W4 Osmosis, osmolarity, tonicity, oncotic pressure - Jason Potas

Question 1

What is Fick’s Law of Diffusion?

Answer 1

States that diffusion (Q) of a substance across a membrane is influenced by a range of factors:

1. Concentration gradient (delta C)
2. Permeability of membrane to substance (P)
3. Surface area of membrane (A)
4. Molecular weight of substance (MW)
5. distance (thickness) of membrane (delta X)

In the equation:
Q = (deltaC.P.A)/(MW.deltaX)

Question 2

Problem:
If two solutions are separated by a semi-permeable membrane impermeable to all solute (i.e. all except water) what will be the change in volume (hydrostatic pressure difference) to reach final conditions if the starting conditions were [solute 1] = 20/120 p/mL and [solute 2] = 10/120 p/mL?

Answer 2

volume = v, solve for v when:

20/(120 + v) = 10/(120-v)

20(120-v) = 10(120+v)
2400 - 20.v = 1200 + 10.v
1200 -20.v = 10.v
1200 = 30.v
v = 40

Question 3

What is osmosis?

Answer 3

The diffusion of water over a semipermeable membrane.

Question 4

What is an aquaporin?

Answer 4

A water channel in the cell membrane, only permeable to water.

Question 5

What is osmolarity?

Answer 5

The total number of particles / L
ie in fluid.
measured as mOsm.

Question 6

What is osmolality?

Answer 6

The total number of particles / kg

i.e. mass

Question 7

If the osmotic coefficient for NaCl is 0.93, and it disperses into 2 particles in solution, what is the osmolarity of a 154 mM NaCl solution?

Answer 7

Osmolarity = 0.93.2.0.154 = 286mOsm

Question 8

If 1 osm lowers the freezing point by 1.87 degrees Celsius, how much does 286 mOsm lower?

Answer 8

286x10^-3 x -1.87 = -0.53 degrees Celsius

Question 9

What is the difference between osmolarity and tonicity?

Answer 9

Osmolarity describes a particular solution relative to normal, whereas tonicity compares a solution relative to a reference solution.
Clinically:
Osmolarity = 2[Na+] + [glucose] + [urea]
Tonicity = 2[Na+] + [glucose]

Question 10

What is the Gibbs-Donnan Equilibria?

Answer 10

A description of the uneven distribution of charged particles across a membrane. States that because protein is negatively charged, more cations (+) enter the cell as fewer anions(-) are needed to neutralise charge on the inside.
r = Donnan factor, and is different various areas of the body due to changes in [protein].

Question 11

How is cell volume maintained?

Answer 11

Through the Na+/K+ ATPase pump, which prevents too much Na+ from entering the cell to prevent cell swelling and lysis by actively pumping Na+ out of the cell.

Question 12

What is Regulatory Volume Increase?

Answer 12

The working of the ionic pumps/channels of the cell membrane to increase the amount of intracellular ions so as to increase volume by osmotic following in after an initial decrease in volume due to extracellular factors (hypertonic condition) causing cell shrinking.

Question 13

What is Regulatory Volume Decrease?

Answer 13

The working of ionic pumps/channels of the cell membrane to decrease the amount of intracellular ions so as to decrease volume by osmotic following out after an initial increase in volume due to extracellular factors (hypotonic condition) causing cell swelling.

Question 14

What happens in isotonic volume replacement?

Answer 14

There is no osmotic flow between to 2 compartments as the solution is isotonic.

Question 15

What happens in hypotonic substitution? Ie addition of pure water to ECF?

Answer 15

Both ECF and ICF will swell, but more so the ICF, to recreate equilibrium of [ECF] and [ICF]on both sides

Question 16

What happens in hypertonic substitution?

Answer 16

The ICF shrinks as it loses water to the ECF in order to recreate equilibrium of [ECF] and [ICF]on both sides