Diffusion and osmosis

Question 1

Define diffusion

Answer 1

Process by which molecules of a substance transfer through a layer or area such as the surface of a solution.

Can also take place without a membrane or a gas liquid layer. e.g. broken gas pipe –> spread

Question 2

What is Fick’slaw

Answer 2

The rate of diffusion of a substance across unit area (such as a surface or membrane) is proportional to the concentration gradient Or ‘tension gradient’ when referring to gases in solution

Question 3

What is the ‘tension’ of a gas in solution

Answer 3

The tension of a gas in solution is the partial pressure of the gas which would be in equilibrium with it

Question 4

State the rate of diffusion equation and explain why hypoxia is more likely hypercapnoea during a diffusion defect

Answer 4

Rate diffusion = Surface area x [gradient] x solubility
____________________________

                              thickness x square root MW

1 RBC takes 0.75 s to transit through a pulmonary capillary

CO2 equilibration complete in 0.1 seconds

O2 equilibration complete ± 0.25 seconds

This is because CO2 is 20 times more soluble than O2.

Question 5

Why does mild diffusion deficit rarely cause symptoms/hypoxia

Answer 5

1 RBC takes 0.75 s to transit through a pulmonary capillary

O2 equilibration complete ± 0.25 seconds

If there is a diffusion defect then there is still 0.5s (buffer time) for O2 diffusion to take place

Question 6

Compared to CO2, how does the diffusion rate of volatile anaesthetic agents compare

Answer 6

Similar. diffusion of volatile agents is complete within a small fraction of a second like CO2

Question 7

What is meant by lung diffusion capacity

Answer 7

DLCO or TLCO is a measurement used to assess the lungs ability to transfer gases.

Single vital capacity breath of 0.3% carbon monoxide, held for 10 seconds and then exhaled.

COinsp - COexp = amount of CO that has diffused across the alveolar-capillary membrane.

CO is diffusion dependent due to its extreme affinity for Hb the partial pressure in plasma does not accumulate as all the CO is bound by Hb. This means that the concentration gradient for diffusion essentially remains the same throughout meaning that the diffusion of CO is perfusion independent and diffusion dependent. So the result is not affected by variations in blood flow to the capillary.

Question 8

How is diffusion capacity altered if pulmonary oedema is present

Answer 8

The oedema accumulates in the interstitial layer –> thickening the alveolar-capillary barrier slowing the rate of diffusion.

Question 9

What is Graham’s law

Answer 9

The rate of diffusion of a gas is inversely proportional to the square root of its molecular weight

Question 10

Why does solubility affect diffusion rate

Answer 10

Increased solubility has a pronounced effect on maintaining diffusion gradients so increasing the rate of diffusion (Fick’s law)

Question 11

Describe osmotic pressure

Answer 11

Osmosis is the passage of solvent molecules through a semipermeable membrane into a solution down its concentration gradient (Fick’s law). As the membrane is semi-permeable the solute molecules cannot move into the solvent hence the solution dilutes the concentration of the solute in solution and would increase the volume of the solution by height ‘h’. The height represents the osmotic pressure.

The same can be done using a closed compartment separated by a semipermeable membrane with a manometer indicating the change in pressure in the solution once the solvent is added to the container and the solvent molecules have moved into the solution through the semipermeable membrane.

If the compartment containing the solution (not the solvent) is 22.4 L and it contains 1 mol of solute at 0 deg C then the osmotic pressure generated will be 1 atm (101.325 kPa)

Question 12

Define the term osmolarity

Answer 12

There are many different types of solutes in body fluids each of which exists with its own molarity (concentration: mmol/L). Osmolarity is a term used to describe the sum of all the molarities within the solution.

E.g. Ringers lactate
Na:          131 mmol/L
Cl:            111 mmol/L
K:                5 mmol/L
Ca:             2 mmol/L
Lactate:    29 mmol/L

Osmolarity: 278 mmol/L –> represents the osmotic pressure of the solution

Question 13

What is the name of the instrument that measure serum oncotic pressure and how does it work

Answer 13

Oncometer. A semipermeable membrane separates the plasma sample from a saline reference solution, and change due to oncotic pressure is measured by a transducer

Question 14

What % of plasma osmolarity is made up by electrolytes vs plasma proteins

Answer 14

Electrolytes (Sodium, Chloride, HCO3-, Urea, glucose) is > 99%

Plasma proteins:
< 1 %
1 mOsmol/L (of 300 mOsmol/L)

Question 15

At what plasma osmolarity would red cells haemolyse due to increasing intracellular hydrostatic pressure

Answer 15

< 200 mOsmol/L

Question 16

Describe and simplify the starling forces and direction of fluid movement over a capillary loop

Answer 16

Arterial hydrostatic pressure = ± 7 kPa
Venous hydrostatic pressure = ± 0 kPa

But arterial and venous oncotic pressures = 3.5 kPa

Arterial side –> positive gradient of 7 kPa - 3.5 kPa moving fluid and solutes into interstitium and hence toward cells

Venous side –> negative gradient of 0 kPa - 3.5 kPa moving fluid and solutes out of interstitium (and hence away from cells) and into capillaries

Simplification small additional hydrostatic and oncotic forces of the interstitium have been omitted for simplicity

Question 17

What is normal plasma versus urine osmolarity and why

Answer 17

Plasma –> 300 mOsmol/L

Urine –> 1000 mOsmol/L

The nephron, via differential permeabilities of the loop oh Henle, the countercurrent exchange mechanism and urea recycling establishes a hyperosmolar mileu in the inner renal medulla. Urine is thus concentrated as it moves down its concentration gradient out of the lumen of the collecting ducts and back into the blood circulation.

Question 18

What instrument is used to measure osmolality and what principle is this based on. Explain this principle

Answer 18

Osmometer

Based on the depression of the freezing point of a solution, a phenomenon related to osmosis.

1 mol of substance added to 1 kg of water depresses the freezing point by 1.86 deg C

Question 19

What is Raoult’s law

Answer 19

Raoult’s law states that the depression or reduction of vapour pressure of a solvent is proportional to the molar concentration of the solute

Question 20

How does introduction of a solute alter the physical properties of a solvent

Answer 20

1. Depresses freezing point
2. Lowers vapour pressure of solvent and hence
3. Increases boiling point

Question 21

What are the colligative properties of a solution?

Answer 21

These are the properties of a solution that depend on the osmolarity of the solution

1. Osmotic pressure
2. Freezing point depression
3. Boiling point elevation (Raoult’s law)
4. Vapour pressure reduction (Raoult’s law)