eLFH - Osmosis

Question 1

Osmosis definition

Answer 1

Process where solvent moves across a semi-permeable membrane from a hypotonic solution (low concentration) to hypertonic solution (high concentration)

Question 2

Semi-permeable membrane definition

Answer 2

Membrane that is permeable to solvent, but not permeable to solute

Question 3

Osmotic pressure definition

Answer 3

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

Question 4

Colligative properties

Answer 4

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)

Question 5

What determines osmotic pressure

Answer 5

It is a colligative property

Therefore depends on the molar concentration of solute rather than the identity of the solute

Question 6

Osmometer definition

Answer 6

Device that measures osmotic pressure of a solution

Question 7

Osmometer mechanism

Answer 7

Uses colligative properties (eg reduction in vapour pressure or freezing point depression) to determine number of osmotically active particles in a solution

Question 8

van ‘t Hoff Equation

Answer 8

Equation to derive osmotic pressure (π)

Question 9

Osmotic pressure units

Answer 9

Pascal

Question 10

Reverse osmosis definition

Answer 10

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

Question 11

Use of reverse osmosis

Answer 11

Water sanitation

Seawater desalination

Question 12

Pressure required for reverse osmosis to occur

Answer 12

Pressure exerted must exceed osmotic pressure

Question 13

Osmole definition

Answer 13

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

Question 14

Osmolarity definition

Answer 14

Number of osmoles per Litre of solution

Question 15

Osmolality definition

Answer 15

Number of osmoles per Kilogram of solvent

Question 16

Which is preferred out of Osmolarity and Osmolality, and why?

Answer 16

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

Question 17

Normal plasma osmolality

Answer 17

280-303 milliosmoles per kilogram of plasma solvent

Question 18

Comparison of composition and osmolality of Plasma vs Hartmann’s vs Normal saline vs 5% Dextrose

Answer 18

Question 19

Types of renal replacement therapy which use Osmosis

Answer 19

Peritoneal dialysis

Haemodialysis

Question 20

Types of renal replacement therapy which use hydrostatic pressure

Answer 20

Continuous veno-venous Haemofiltration

Question 21

Peritoneal dialysis

Answer 21

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

Question 22

Haemodialysis

Answer 22

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

Question 23

Continuous veno-venous haemofiltration

Answer 23

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

Question 24

Mannitol composition

Answer 24

Unmetabolised sugar

Question 25

Mannitol administration

Answer 25

IV

Question 26

Mannitol mechanism of action

Answer 26

Filtered freely by glomerulus
Not reabsorbed from renal tubule

Increases osmolality of glomerular filtrate reducing volume of water reabsorbed

Question 27

Use of mannitol

Answer 27

Raised intracranial pressure

Raises plasma osmolality to draw water out from oedematous brain cells

Question 28

Tonicity vs Osmolality

Answer 28

Tonicity refers to solutions in the context of a specific semi-permeable membrane

Osmolality simply describes the osmotic pressure of two or more solutions