Diffusion, mixtures and solutions Flashcards
Diffusion
The process of net movement of one type of molecule through space as a result of random motion (entropy)
Diffusion through a permeable membrane
Two divisions of a container separated by a permeable membrane of thickness
Higher [O2]- alveolar side (west) drives net movement to capillary side (east)
Continuous capillary blood flow causes O2 to be removed maintaining gradient movement west to east
Diffusion through a semi-permeable membrane
Dividing membrane semi-permeable, our example not permeable to proteins
Water is freely permeable and attempts to (distribute equally) dilute the concentration gradient equally Osmosis.
Osmotic Pressure
is related to the number of non-permeable molecules (solute)
Osmotic pressure is the pressure required to stop osmosis.
Oncotic Pressure
In capillaries osmotic pressure when caused by plasma proteins is called Oncotic pressure
Oncotic pressure =24-27mmHg
OSMOLE
a gram molecular weight of undissociated solute
ie. 1 mole (gram molecular weight) NaCl= 58.5 Gms (23+35.5=58.5)=2 osmoles
1 osmole Na+ and 1 osmole Cl-
ie. 1 mole KCL=74.55 gm (39.10 + 35.45)=2 osmoles 1 osmole K+ and 1 osmole Cl-
OSMOLALITY
Concentration of solute in terms of osmoles per kilogram water
Reflects the total number of ions or molecules in a solution regardless of size or weight
Normal Serum Osmolality
285-295 mOsm/kg
Serum Osmolality formula
(2 x (Na + K)) + (BUN / 2.8) + (glucose / 18)
OSMOLARITY
Concentration of solute in terms of osmoles per liter water
A mole of any solute in 22.4 liters of solution at 0C will exert an osmotic pressure of?
1 atm
in a mixed solution the osmotic pressure is the sum of ?
the individual molarities
over X% of plasma osmolarity is due to X?
99%
electrolytes
plasma proteins account for approx ?
1 milliosmole/liter
in dilute solutions such as X, differences between osmolality and osmolarity are ?
less than 1%
RBCs lyse at omolarities below?
200 mOsm/L
(blank) are impermeable to plasma proteins and this causes (blank blank blank) between plasma and interstitial fluid?
Capillaries
osmotic pressure difference
Oncotic pressure is X mmHg?
or X mOsm/kg?
24-27 mmhg
285-295 mOsm/kg
NaCl osmolarity
308 mOsmol/Liter
LR osmolarity
273 mOsmal/liter
requirment for Osmosis across cell membrane
- Must have membrane that is impermeable to one or more solutes.
- Must have a difference in the concentration of solutes.
requirement for Osmosis across capillary walls
Most capillary walls are permeable to small solutes (sodium, chloride, etc).
Albumin doesn’t permeate the capillary wall.
Fick’s Law of Gas diffusion across a fluid membrane formula
J = solubility constant for oxygen x diffusivity / membrane thickness x (PAO2- PcapO2)
PcapO2
pulmonary capillary oxygen partial pressure
PAO2
alveolar partial pressure
Ficks Law of Gas diffusion accross a fluid membrane
1) Diffusion rate of gas is directly proportional to:
- Partial pressure gradient
- Membrane area
- Solubility of gas in membrane
2) Diffusion rate of gas is inversely proportional to:
- Membrane thickness
- Square root of the molecular weight
Ficks Law explains
Concentration effect
Second gas effect
Diffusion hypoxia
Why turning on N2O leads to an increase of volume in gas spaces of the patient’s body
Factors determining diffusion rate across membranes for non-gases:
Concentration gradient for un-ionized substances
Electrochemical gradient for ions
Lipid solubility
Size
Ion Diffusion through a semi-permeable membrane
When a semi-permeable membrane is permeable to one ion but not the other oppositely charged ion, a charge imbalance is produced.
This transmembrane potential can be calculated by the Nernst Equation
Graham’s Law
The diffusion rate of a gas (or liquid) is inversely proportional to the square root of it’s molecular weight
In short, Big molecules diffuse slower than Small molecules
Release a liter each of helium and xenon at the same time in a closed room- helium will achieve greater randomness faster
Grahams Law
Diffusion is inversely related to the square root of it’s molecular weight
r = 1/ square root of MW
r= rate of diffusion
mw=molecular weight
Laws that apply to Diffusion Through a Tissue Sheet
Graham’s law, Henry’s law and Fick’s law all apply to tissue diffusion
N20 is x times more soluble than x thus we do not use when X
N2O is 36 times more diffusible than N, thus do not use when an air filled cavity’s expansion would be undesirable
CO2 and N2O are approx X times more diffusible than O2
20
Main determinant of gasses diffusion when all other factors are equal is
Concentration gradient
Heterozygous mixtures
Uneven distribution of substances (example=earth soil)
Homogenous mixtures
Even distribution of substances, known as solutions (example=saline, 5% albumin)
Solute
the dissolved substance
Solvent
that which the solute dissolves into
Molality formula
moles of solute / kg solvent
molarity formula
number of moles solute/ 1 liter solution
Mole
Molar weight: a gram molecular weight of a substance
Milimole
miligram molecular weight (1/1000 mole)
Equivalent weight
gram wt. of a substance which will combine or replace one gram (one mole) of hydrogen; an equivalent wt. can be determined by dividing the molar wt. of a substance by its ionic valence.
Milli equivalents
1/1000 equivalent
Ionic solutions are
solutions of atoms or molecules in the ionized form
Ie. NaCl is table salt, a white crystalline substance Na+ Cl- is dissociated or ionized table salt
- Adding table salt to water allows the weak polar H2O molecules to pull Na away from Cl
- The salt molecule is thus dissociated (ionized) leaving Na+ wanting an electron and Cl- having an electron alone in its outer shell (orbit)
Are Ionized drugs lipid soluble?
NO they DO NOT cross lipd membrane layers
Are unionized drugs lipd soluble?
YES, they do cross lipid membranes
if blood/gas coefficient is >1 where is the greater affinity?
the blood
Partition coefficient
the ratio of the amount of substance present in equal volumes of two phases at a stated equilibrium temperature
If one of the phases is gaseous the partition coefficient and the Ostwald coefficient are the same
Partition coefficients frequently used when phases are both solid & liquid ie. tissue-blood coefficient
Osmotic pressure
all about electrolytes, the pulling power of electrolytes. (the pressure required to stop osmosis).
anytime you hear diffusion
THINK FICK
Diffusion rate of gas is directly proportional to:
Partial pressure gradient
Membrane area
Solubility of gas in membrane
Diffusion rate of gas is inversely proportional to:
Membrane thickness
Square root of the molecular weight (Grahms law)
Factors determining diffusion rate across membranes for non-gases
Concentration gradient for un-ionized substances
Electrochemical gradient for ions
Lipid solubility
Size
