eLFH - Osmosis Flashcards
Osmosis definition
Process where solvent moves across a semi-permeable membrane from a hypotonic solution (low concentration) to hypertonic solution (high concentration)
Semi-permeable membrane definition
Membrane that is permeable to solvent, but not permeable to solute
Osmotic pressure definition
Hydrostatic pressure that arises within a closed container of solution separated by a semi-permeable membrane from a hypotonic solution
When osmotic pressure is reached, osmosis ceases even if solutions are still not isotonic
Colligative properties
Changes in molecule interactions dependent upon the total number of solute molecules within the solution (not just the presence of the solute alone but the amount of it)
What determines osmotic pressure
It is a colligative property
Therefore depends on the molar concentration of solute rather than the identity of the solute
Osmometer definition
Device that measures osmotic pressure of a solution
Osmometer mechanism
Uses colligative properties (eg reduction in vapour pressure or freezing point depression) to determine number of osmotically active particles in a solution
van ‘t Hoff Equation
Equation to derive osmotic pressure (π)
Osmotic pressure units
Pascal
Reverse osmosis definition
External pressure exerted upon osmotic system to push fluid through semi-permeable membrane against osmotic gradient
Solvent can pass through membrane but solutes can’t
Use of reverse osmosis
Water sanitation
Seawater desalination
Pressure required for reverse osmosis to occur
Pressure exerted must exceed osmotic pressure
Osmole definition
Number of moles of a chemical compound that contribute to a solution’s osmotic pressure
In calculating osmoles present in solution, salts are presumed to have separated into component ions
Eg. NaCl gives 2 osmoles in solution, Glucose gives 1 osmole
Osmolarity definition
Number of osmoles per Litre of solution
Osmolality definition
Number of osmoles per Kilogram of solvent
Which is preferred out of Osmolarity and Osmolality, and why?
Osmolality preferred
Volume of solution changes with temperature but mass does not change
Therefore osmolarity of a solution changes with temperature but osmolality remains constant with temperature
Normal plasma osmolality
280-303 milliosmoles per kilogram of plasma solvent
Comparison of composition and osmolality of Plasma vs Hartmann’s vs Normal saline vs 5% Dextrose
Types of renal replacement therapy which use Osmosis
Peritoneal dialysis
Haemodialysis
Types of renal replacement therapy which use hydrostatic pressure
Continuous veno-venous Haemofiltration
Peritoneal dialysis
Sterile solution runs into peritoneal cavity via Tenckhoff catheter
Fluid and toxins move via osmosis out of circulating plasma across semi-permeable membrane of peritoneum
Removed as dialysis fluid runs out of catheter
Haemodialysis
Blood pumped out and exposed to semi-permeable membrane with dialysate solution on other side
Small solute move by osmosis out, larger proteins remain in blood
Dialysate and blood run in countercurrent system to improve efficiency, efficacy and speed of filtration
Continuous veno-venous haemofiltration
Used in ITU
Uses hydrostatic pressure, rather than osmotic pressure to drive fluid and small solutes across semi-permeable membrane out of blood
Dialysate and blood run in same direction
Slower than haemodialysis but slower fluid shifts with CVVHF
Mannitol composition
Unmetabolised sugar
Mannitol administration
IV
Mannitol mechanism of action
Filtered freely by glomerulus
Not reabsorbed from renal tubule
Increases osmolality of glomerular filtrate reducing volume of water reabsorbed
Use of mannitol
Raised intracranial pressure
Raises plasma osmolality to draw water out from oedematous brain cells
Tonicity vs Osmolality
Tonicity refers to solutions in the context of a specific semi-permeable membrane
Osmolality simply describes the osmotic pressure of two or more solutions
