Lecture 5: Electrolytes Flashcards
Total body water distribution
Total body weight: 40% dry, 60% water
Total body water: 66% ICF, 33% ECF
Extracellular water: 20% plasma, 80% interstitial
See figure
What % of total body weight are plasma volume and blood volume?
Plasma volume: 4% of total body weight
Blood volume (40% Hit): 7% total body weight
What % of total body weight is made of total body water?
45 - 75%
Variability depends on sex, % body fat, % skeletal muscle, age
What % body weight is represented by total body water for males vs females
Males: 60%
Females: 50%
Females have increased fat, which decreases % water
Males have increased muscle, which increases % water
What does age do to the % body weight of total body water
Increasing age decreases % body weight
Newborn: 75%
1 year: 65%
adult 50 - 60%
Relationship of ECF (plasma, interstitial) and ICF
See figure
What other system drains into the interstitial?
Lymphatic fluid
Relative ion amounts in extracellular vs intracellular fluid
High in extracellular: Na, Ca, Cl, HCO,
High in intracellular: K, Mg, HPO4, Protein
See figure
What are the extracellular and intracellular concentrations of Na
Extracellular: 140 (plasma), 146 (interstitial)
Intracellular: 12
What are the extracellular and intracellular concentrations of K
Extracellular: 4 (plasma), 4 (interstitial)
Intracellular: (150)
What are the extracellular and intracellular concentrations of Cl
Extracellular: 103 (plasma), 104 (interstitial)
Intracellular: 3
What happens once gradients are established in the ECF and ICF?
little movement of ions between ECF to ICF
water moves easily across plasma membranes based on osmotic forces
if sodium is added to the ECF – amount of Na in the ECF increases; amount of Na in ICF is unchanged
Distribution of anions and cations in the ECF and ICF
See figure
Control of ICF and ECF
ICF is tightly controlled
ECF is less tightly controlled
What movement does the capillary wall allow?
Fluid and electrolytes
Movement across cell membrane
Plasma membrane contains channels and pumps which control the movement of electrolytes
Water moves easily across via osmotic forces
What controls movement between plasma and interstitial fluid?
Starling forces
See figure
What forces move fluid out of the capillaries?
PC = hydrostatic pressure in capillary (changes along capillary, 37 to 17mm Hg)
pi = oncotic (colloid) pressure in interstitium (~0-1 mm Hg)
What happens to movement between plasma and interstitial when venous pressure increases?
Increased PC may produce edema since fluid can’t return.
What focus move fluid into capillaries?
Pi = hydrostatic pressure in interstitium (~1-2 mm Hg)
pC = oncotic (colloid) pressure in capillary (25 mm Hg)
What happens if the lymphatic system doesn’t drain the interstitial and return fluid to circulation?
Edema
The arterial ends of the capillaries have sphincters to control pressure
The venous end of the capillaries do not have these sphincters
See figure
What determines fluid movement between plasma and interstitial space?
Hydrostatic pressure
changes in osmolality play a minor role (but still important)
What does an increase in arterial pressure (BP) do to the PC?
does not increase PC (at least directly) since pre-capillary sphincters control the pressure in the capillaries
What does an increase in venous return to the heart do to the PC?
if the heart is unable to pump blood forward, the pressure backs up into veins
the increased venous pressure increases PC, leading to edema
See figure
What happens if lymphatic system is not working properly?
Fluid accumulates in the lymphnodes
How is the electrochemical gradient regulated?
The Na/K ATPase establishes/maintains the electrochemical gradient
Ion channels regulate ion movement across membranes
This allows alteration of membrane potential – e.g. action potential
Water movement between ICF and ECF
Water moves easily/freely between the two compartments (ICF:ECF)
Water movement is driven by differences in osmolality
What are the osmolalities of the ICF and ECF at steady state>
They are the same
this is due to the bidirectional movement of H2O, not solute movement
What is the major osmotically active cation outside the cell? Can its concentration change?
Sodium
Concentration can change a fair amount (diet, dehydration)
What is the major osmotically active cation inside the cell? Can its concentration change?
Potassium
its concentration should not change very much in or out of the cell
otherwise, arrhythmia/death can occur- especially with a change in extracellular fluid
What are the main consequences OF changes in sodium and potassium?
Sodium produce osmolality changes and cause water movement between ICF and ECF
Potassium produce changes in resting membrane potential (RMP) and cellular excitability
What is osmolality?
instrument measurement of solutes in a solution
reported as
mOsmol/Kg of solvent
What is osmolarity?
calculated measurement of solutes in a solution and reported as mOsmol/litre of solvent
What is osmotic pressure?
the pressure needed to prevent the movement of water across a semipermeable membrane
What is tonicity?
osmotic pressure gradient of two solutions separated by a semipermeable membrane, usually in reference to plasma.
Where does osmosis occur?
between two separated compartments if the membrane allows water but not solute movement (e.g. ECF and ICF)
What produces osmotic pressure?
Difference in osmolality on two sides of membrane
since solutes can’t cross the membrane but water can, the hypertonic side appears to “pull” water out of the hypotonic side.
Is it better to drown in fresh water or salt water?
Freshwater (hypotonic) drowning: cells will burst
Seawater (hypertonic) drowning: cells shrink
Shrunken cells can still be restored
So, it’s better to drown in seawater
What solutes play a major role in plasma osmolality?
Sodium and it’s associated anions
Formula to calculate osmolality in mOsm/L
calculated osmolality (CO) = (2 x [Na]) + [glucose] + blood urea nitrogen (BUN)
in a healthy person, the calculated osmolality is predictably about 10 mOsm/L less than the actual measured osmolality
PNa (and hence osmolality) and water movement
See figure
Why is sodium important in determining total body water?
changes in plasma sodium concentration change plasma osmolality
when plasma osmolality changes:
water distribution changes (intracellular and extracellular) - acute and small correction normally
What hormone levels change for water and sodium regulation?
vasopressin and aldosterone
required for chronic correction
Vasopressin (site of release, effect, when would we want it)
released from posterior pituitary
increases water reabsorption in collecting duct of kidney
Want this in hypertonic conditions
Aldosterone(site of release, effect, when would we want it)
released from adrenal medulla
increases sodium reabsorption in distal tubule of kidney
would want this in hypotonic conditions
What happens if you ingest large volumes of water?
could be due to excessive water reabsorption - SIADH)
water absorbed from gut into the plasma
dilution of PNa
hyponatremia and decreased osmolality ê
osmotic gradient now favors movement of fluid into cells
water moves (is drawn) into cells to balance osmotic gradient
cell size increases
normally not a problem (except in the brain)
What happens when you ingest or infuse sodium and little/no water
INCREASED NA IN PLASMA (increases vascular volume and TBW)
If you ingest or infuse sodium and little/no water
(increases total body sodium but only in ECF)
plasma sodium concentration rises, increasing plasma osmolality (osmol receptors in hypothalamus - increases vasopressin release)
water moves (is pulled) out of cells to balance osmolality
plasma osmolality still slightly elevated when balanced
vasopressin retains water in the kidney and helps dilute plasma back to normal osmolality
Now have increased extracellular (interstitial and plasma) volume (Na and H20). Cells returned to previous volume (and osmolality)
What happens when you lose sodium or have not enough loss of water?
DECREASED VASCULAR VOLUME AND TBW
plasma sodium concentration drops, decreasing plasma osmolality (this turns off vasopressin but increases aldosterone release)
water moves (is pulled) into cells to balance osmotically
fluid shifts from ECF to ICF: decreases plasma volume
plasma osmolality remains slightly decreased aldosterone retains sodium in the kidney helps correct low sodium
now have decreased extracellular (interstitial and plasma) volume (sodium and water)
Why do we calculate the plasma osmolality (CO) when it can be measured?
This equation shows the major importance of Na.
If the measured osmolality (MO)»_space;> calculated osmolality (CO), this indicates that there is a foreign substance in the blood
Osmolar gap calcularion
OG = MO - CO
Normally about 10 mOsmol/L
What does MO measure?
Measures the osmolar effects of all substances in the blood including sodium, glucose, BUN and any foreign substances if osmotically active
What does CO measure?
Only measures osmolar effects of sodium, glucose and BUN
What does a high osmolar gap indicate?
poisoning with alcoholic beverages, ethanol, wood alcohol (methanol), rubbing alcohol or antifreeze
(osmotically active in blood and blood levels can be high)
Where is the OG most helpful?
narrowing down causes of metabolic acidosis (later)
What is the electrical charge of plasma?
Electroneutral (cations = anions)
What are the normal major cations and anions in blood?
cations: Na and K (less so K)
anions: HCO3 and Cl
How to estimate the normal calculated anion gap
Na+ - (HCO3- + Cl-)
Eq’n says NaHCO3 and NaCl account for most cation/anion associations
will also account for Na associated with other anions (small in number)
What happens if anion gap increases when estimated?
Something other than HCO3, Cl and the small number of unmeasured anions is also associated with Na
The level of sodium doesn’t change but the anions it is associated with does
Tells you there is more of an unmeasured anion present that is linked to sodium
Normal plasma sodium concentration
135-145 mmol/litre
What does low sodium concentration in the plasma tell you about the movement of water between the ECF and ICF?
Water moves into the ICF
What does low sodium concentration in the plasma tell you about the relation of sodium to water in the ECF?
More water
What does low sodium concentration in the plasma tell you about the plasma volume (low, high or normal)?
Requires more info
What does a high sodium concentration in the plasma tell you about the movement of water between the ECF and ICF?
Moves into ECF
What does a high sodium concentration in the plasma tell you about the relation of sodium to water in the ECF?
More sodium
What does a high sodium concentration in the plasma tell you about the plasma volume (low, high or normal)?
Requires more info
What is hyponatremia?
(only tells you more water than sodium in ECF)
low sodium concentration in plasma
decreased plasma osmolality
increased water in relation to sodium
water would move from plasma to enter cells
can be problematic in brain, producing cerebral edema
What is hypernatremia?
(only tells you more sodium than water in ECF)
increased sodium concentration in the plasma
increased plasma osmolality
decreased water in relation to sodium
water would move from cells to plasma
may be useful, to treat cerebral edema caused by head injury - may be harmful, decreases brain volume
Why is plasma volume important?
allows the heart and circulation to function properly!
If plasma volume is too high or low - cardiovascular problems may occur which may be life threatening.
What is the total body water or plasma volume of a patient with a chronic plasma sodium concentration of 125 mmol/L
Is plasma volume high, normal or low?
ANSWER – we don’t know
Additional information required: volume status of patient to go along with the PNa
What is hypovolemia? How is it assessed?
insufficient vascular volume
history (vomiting, diarrhea, fluids,
physical (BP, HR, skin turgor, mucous membranes, etc)
urine output (volume, colour)
What is hypervolemia? How is it assessed?
excessive vascular volume
increased weight
indications heart can not adequately deal with the increased volume (backs up into venous system, edema in lower extremities to start
possible ascites (fluid in abdomen), pulmonary edema
How to identify problem using PNa and volume status
See figure
What is plasma potassium used for?
important in determining intracellular osmolality
cells tightly regulate their internal milieu (K changes little)
Why are changes occurring in extracellular potassium relative to intracellular potassium critical?
These can produce major effects on the resting membrane potential.
this can cause problems in electrically conducting tissues (nerves, heart)
Intracellular and serum concentration of K+
Intracellular: 140-150 mmol/L
serum: 3.5-5 mmol/L
even small change in serum levels can be life-threatening
What can be used to evaluate changes in heart function?
EKGs
if changes are chronic and gradual, some patients may be asymptomatic at 6.5 mmol/L whereas others might not survive this as an acute change
How does the body respond to changes in plasma potassium?
acutely: drive potassium into or out of cells (hide or release potassium?)
chronically: the kidney becomes important (distal nephron sodium delivery , renal actions of aldosterone)
What alters the RMP? What alters the threshold?
plasma potassium alters the resting membrane level
plasma calcium alters the threshold level
do not want the RMP getting close to the threshold potential
How to treat low plasma potassium (hypokalemia)
can administer K slowly (diet, intravenously)
must be cautious against producing hyperkalemia
How to treat high plasma potassium (hyperkalemia)
can drive K into cells (short term solution) - in emergencies give insulin, β-adrenergic receptor stimulation, alkalosis (H leaves the cell in exchange for K entering)
Aldosterone (kidney, long term) - sodium reabsorption and potassium secretion (loss)
calcium to depolarize the threshold potential (i.e. - more positive potential)
How do IV solutions work?
Fluids and electrolytes are administered to correct imbalances in electrolytes and to correct plasma volume, ECF volume and/or total ECF and ICF volume.
The solvent (water) will follow the solute (glucose or saline)
5% dextrose in water (IV solution) distribution
D5W
will distribute through all fluid compartments. Less than 10% will remain in the plasma – it distributes through all body water
Normal saline (IV solution) distribution
distributes in extracellular fluid with only 20% remaining in plasma
Dextran, mannitol, albumin IV solution distribution
larger molecules that remain in the vascular space
will expand the plasma/blood volume
What happens when dextrose in water is given as an IV
glucose is useful, but rapidly metabolized to CO2 + H2O
so essentially only giving water (easily distributes)
volume distributed into intracellular as well as extracellular (distributes in total body water)
D5W, D10W (5 or 10% dextrose in water)
Volume replacement and caloric supplement (no electrolytes)
See figure
What happens when saline solution is given as an IV
come in a variety of concentrations: hypotonic (0.2%), isotonic (0.9%), and hypertonic (5%).
provides sodium, chloride, water
sodium maintains osmolality
water helps to maintain blood volume and organ perfusion
increases interstitial volume
What happens when dextran solution is given as an IV
Dextran: a long chain polysaccharide (a long sugar)
solutions are confined (large molecule) to the vascular compartment- preferentially expand this portion of the ECF.
albumin (protein) solutions are much the same
