Acid base physiology Flashcards
why is pH homeostasis important?
Change in [H+] will effect ionisation state of proteins and can cause denaturing - enzymes, receptors, structural proteins = disruption to cellular functions. including O2 carrying capacity of Hb
disruption to membrane potentials - positive charge will effect the balance inside and out and hence can cause excitability or reduce this. this can alter myocardial contractility and neuronal transmission
H+ ions also cause vasodilation in most vascular beds and vasoconstriction in pulmonary vasculature.
H+ **exchanges with K+ **and causes hyperkalaemia too. which has its only problems in causing arrhythmias and increasing neuronal excitability
define pH
pH is the negative logarithm for hydrogen ion concentration.
pH = -log [H+]
it is used to allow easier interpretation of acidity as H conc is in nanomoles.
since the relationship is logarithmic it is not linear and a 1 fold change in pH is equivalent to 10 fold change in H+
for example a pH of 7 = 10^-7 M
and pH of 8 = 10^-8 M
state the henderson hasselbach equation…
how is anion gap calculated?
anion gap = [Na + K] - [Cl +HCO3]
what is pKa
the pH at which 50% of ions are in dissociated form and 50% are undissociated.
it is the negative log of Ka , which is the dissociation constant ([H][A]/[HA])
pKa = - log [Ka]
it is the measure of an acids strength.
the larger Ka the more dissociated, take the negative logs mean the smaller the pKA, the more dissociated, stronger the acid.
what is the normal pH
7.35 to 7.45
equivalent to 34nM to 46nM
what is normal values for bicarb and CO2?
bicarb = 22 to 26 mM
CO2 = 4-6kpa (5.3kpa)
what is the pKA of different buffer systems?
bicarb = 6.1
phosphate = 6.8
Hb buffer = 7.8
plasma proteins = 7.4
define an acid
by Brownsted lowry definition
this is a proton donor
a molecule that dissociates to form conjugate base and H+ ions
a strong acid fully dissociates
a weak acid partially dissociates
define a base
by Brownsted lowry definition
this is a proton acceptor
a molecule that associates with a H+ to become ionised
(an alkali is a base that produces OH- ions)
define a buffer
A solution that can resist pH changes by reacting with H+ ions or releasing H+
usually consists of weak acid and its conjugate base
as pH / H+ conc chnages it will shift reaction in one direction to buffer the chnages.
what is standard bicarbonate?
plasma conc of bicarb when pCO2 is corrected to 5.3kpa , at 37 degrees and when Hb is fully saturated
it represents the bicarb that there would be once CO2 deviations are eliminated. Gives a cleaner picture of metabolic acid-base disturbances without resp influence
what is meant by the base excess / deficit?
the amount of base required to restore 1 litre of blood to normal pH (7.4) at CO2 of 5.3kpa and body temp of 37 degrees.
marker of severity of metabolic acidosis
if negative e.g. -3 then 3mmol/l of base required to reach pH of 7.4
normal BE = -2 to +2
what does the pKa tell you about when acids and bases ionise?
acids ionise ABOVE their pKA
bases ionise BELOW their pKA
what is the difference between acidosis and acidaemia ?
acidosis = an underlying process that causes acidaemia
acidaemia = blood pH of less than 7.35
can be acidotic without acidaemia
describe how a buffer solution works
slightly wrong in diagram - should say if H+ fall more acid disocciates.
this is know as Le Chatelliers principle = equilibrium shifts in order to minimise change.
derive the henderson hasselbach equation
- write out the equilirbium
- state K1 = K2 at equilibrium i.e the rates of forward and back
- Ka is the dissociation constant i.e. how much of the forward product exists at equilirbium / how shifted to the right
- Ka = [H+][A-} / [HA}
- hence the bigger Ka, the more dissociated, the stronger the acid
- Rearrange this and take logs off both sides
should end up with
pH = pKA + log [A]/[HA]
proove using HH equation that when the pKA is equal to pH the dissociation is 50%
pH = pKa + log [A]/[HA]
pH-pKa = log [A]/[HA]
when they are equal..
0= log [A]/[HA]
10^0 = 1 = [A]/[HA]
hence ratio is 1 so top and bottom are equal
what is a buffer titration curve?
tells you at which pH ranges a buffer is most effective.
pH on y axis
% of acid/base added to buffer on x axis
can read across y axis at what pH the buffer gets exhausted.
what are the characteristics of an ideal buffer?
An ideal buffer system in the blood is one ..
1. present in high enough quantities to work effectively - the more present the buffering capacity
2. pKa is close to physiological pH such that it works optimally around deviations to norm.
3. open system - can be removed at one end/ added at one end. e.g. CO2 removed, HCO3 added to maintain the buffer even when used up.
explain how the bicarbonate buffer system works..
H20 + CO2 reaction catalysed by carbonic anhydrase which is present in RBC. HCO3 leaves RBC in exchange for chloride
open at both ends - CO2 removed, HCO3 added.
rate of HCO3 reabsorption by kidney depends on H+ secretion which is higher at lower pH.
how do the kidneys handle acid?
PCT, DCT and collecting duct are main regions for acid/base handling
1. HCO3 reabsorption
2. H+ secretion
3. production of ammonium to act as a buffer
HCO3/ H+
* H+ secreted by primary and secondary AT
HCO3 is freely filtered at glomerulus and then reabsorbed at PCT
* The more acidic the blood, the easier to secrete H+ into lumen and hence reabsorb HCO3
* in DCT and collecting duct similar mechanism occurs however H+ exchanged for K+ instead of Na.
the H+ secreted is buffered by ammonia
* ammonium ions are produced by PCT cells by deamination of glutamate to glutamic acid and oxoglutarate (a-ketoglutarate)
* oxoglutarate reacts with H+ and forms glucose which is reabsrobed by blood (gluconeogenesis)
* NH3 enters tubules and buffers H+ to aid further secretion of H+ into tubules.
* oxoglutarate also indirectly produces HCO3 for reabsorption
how do urinary buffers contribute to acid /base homeostasis?
there are a number of urinary buffers - ammonia and phosphate are the main ones.
- ammonium ions are produced by PCT cells by deamination of glutamate to glutamic acid and oxoglutarate (a-ketoglutarate)
- oxoglutarate reacts with H+ and forms glucose which is reabsrobed by blood (gluconeogenesis)
- NH3 enters tubules and buffers H+ to aid further secretion of H+ into tubules.
- oxoglutarate also indirectly produces HCO3 for reabsorption
explain how a pH electrode works?
2 half cells
reference electrode = silver/silver chloride + KCl solution. AgCl + e- –> Cl- + Ag
measuring electrode = silver / silver chloride electrode + 0.1M HCl buffer solution. pH sensitive glass in contact with blood. pH sensitive glass is negativley charged on both sides(silica) , H+ ions bind and need to be equal on both sides, causes HCL to dissolciate and Cl free to react with Ag. produces AgCl + e- .
kept at a constant temp
potential difference betwen 2 half cells is measured.
what is the siggard anderson nonogram?
a graphical representation of pCO2 and pH and HCO3
helps to derive one if others are known
can be used to derive base deficit and therefore helps in diagnosis of metabolic acidosis.
