BT_PO 1.76 - Regulation of osmolality Flashcards
(18 cards)
Definitions
Mole
A mole is the amount of a substance that contains the number of molecules equal to Avogadro’s number (6.02 x 10). The mass in grams of one mole of a substance is the same as the number of atomic mass units in one molecule of that substance (ie the molecular weight of the substance expressed as grams)
Molality of a solution
is the number of moles of a solute per kilogram of solvent
Molarity of a solution
is the number of moles of solute per litre of solution
Osmole
This is the amount of a substance that yields, in ideal solution, that number of particles (Avogadro’s number) that would depress the freezing point of the solvent by 1.86K
Osmolarity
of a solution is the number of osmoles of solute per litre of solution
mOsml/L
Osmolality of a solution
is the number of osmoles of solute per kilogram of solvent
Osmolality depends on the mass of the solvent which is independent of temperature and pressure.
mOsmoles/kg
a measure of the number of particles present in solution and is independent of the size or weight of the particles.
It can be measured only by use of a property of the solution that is dependent solely on the particle concentration. These properties are collectively referred to as the colligative properties: vapour pressure depression, freezing point depression, boiling point elevation, osmotic pressure
Normal ECF osmlality: 285 to 290 mOsmoles/kg.
Both osmolality and osmolarity are __________ properties of the solution. What does this mean?
colligative properties of the solution, i.e. they depend on the number of particles dissociated in the solution, but not on the characteristics of the particles themselves.
Why are the numerical values of osmolality and osmolarity obtained for dilute aqueous solutions are almost the same.
because a litre of water weighs about a kilogram,
What are the two different types of osmoles
“Effective” osmoles:
Substances capable of exerting an osmotic force across a cell membrane b/c they cannot cross it easily (most solutes such as Na+, Cl-, Ca2+)
Thus → ↑ osmolality (and tonicity) of solution containing them → creates POSMOTIC gradient → results in osmosis of H2O from ↓ to ↑ POSMOTIC until the gradient dissipates
“Ineffective” osmoles
Substances incapable of exerting an osmotic force across a cell membrane b/c the can cross it easily (Eg. Urea, but not in the brain)
Thus → substance diffuses across membrane until its [ ] gradient is equal in both solutions → so the osmolality of both solution ↑ proportionately and no POSMOTIC gradient is generated (Ie. Both solutions isotonic) → no osmosis of H2O
What is ECF osmolality and how is it determined? What is ICF osmolality?
ECF osmolality = 285-290 mOsmoles/kg → determined 1°ly by ECF Na+ content (See above) → urea and glucose contributes a small amount (3%) only
ICF osmolality is SAME as ECF osmolality → because H2O easily diffuses across the membrane to dissipate any POSMOTIC gradient between ICF and ECF
Summary of how ECF / plasma osmolality is regulated
ECF/plasma osmolality is determined by ECF Na+ contents→ it is thus determined by mechanisms that regulate ECF Na+ → relies on sensing ∆ ECFV and regulating it
It is also regulated by ADH/thirst response that modulates TBW content according to plasma osmolality
How do we calculate/measure serum osmolality
can be measured by use of an osmometer or it can be calculated as the sum of the concentrations of the solutes present in the solution.
The value measured in the laboratory is usually referred to as the osmolality
Calculating sum of plasma solute [ ] = 2x [Na+] + [glucose] + [urea] = 2x140 + 5 + 5 = 290 mosm/kg
The value calculated from the solute concentrations is reported by the laboratory as the osmolarity
The two values usually don’t match exactly for various reasons:
there are a number of formulas that can be used and they all give slightly different results;
the formulas typically use the concentrations of only 3 solutes (Na, glucose, urea) in the calculation so contributions from abnormal small MW uncharged substances will be missed so the calculated value will be low;
use of osmometers that use the vapour pressure method are unreliable in the presence of volatile chemicals.
Osmolar gap: normal vs abnormal, what does an elevated gap mean, possible contributors
normal = < 10
note that is a pragmatic clinical aid – the units are different (osmolality =mOsm/kg and osmolarity = mOsm/L) so it doesn’t make mathematical sense!
Meaning of a high osmolar gap
presence of an abnormal solute present in significant amounts
must have: a low molecular weight and be uncharged -> can elevate the osmolar gap
if the ethanol levels are measured they can be added to the calculated osmolarity to exclude the presence of an additional contributor to the osmolar gap. [NB: To convert ethanol levels in mg/dl to mmol/l divide by 4.6]
Causes of a high osmolar gap
if elevated consider presence of other osmotically active particles
mannitol
methanol
ethylene glycol
sorbitol
polyethylene glycol (IV lorazepam)
propylene glycol (IV lorazepam, diazepam and phenytoin)
glycine (TURP syndrome)
maltose (IV IG – Intragram)
Why does osmolality need to be ‘corrected’ for the type of solute
Most solutes (eg Na+, Cl-, Ca2+) do not cross cell membranes easily and are effective at exerting an osmotic force across a cell membrane.
Other solutes (eg urea, glucose) can cross membranes easily and are ineffective at exerting an osmotic force across cell membranes.
Urea / other solutes that can cross cell membranes (?Cl-)
If excess urea was added to the ECF sufficient to ↑ ECF osmolality, we would expect that there would be net movement of water out of the cell until there was no osmolar gradient across the cell membrane.
However urea itself crosses the cell membrane (until the urea concentrations are the same on both sides of the cell membrane).
Any water that we predicted would have moved out of the cell in response to the initial ↑ ECF (due to urea) would now move back into the cell. The final result of adding urea extracellularly is an ↑ [urea] in ECF and ICF but no change in the final fluid distribution across the cell membrane.
∴ osmolality needs to be corrected for this type of solute
Tonicity
Definition
Definition → the “effective” osmolality of a solution (Ie. part of total osmolality due to “effective” osmoles) → a measure of only the particles in solution that can exert an osmotic force (and produce an osmotic pressure gradient) across a semi-permeable membrane
Strictly speaking, tonicity should be defined in relation to a specific semi-permeable membrane (as whether an osmole is effective or not depends on the membrane)
If not specified, then it is assumed that it is in relation to a generic cell membrane
It is often defined in relation to another fluid across a membrane:
Isotonic (both fluids have same tonicity or “effective” osmolality)
Hypertonic (↑ tonicity or “effective” osmolality cf. other fluid)
Hypotonic (↓ tonicity or “effective” osmolality cf. other fluid)
Is tonicity more or less than osmolality? Why?
tonicity is less then osmolality by the total concentration of ineffective solutes it contains
Estimating serum tonicity
Plasma tonicity = (Plasma osmolality) – [urea] – [glucose]
Because urea and glucose are the only two ineffective solutes ordinarily present in any significant concentration.
How can hypertonic urea be used to ↓ ICH if urea is an ineffective osmole
Urea is isotonic to most cells in body as it is permeable to their membranes (Ie. An ineffective osmole) and does not cause any H2O transfer → EXCEPT for brain cells for which it is hypertonic to as urea is impermeable to BBB (Ie. now an effective osmole) → generates an osmotic pressure gradient across it and causes H2O transfer out of brain → thus hypertonic urea used to treat ↑ ICP!
Is glucose hyper or isotonic for patients?
For patients with normal inuslin levels glucose is isotonic as glucose is an ineffective osmole, will move into cells
For DM patients, glucose is hypertonic (Ie. an effective osmole) to insulin-sensitive cells (fat/muscle) that cannot take up glucose → generates osmotic pressure gradient that draws H2O out of these cells → cause dehydration as fat/muscle is large % of body
