Osmosis Flashcards
What is osmosis?
Osmosis is the movement of a solvent across a semi permeable membrane from a low concentration (hypotonic) solution to a higher concentration (hypertonic) solution
What is osmotic pressure?
Osmotic pressure is the hydrostatic pressure exerted by a solution in a container across a semi-permeable membrane.
It is osmotic pressure which drives osmosis, once the osmotic pressure has been reached then osmosis will stop.
How do osmometers work?
Osmometers use colligative properties such as the reduction in vapour pressure or freezing point
depression to indirectly determine the number of osmotically active particles in a solution.
What is the Van ‘t Hoff Equation?
π= iMRT
i= Van ‘t Hoff factor
M= molarity
R= ideal gas constant
T= temperature (in kelvin)
The fact that the equation is related to molarity and not the type of solute demonstrates that osmotic
pressure is a colligative property.
Van’t Hoff factor is the number of particles in solution/moles dissolved in solution. I.e for NaCl the Van’t Hoff factor would be 2 as when dissolved NaCl dissociates into a Na and Cl ion.
What is Avogadro’s hypothesis?
Avogadro’s hypothesis states that equal volumes of all gases at the same temperature and pressure contain equal numbers of particles.
Therefore 1 mole of a gas will exert 1atm pressure in 22.4L volume.
How does reverse osmosis work?
It is where a pressure is applied to overcome the osmotic pressure and force the solvent to moved against the concentration gradient to the area of low solute.
Define an osmole?
Osmole is the number of moles of a compound contributing to the total osmotic pressure.
1 mole of NaCl gives 2 osmoles whilst 1 mole of glucose gives 1 osmole in solution.
What is the difference between osmolality and osmolarity, and why is one term preferred?
Osmolality is the number of osmoles per Kg of solution.
Osmolarity is the number of osmoles per litre of solution.
Osmolality is the preferred term as osmolarity is more effected by a change in temperature.
What is tonicity?
Tonicity is the osmotic property of a solution in relation to the specific semi-permeable membrane.
What is the normal range of plasma osmolality?
287.4 mosmol/kg
Normal range 280-303 mosmol/kg.
List the osmolality of the following solutions: Hartmanns, Normal saline, 5% dextrose?
Hartmanns 278 mosmol/kg
5% dextrose 278 mosmol/kg
NaCl 0.9% 308 mosmol/kg
Describe the different methods of renal replacement therapy?
Peritoneal dialysis: Dialysis fluid is pumped into the peritoneal cavity via a permanent in dwelling catheter.
Water and toxins move via osmosis and diffusion out from the circulating plasma across the semipermeable membrane of the peritoneum.
After a period to allow the osmosis and diffusion to take place the dialysis fluid is removed.
Haemodialysis: Blood is pumped out of the body via a AV fistula. In the dialysis machine it is separated from the dialysis fluid via semi-permeable membrane, allowing fluid and toxins to be removed via osmosis and diffusion.
The blood runs countercurrent to the dialysis fluid, this ensures that there is a high concentration gradient improving efficiency.
Continuous veno-veno haemofiltration: Haemofiltration uses hydrostatic pressure, rather than osmotic pressure, to drive fluid and small solutes across a semi-permeable membrane and out from the circulating blood.
