Download Various basic scientific laws fundamental to physiology

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THE VARIOUS BASIC SCIENTIFIC LAWS important to physiology
MOLES
A mole is 6 x 1023 molecules of something. It’s the standard SI unit for expressing an amount of a substance.
A mole of NaCl is the weight of 1 mole of Na and 1 mole of Cl; which weigh 23 and 35.5 grams ( thus, 58.5 grams)
A millimole is one thousands of a mole. Thus a millimole of Na+ in solution weighs 23 milligrams.
Thus, a litre of normal blood contains 3.22 grams of sodium
DALTONS
A dalton is a unit of measuring the molecular mass of a substance. It’s the ratio of the mass of the molecule you are
measuring to the weight of 1/12th of a carbon atom (which, if its carbon 12 (12C), has the atomic weight of around 12.)
Measuring large molecules requires kilodaltons.
A dalton is a dimensionless ratio. Thus, for some reason it is incorrect to say that a protein weighs 64 kDa. You say, “this
protein’s molecular mass is 64 kDa”.
EQUIVALENTS
These are electrical equivalents; 1 equivalent (eq) of a substance is one mole of the substance divided by its valence.
Thus, lets take ionized sodium. One mole of it will dissociate into 1 eq because it has the valence of 1. Thus, 1eq of
sodium weighs 23 grams. Calcium, however, has a valence of 2 ( Ca++) and so 1 eq of calcium only weighs 20 grams
(whereas a mole of calcium weighs 40 grams).
TO CONFUSE THINGS FURTHER:
There are also chemical equivalents, which are totally different to electrical equivalents. A gram equivalent is the weight
of a substance which is chemically equivalent to 8 grams of oxygen. It’s the mass of a given substance that will supply or
react with 1 mole of hydrogen ions in an acid base equation, for example. It determines the normality (N) of a solution; for
example a 1N solution of hydrochloric acid will have 1 gram equivalent of hydrogen and 1 gram equivalent of chloride
ions (which actually weighs 35.5 grams). This is of interest to chemists, but rarely to doctors.
pH AND BUFFERING
pH is the negative (inverse) base 10 logarithm of the H+ concentration in water.
Acids donate more H+ , thus increase the H+ concentration, and thus DECREASE the pH (see, its inverse)
At 25o , the pH of pure water is 7.0 (i.e. the H+ concentration is 10-7 moles per litre)
for every pH unit below 7, the H+ concentration is increased tenfold
stomach juice is pH 1.5 – 2
pancreatic juice is around 8.0
normal plasma has to stay slightly alkaline, around pH 7.35 – 7.45. Otherwise enzyme activity goes to hell. This is
because the enzymes are made of amino acids, and keep their shape only because the amino acids interact – ionized ends
of amino acids attract other ionized ends and thus the protein bends and twists. Acidic amino acids have carboxyl functional
groups, and basic amino acids have amine functional groups in their side chains. Changes in pH cause the amino acids to
become relatively more or less acidic; they cause them to be more or less ionized. This causes the enzyme protein to lose its
shape; and this is bad because it is the distinct shape of the enzyme which makes it specific to its substrate. The substrate’s
charge properties will also be affected by the pH, and so nothing is going to work unless the pH is just right
BUFFERING
-
a BUFFER is something that can absorb or release H+ ions to keep the pH of a solution constant.
There are numerous buffers in body fluids, but they
all buffer the same H+ concentration (this is called
the Acid Dissociation Constant; the larger the
the ISOHYDRIC PRINCIPLE)
value, the stronger the acid i.e. the more of the acid
Most acids in the body are weak acids (i.e. they
molecules have donated their protons.
don’t completely dissociate in solution; some acid
remains floating around- like H2CO3 which prefers to The pKa is the pH at which concentration of ionized and
remain as H2CO3 and not dissociate into H+ and
non-ionised forms is equal.
HCO3 The buffering capacity of a weak acid is best when the pKa of that acid is equal to the pH of the solution.
Carbonic acid (H2CO3) is a weak acid – pKa around 6.36 – and thus only partly dissolved:
If H+ is added, the solution becomes more acidic and the carbonic acid absorbs some H+ (equation shifts left) to
maintain equilibrium.
If OH- is added, the extra H+ in the carbonic acid solution combines with the OH- and forms H2O, so the acidity drops
-so to maintain equilibrium, the carbonic acid dissociates some more, releasing H+ ions (equation shifts to the right)
pKa:
-
-
REFERENCES: Ganong’s review of medical physiology, 23rd ed: Section 1; as well as Guyton and Hall (11th edition), and, sadly, Wikipedia.
This document was created by Alex Yartsev ([email protected]); if I have used your data or images and forgot to reference you, please email me.
DIFFUSION
If asked in a viva exam situation, one would be forced to say that DIFFUSION IS A PROCESS BY WHICH A GAS
OR SUBSTANCE IN A SOLUTION EXPANDS, BECAUSE OF THE MOTION OF ITS PARTICLES, TO
FILL ALL THE AVAILABLE VOLUME.
How fast will it do this?
-
the higher its concentration, the faster it will diffuse
the farther it has to travel, the slower it will diffuse (proportionally to the square of the distance)
the wider an area it is diffusing across, (the wider the membrane) the faster it will diffuse
THIS IS FICK’S
LAW OF DIFFUSION:
(high concentration minus low concentration) x
Area x Permability coefficient
Thickness
OSMOSIS
-
-
if you dissolve something in water, the concentration of water in that solution is LOWER than in pure water.
Thus, if given half a chance, pure water will attempt to diffuse into this solution, from a higher concentration of pure
water into the area of lower concentration (where water is impure and something is dissolved)
Thus, in a viva, one is compelled to spout that OSMOSIS IS THE DIFFUSION OF SOLVENT MOLECULES
INTO A REGION WHERE THERE IS A HIGHER CONCENTRATION OF SOLUTE.
This can be prevented- if you apply pressure to the impure solution, pure solvent will be
discouraged from going there.
THE PRESSURE REQUIRED TO PREVENT SOLVENT MIGRATION IS THE OSMOTIC PRESSURE
Osmotic pressure depends on the number of particles of solute in solution, rather than their
type. It doesn’t matter what is dissolved. Like the partial pressure of a gas, it is a constant,
modified only by temperature and volume.
In an ideal situation, at constant temperature, the osmotic pressure is proportionate to the
number of particles per unit volume of solution.
For this reason, the concentration of osmotically active particles is expressed as OSMOLES.
o one osmole (Osm) is the gram-molecular weight of a substance divided by the number of
particles it releases in solution
o for non-ionising substances like glucose, it is the number of molecules that matters
o for ionizing sustances like NaCl, each ion is a particle; so 1 mole of NaCl forms 2 osmoles
o of course, in body fluids, ions interact, and so the actual number is slightly less than 2.
The freezing point of normal human plasma is -0.54 Co
o
o
o The osmolal concentration of a substance
is measured by the degree to which it
depresses the freezing point of the
solution: 1 mol of an ideal solution
depresses the freezing point by 1.86 Co
The OSMOLARITY is the number of osmoles per litre of solution
The OSMOLALITY is the number of osmoles per kilogram of solvent.


Thus, osmolarity is affected by temperature and volume of the solution, while osmolality is not.
Seeing as 1 litre of water weighs 1kg, osmolality can also be expressed as osmoles per
litre when discussing substances dissolved in body fluids.
TONICITY
The normal osmolal concentration of plasma is 290 mOsm/L. One might expect it to be around 300, but some of the ions
interact, forming complexes and reducing the total number of dissolved particles.
The osmotic pressure against pure water is 7.3 atmospheres.
SOLUTIONS WHICH ARE AROUND 300 mOsm ARE SAID TO BE ISOTONIC.
All but 20 mOsm of plasma are contributed by Na+ and Cl-, with a little help from HCO3Proteins contribute 2 mOsm; urea contributes 5 mOsm and glucose contributes 5mOsm
REFERENCES: Ganong’s review of medical physiology, 23rd ed: Section 1; as well as Guyton and Hall (11th edition), and, sadly, Wikipedia.
This document was created by Alex Yartsev ([email protected]); if I have used your data or images and forgot to reference you, please email me.
NON-IONIC DIFFUSION
This applies to substances which are lipid-soluble in their undissociated form; and are thus able to
dissolve through the lipid cell membrane and get to the other side – where they dissociate.
DONNAN EFFECT and the Gibbs-Donnan equation
Cells are full of ions which occasionally aren’t able to diffuse out of the cells. However, these ions
are still osmotically active, and have a charge.
Donnan and Gibbs state that in the presence of a non-diffusable ion, the diffusable ones will
distribute themselves so that their concentration ratios will be equal; i.e.
(sum of concentrations of diffusable ions inside) = (sum of concentrations of diffusable ions outside)
Cell
BUT: there end up being more osmotically active solutes inside the cell.
Thus, water is constantly attracted into the cell by
osmosis. The cell needs to constantly pump ions out of
Proteins: constant source of negative
itself to reduce the number of osmotically active
change and osmotic activity
particles inside itself.
Cl- 9 mmol
-70mV
Cl- 125 mmol
K+ 150 mmol
Na+ 15mmol
-90mV
K+ 5.5 mmol
+60mV
Na+ 150mmol
IF IT WERE NOT FOR Na+/K+ ATPase, ALL OF
OUR CELLS WOULD SWELL UP AND EXPLODE.
Because of the constant pumping, the concentration of
ions inside vs. the concentration outside is unequal; and
because the ions are charged, AN ELECTRICAL
POTENTIAL DIFFERENCE EXISTS BETWEEN THE
INSIDE AND THE OUTSIDE
FORCES ACTING ON IONS
Chloride ions are present in a higher concentration outside the cell; they would diffuse
inside if they could, but they are REPELLED by a net negative charge inside the cell (provided by the
proteins in there). The diffusion force sucking it in is balanced by the repulsion force pushing it out.
The membrane potential at which this equilibrium exists is the equilibrium potential.
The equilibrium potential for Cl- is -70millivolts.
K+ is the reverse: it is being attracted inside the cell by the negative charge, but sucked back out of
the cell by the concentration gradient.
For Na+, the chemical gradient promotes suction into the cell (where there is less sodium). The
electrical gradient is also sucking it into the cell (because the charge is nicely negative in there).
One might expect the cell to fill up with an absurd amount of sodium. Thankfully, owing to the
constant thankless efforts of the Na+/K+ ATPase, the intracellular concentration of sodium is kept at a
steady low 15mmol.
MEMBRANE POTENTIAL
At equilibrium, there is a slightly increased concentration of anions on the inside. This condition is
maintained by the the Na+/K+ ATPase. The difference in concentration is miniscule, but it creates a
potential difference across the membrane – about 70 mV in total. The condition of this electrical
difference is present only around the membrane (everywhere else the charges are mixed well
enough).
REFERENCES: Ganong’s review of medical physiology, 23rd ed: Section 1; as well as Guyton and Hall (11th edition), and, sadly, Wikipedia.