osmosis Flashcards
what is osmosis?
It’s net movement of H2O from regions of high H2O concentration to regions of low H2O concentration
what is the osmotic pressure?
the pressure that must be applied to oppose osmosis
It is the number of solute particles, not molecules, which determine the osmotic effect on [H2O] and this can be misleading because some molecules dissociate in solution.
What is the osmolarity of normal human plasma?
285 (round up to 300), which is the same as within cells
What are penetrating and non-penetrating solutes?
Non penetrating - Na+ and Cl- in the ECF
Penetrating - K+ and organic anions in the ICF
what is osmolarity?
It describes the total number of solute particles
What does isosmotic, hypoosmotic and hyperosmotic solutions mean?
Isosmotic – the same total number of solute particles as normal ECF (plasma)
Hypo osmotic – solution with fewer total solute particles
Hyperosmotic - solution with more total solute particles
What does tonicity mean?
It describes the total number of non penetrating solute particles
What does isotonic, hypotonic and hypertonic solutions mean?
Isotonic – the same number of non penetrating solute particles as normal ECF (plasma)
Hypotonic – solution with fewer non penetrating solute particles
Hypertonic – solution with greater number of non penetrating solute particles
What is the clinical importance of osmolarity and tonicity?
Because in hospitals, plasma samples may only give info about osmolarity and we must know the composition of that sample to predict tonicity and cell volume.
This needs to be taken into consideration when doing intravenous fluid transfusions.
Inappropriate fluid administration can cause Intravascular Haemolysis (cell bursting in a hypotonic solution) which can lead to death.
Cells bursting introduce proteins to ISF, increasing the tonicity of ECF uncontrollably.
Fluids must have the appropriate tonicity - 0.9% saline (150mM) - isotonic
Why is the osmolarity of both the ECF and ICF at 285 mOsmol/L?
Because there are no barriers to the movement of water between them.
The number of particles per litre on both sides of the membrane is the same,
but the composition of particles (non-penetrating/ion species) is vastly different.
What happens if you place red blood cells in a hyperosmotic aqueous urea solution? and why?
They will swell up and burst.
aqueous + urea = hypotonic solution because of more penetrating solutes in comparison with the RBC.
Because urea is a penetrating particle it enters the cell until equilibrium is reached.
But the solution is still hypotonic so water moves into the cell causing it to swell and burst.
All in all, urea (penetrating particle) makes no difference to the movement of water across membrane, only tonicity has to be looked at.
What happens to the volume of red blood cells in patients with ureamia (excess urine in their plasma)?
Very little to no change.
plasma = isotonic solution
The urea will distribute across the membrane, reaching equilibrium, changing the osmolarity of the ICF and ECF.
But the number of non-penetrating solutes remains unchanged so the plasma is still isotonic.
So there will be no net movement of water.
Why is saline (150mM) an isotonic solution?
Saline is a mixture of sodium chloride in water.
NaCl dissociates into Na+ (150mM) and Cl- (150mM)
so a 150mM solution of saline has an osmolarity of 300 mOsmole/L which is approximately the same as plasma, so it is considered isotonic.
