electrolytes 1 Flashcards
references ranges for osmolality
- serum
- osmolar gap
- critical values
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
what is the ultimate regulator of water in the body
Kidneys
hormones and water balance
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
body weight % of water
amount that is intracellular & extracellular
40-75% water
2/3 is intracellular
-40-50%
1/3 is extracellular
- intravascular (plasma) 5%
- interstitial (lymph & tissue) 15%
anions and anode
cations and cathode
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
main anion and cation of intracellular fluid & extracellular fluid
ICF
- main cation is K+
- main anion is HPO4(2-) aka phosphate
ECF
- main cation is Na+
- main anion is Cl-
what should you do if you have a high K+ result on a patient
first check for hemolysis ( hemolysis falsely increase K+)
then rerun
then report
what has osmolality replaced for testing
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 )
Colligative properties
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
Na+ accounts fro what % of osmotic activity in plasma
• 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.
water deficit
- 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
water excess
• 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
diabetes insipidus
• 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.
Regulation of blood volume - decreased blood volume or pressure (hypovolemia)
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 )
Regulation of blood volume - increased blood volume/ pressure ( hypervolemia )
- Atrial natriuretic peptide (ANP) is released from myocardial atria
- Promotes Na+ excretion in kidney
specimens for osmolality
• 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)
common methods for analysis of osmolality
- Freezing point depression
- Vapor Pressure decrease
-Freezing point osmometers use sodium chloride as reference
solutions (calibrators).
Osmolality Measurement: Freezing Point Depression Osmometer
- 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)
Calculated Osmolality
- 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)
Oamolal Gap
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
Electrolyte functions
- maintains osmotic pressure & water distribution
- maintains pH
- regulates the function of heart & other muscles
- oxidation- reduction rxns
- 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
Ion-Selective Electrode Potentiometry - Key Words
oxidation , reduction , anode, cathode, electrochemical cell
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
potentiometry & principle
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
• 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.
indicator electrode
• Measuring electrode constructed to detect a specific ion of interest
• Generates electrical potential when placed in a solution containing
the ion to be measured.
• The potential generated is proportional to the concentration of the ion
(activity)
reference electrode
- Surrounded by a solution of constant composition
- At constant pH; generates constant potential; compensates for changes in temp., electrical noise, age of electrodes, etc.
- Common reference electrodes:
- Silver/ silver chloride (Ag/AgCl)
- Calomel (mercury/ mercury chloride) Hg/HgCl
Nernst Equation
𝐸= (𝐸𝑜+2.39𝑅𝑇 / 𝑛𝐹 ) log𝑎
in brackets is the constant
slope = 2.3 RF/ nF
reduced equation = E~log a
i.e. measuring potential (mV) ~ activity (concentration) of the ion
sloping of an electrode ( calibrating using 2 standards )
when conc & potential are plotted against each other on a semi log paper
- straight line
For a monovalent ion (eg. K+) Calibration Standards with tenfold difference have
voltage difference of 59.1mV
When the sample is analyzed, the mV reading is used to extrapolate the
concentration of ion from the slope
Ion-Selective Electrodes selectivity
- ISEs are selective but not specific
- Other interfering ions are also measured to a slight degree
• Each ion has a selectivity factor to determine the amount of interference it
will cause
• Electrodes are designed to have the lowest possible selectivity factors for
interfering ions.
classes of electrodes
- Solid material (eg. Glass)• Specific types of glass used (eg. H+, Na+)
- Can be modified using a gas-permeable membrane (eg. CO2)
- Liquid (Ion-exchange)• Binds with ion being measured (eg. K⁺)
- Solid state• Insoluble inorganic salts in a membrane; potential due to the ion exchange process. (eg. Cl⁻)
- Enzyme• Enzyme bound to surface; reacts with a non-ion to produce ionic species which can then be measured (urea/urease = NH₄+)
