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UG Biochemistry > 5. Fluid + Electrolyte Na+ > Flashcards

5. Fluid + Electrolyte Na+ Flashcards

1
Q

What are electrolytes?

A

Ions capable of carrying electric charge

Anions -
Cations +

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2
Q

What are the main physiological electrolytes to consider?

A 
Sodium Na+
Potassium K+
Calcium Ca2+
Magnesium Mg2+
Chloride Cl-

Acid base;
Hydrogen phosphate HPO42-
Hydrogen carbonate HCO3-

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3
Q

What do the electrical charge symbols on an ion indicate?

A

Indicate substance is ionic in nature + has unbalanced distribution of electrons = result of chemical dissociation in solution

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4
Q

What are the main functions of electrolytes in physiology?

A

Conduct energy;
- neurons = electrical tissue, activated by electrolyte activity b/n ECF + ICF via ion channels

Regulate fluid balance + pH;

  • Na (main in ECF) + K (main in ICF) involved in fluid balance + BP
  • osmotic gradients affect + regulate hydration + blood pH = critical for nerve + muscle function

Support muscle function;

  • muscle tissue = electrical tissue, activated by electrolyte activity b/n ECF + ICF via ion channels
  • muscle contraction dependent on presence of Ca2+, Na+, K+: deficiency = muscle weakness/severe contractions
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5
Q

What physiological processes are electrolytes essential for?

A

Volume + osmotic regulation (Na+, Cl-, K+);
- osmoregulation = physiological process maintaining fixed conc of cell-membrane impermeable mol + ions in the interstitial fluid

Myocardium rhythm + contractility (K+, Mg2+);

  • contraction beings with characteristic flow of ions across cell membrane = AP
  • triggers muscle contraction by increased Ca2+ in cytosol

Enzyme cofactors (Mg2+, Ca2+, Zn2+);

ATP production + ion pumps (Mg2+, PO4-);

  • binding of divalent cation (almost always Mg2+) strongly affects interaction of ATP with various proteins
  • ATP exists in cell mostly as complex with Mg2+ bonded to phosphate oxygen centres

Neuromuscular excitability (K+, Ca2+, Mg2+)

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6
Q

How are electrolytes balanced in the body + why?

A

Electrolyte homeostasis must be maintained to allow proper function

Depends on integration of respiratory, renal + behavioural systems

H2O + Na2+ regulation maintained against variation in volume + osmolality

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7
Q

What contributes to electrolyte intake?

A

Food
Fluids
Metabolic reactions

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8
Q

What contributes to electrolyte output?

A

Respiration (H+/HCO3-)
Excretion (H2O/electrolytes)
Behaviour (alcohol = increased urination)

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9
Q

Describe the overall fluid state of the human body

A

Average H20 content in body = 40-75% total body weight

  • H2O = solvent for body processes
  • women >H2O as body comp different

2/3 body H20 = ICF
1/3 body H20 = ECF

Plasma (ECF component) = 93% H2O + lipids + proteins

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10
Q

What are the different tonicities?

A

Hypertonic = increased osmotic pressure aka more solutes in a solution

Hypotonic = decreased osmotic pressure aka less solutes in a solution

Relative to another solution

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11
Q

Describe the fluid shift between ICF + ECF

A

ECF + ICF are dynamic;

  • link b/n external/internal environments = plasma (only fluid circulating whole body)
  • osmolalities of all body fluids equal: changes in solute concentration quickly follow by osmotic changes
Water movement;
ECF hypertonic (more solute) = water moves ICF -> ECF
ECF hypotonic (less solute) = water moves ECF -> ICF (cells)

Solute movement;
Ion fluxes restricted + move selectively by active TP

Opposing conc of Na+ and K+ in ECF + ICF;
- maintained by activity of cellular ATP-dependent Na+/K+ pumps

Nutrients/resp gases/wastes move unidirectionally

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12
Q

Define + describe the ECF

A

ECF (1/3 BF) = all body fluid outside cells of multi cell org

Components;

  • interstitial fluid: main, bathes cells: provides nutrients/ removes waste
  • blood plasma (both = 97% ECF)
  • transcellular fluid: smallest, contained within epithelial lined spaces e.g. CSF, aq humour, serous fluid, peri/endo lymph, joint fluid (2.5% ECF)

Plasma + IF v similar as water, ions + small solutes constantly exchanged b/n across walls of capillaries through pores + capillary clefts

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13
Q

Define + describe the ICF

A

ICF (2/3 BF) = the cytosol, the fluid contained inside cells

The matrix in which cell organelles are suspended

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14
Q

Define osmolality + its use in physiology

A

Osmolality = physical property of a solution based on concentration of solutes (mOsm/kg) per kg of solvent

Controlled by release of arganine vasopressin hormone (AVH/ADH) from post pit - changes water output in urine

Used to assess water balance + whether body is regulating properly

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15
Q

Define osmolarity + its use in assessing fluid balance

A

Osmolarity = concentration of solutes (mOsm/L) per litre of solvent

Can be inaccurate as affected by temp, H20 content, pressure

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16
Q

Define the osmolar gap + its use in assessing fluid balance

A

Osmolar gap = difference b/n measured osmolality + calculated osmolarity

Due to differences in the way blood solutes are measured + their calculation method

Normally osmolality contributing particles in serum = sodium, potassium, salts, glucose, urea

Osmolar gap calculates difference in osmolarity + osmolality to see if OTHER particles are contributing to measured serum osmolality

> 10 indicates for e.g. alcohol, aspirin, manitol

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17
Q

What is the normal plasma osmolality maintained at + how sensitive is it to change?

A

Maintained at ~275-295 mOsm/kg of plasma H2O

Very sensitive to even 1-2% changes in osmolality

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18
Q

What is the main osmotic agent in ECF?

A

NaCl is the main osmotic agent in ECF - cannot alter one aspect of sodium/water balance without triggering homeostatic mechanisms which influence the other

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19
Q

How is plasma osmolality regulated?

A

Kidney function: H2O output in urine/salt output or reabsorption

Kidney function regulated by anti-diuretic hormone (ADH) (aka arganine vasopressin hormone/vasopressin)

  • ADH produce in hypothalamus
  • ADH stored in + released from posterior pit

Primary function of ADH = decrease water loss in kidneys = decrease conc of electrolytes in body fluid

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20
Q

Describe the ADH loop regulating osmolality

A

Increase in salt concentration;

  • sensed by receptors in hypothala > signals post pit
  • post pit increases ADH secretion
  • blood reabsorbs more water from filtrate in kidney
  • urine is more concentrated
  • salt concentration of body fluid is decreased

Decrease in salt concentration;

  • sensed by receptors in hypothala > signals post pit
  • post pit decreases ADH secretion
  • blood reabsorbs less water from filtrate in kidney
  • urine is less concentrated
  • salt concentration of body fluid is increased
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21
Q

Describe the state of sodium in the body + its relation to osmolality

A

Normal plasma osmolality = 270-295mmol/L due to Na+ and assoc anions

90% of all cations in the ECF

ECF = high Na+/low K+
ICF = low Na+/high K+
Cell balance is maintained by Na+/K+ ATPase ion pumps;
- 3 Na+ pumped out of cell in exchange for 2 K+ ions
- ATP>ADP

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22
Q

How is sodium measured?

A 
ISE - routine in clinical lab
Colorimetric - useful for POCT
Enzyme modulation
Flame emission spectroscopy (FES)
Atomic absorption spectroscopy(AAS)

Some methods require large sample volume

23
Q

Describe ISE as a method for sodium measurement

A

Uses semi-permeable membrane allowing particular ions to pass through;

  • potential difference builds up b/n outside + inside of membrane depending on ion conc
  • difference measured by 1 test electrode in respect to 2nd reference electrode

Measures ion activity not conc

ISE for sodium;

  • ISE calibrated with Na+ soln of known concentrations
  • 2 types of ISEs with no significant difference b/n results
  • DIRECT ISE: reqs larger, undiluted sample
  • INDIRECT ISE: reqs smaller, dilute sample (dilution error)

Advantages;

  • cheap
  • easily automated
  • quick

Disadvantages;

  • dilution error
  • protein build up on membrane
  • nonspecific
24
Q

Discuss the possibility of dilution errors with ISE

A

Dilution + calculations to produce ISE depend on assumption sample contains normal fat levels

In v lipidemic samples = apparent hyponatremia even when biologically active sodium conc in aq phase is normal

25
Q

What are the types of water balance disorders?

A

Dehydration

Hypotonic hydration

26
Q

Describe dehydration, its signs + symptoms, and causes

A

Water loss> water intake

S+S;

  • thirst
  • dry/flushed skin
  • oligouria (decreased urine production)

Causes;

  • hemorrhage
  • severe burns
  • prolonged vomiting/diarrhea
  • profuse sweating
  • water deprivation
  • diuretic abuse
27
Q

Describe hypotonic hydration, its complications, and causes

A

Cellular overhydration/water intoxication

  • diluted ECF: normal [Na+] but excess [H2O] = hyponatremia
  • net osmosis into tissue cells (ECF>ICF)

Causes;

  • renal insufficiency
  • large volume water ingested

Complications;
- if not quickly reversed = severe metabolic disturbances, especially in neurons

28
Q

What is referred to when discussing electrolyte balance in physiology?

A

Electrolytes = salts, acids + bases

But electrolyte balance usually refers to salt balance only

29
Q

How can osmolality be used to classify + identify causes of hyponatremia?

A

Na+ = major contributor to osmolality therefore levels can be used to help identify cause of hyponatremia;

Hyponatremia with;

  1. low osmolality: increased Na+ loss/H2O retention
  2. normal osmolality: increased non-sodium cations/lithium excess/hyperlipidemia/ lots of others
  3. high osmolality: hyperglycemia, mannitol infusion
30
Q

How does urea + electrolytes test help assess fluid status?

A

U+E = most commonly performed biochem lab test

Analytes;

  • electrolytes: Na, K, Cl, HCO3
  • markers of renal function: urea, creatinine

Provides info on renal function + dehydration

  • can exclude serious imbalance of salts in blood
  • creatinine levels = eGFR (gold std marker kidney health)

Abn very common in hospitalised pts

Conditions detected;

  • hyper/hyponatremia
  • hyper/hypokalemia
  • metabolic acidosis/alkalosis (HCO3-)
  • dehydration (increased urea/creatinine)
  • GI bleed (increased urea)
  • hyper/hypoglycemia (glucose)
31
Q

Describe some of the main causes of fluid imbalance

A 
Vomiting/diarrhea
Unable to eat/drink
Blood loss
Kidney disease
Water tablets for hypertension
Heart disease
Failure to pump fluid (blood) where it needs to be
Iatrogenic disease (diag + therapeutic procedures)
32
Q

When do U+E values need validated?

A

If result grossly abn

If an unlikely change b/n one day and next

33
Q

What is hyponatremia?

A

Serum/plasma Na+ <135mmol/L

Common;

  • most common disturbance in blood chem
  • most common electrolyte disorder in hosp + non-hosp pts

Usually mild, self limiting condition
- pts asymptomatic

Severe can occur in serious conditions + associated with neurological symptoms

34
Q

List the symptoms of hyponatremia

A

Depend on severity + speed of onset

120-125mmol/L = primarily GI
<120mmol/L = nausea, vomiting, muscle weakness, headache, lethargy, ataxia, confusion

Severe symptoms = seizures, coma, resp depression
<120mmol/L for 48H or less = medical emergency

35
Q

When should the ward be phoned for a hyponatremic pt?

A

<125mmol/L Na+

36
Q

What are the 3 broad causes of hyponatremia?

A
  1. Increased sodium loss - loss of sodium rich fluid
  2. Increased H20 retention - failure to excrete fluid load
  3. Water imbalance - admin of hyponatremic fluids/failure to excrete water normally
37
Q

Describe the causes of increased sodium loss

A

GI loss with inappropriate replacement

Decreased aldosterone production (hypoadrenalism)

Diuretics/medication

Salt losing nephropathy in renal tube disorders (renal salt excretion regulated primarily by tubular reabsorption rather than glomerular filtration)

Low K+: tubules conserve K+ and excrete Na+

38
Q

Describe the causes of increased H20 retention

A

Failure to excrete water

Heart/liver failure

Dilution of serum/plasma Na+ in renal failure

Decreased plasma proteins = decreased colloid osmotic pressure = intravascular fluid (plasma) migrates to tissue causing edema

Failure to pump enough blood to kidney for excretion

39
Q

Describe the causes of water imbalance

A

Syndrome of inappropriate AVP/ADH secretion (SIADH)

Pseudo-Na+ measured by indirect ISE in hyperprotein/lipidemic pts

Excess water intake

Inappropriate IV fluids

40
Q

How is hyponatremia classified?

A
  1. True hyponatremia: serum Na+ conc truly less than normal (<135-145 mmol/L)
  2. Pseudo-hyponatremia: serum Na+ conc actually normal but erroneously reported low due to either hyperlipidemia or hyperproteinemia
41
Q

What is pseudohyponatremia caused by hyperlipidemia?

A

Serum Na+ conc normal but falsely reported low due to hyperlipidemia

Not common, error usually small

42
Q

Why does pseudo-hyponatremia occur?

A

Modern analysers used indirect ISEs to estimate electrolyte conc in serum;

  • reqs 1:10 dilution ratio before measuring Na+
  • assumes sample = 93% H2O

Water concentration is altered by presence of increased lipid/protein content = decreased water fraction in sample

  • smaller sample in same dilution = increased dilution vs normal sample
  • dilution step + calculation of conc by analyser then gives false low Na value
43
Q

What is the problem with false low sodium value and how can it be avoided?

A

Pt at risk, especially if false result informs clinical decision

Avoided by;
Checking sample;
- hypertriglyceridemia usually grossly evident
- check protein level
- check osmolality: if Na+ low but osmolality normal then problem not with Na+ balance

Use direct ISE: no dilution step
- directly measures activity of Na+ in water phase rather than total Na+ conc

Calculate corrected value from serum water fraction if indirect ISE not available;
- equation for this

44
Q

When should pseudo-hyponatremia be suspected?

A

Hyperproteinemia
Hyperlipidemia
Low sodium in diabetes mellitus with hyperlipidemia
Discrepancy b/n calculated + measured osmolality

45
Q

What other investigations are carried out for hyponatremia after ISE serum?

A

Urinary Na+ and osmolality

46
Q

How is hyponatremia treated?

A

Correct cause, either: increased H20 or decreased Na+

Depends on severity + onset

Strict fluid management, commonly fluid restriction + hypertonic saline

AVP/ADH receptor antagonists;

  • conivaptan for treatment of euvolemic, hypervolemic hyponatremia
  • blocks action of AVP in collecting ducts of nephron to decrease H2O reabsorption = pee out more water!
47
Q

What is hypernatremia?

A

Hypernatremia = elevated serum Na+ conc (>145mmol/L)

Not usually caused by excess of Na+ but relative deficit of water so often coincides with dehydration

Less common than hyponatremia

48
Q

List the symptoms of hypernatremia

A

Most commonly involve CNS;

  • altered mental status
  • lethargy
  • irritability
  • seizures
  • muscle twitching
  • nausea
  • vomiting

Serum Na+ >160mmol/L = mortality rate of 60-75%

Pts obviously unwell with severe hypernatremia

49
Q

When should the ward be called for a hypernatremic pt?

A

If Na+ >160mmol/L

50
Q

What are the causes of hypernatremia?

A

Excess water loss;
- any condition causing excess water loss

Diabetes insipidous;
- deficiency of pit hormone ADH which regulates kidney function

Decreased water intake;

  • pt with stroke/dementia
  • increased urea/creatinine

Increased intake/retention;

  • inappropriate IV prescribing
  • hyperaldosteronism
51
Q

How is hypernatremia diagnosed?

A

ISE

Measurement of urine osmolality needed to evaluate cause of hypernatremia;

<300mOsm/kg;
- diabetes insipidous (impaired secretion of ADH or kidneys cannot respond to ADH)

300-700mOsm/kg;

  • partial defect in ADH release/response to ADH
  • osmotic diuresis

> 700mOsm/kg;

  • loss of thirst
  • insensible loss of water (breathing/skin)
  • GI loss of hypotonic fluid
  • excess intake Na+
52
Q

How is hypernatremia treated?

A

IV fluids to reduce Na+ levels at carefully controlled rate

Management: careful monitoring necessary as rapid reduction can cause cerebral edema, convulsions + permanent brain injury

53
Q

List the important serum sodium measurements from hyponatremia to hypernatremia

A 
Hyponatremia <135mmol/L
Normal 135-145mmol/L
Mild hypernatremia 140-149mmol/L
Moderate hypernatremia 150-169mmol/L
Severe hypernatremia >=170mmol/L

UG Biochemistry (14 decks)

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