all the components (indicator electrode, reference electrode and salt bridge) in one unit.