Acid Base Balance 1 & 2 Flashcards
(96 cards)
why are acid/base balance important and what happens if it is disturbed?
Metabolic reactions are exquisitely sensitive to the pH of the fluid in which they occur. Relates to the high reactivity of H+ ions with Pr- = changes in configuration and function, especially enzymes
Acid/Base disturbances = all sorts of metabolic disturbances
pH of ECF is very closely regulated
Normal pH of arterialized blood is 7.4 = free [H+] of 40 x 10-9 moles/l or 40 x 10-6 mmoles/l
what is the important of free H+?
Only free H+ ions contribute to pH
Other plasma constituents are present in mmoles eg Na+, K+, Cl-, glucose etc. So [H+] is one millioneth that of other plasma constituents
does hte body produce H+?
yes
what ar ethe 2 main sources of H+?
respiratory acid
metabolic acid (non-respiratory acid)
how is respiratory acid a source of H+?
CO2 + H2O <=> H2CO3 <=> H+ + HCO3-
Formation of carbonic acid is not normally a net contributor to increased acid because any increase in production = decrease in ventilation
Problems occur if lung function is impaired
hwo is metabolic acid a source of H+?
via metabolism
a) inorganic acids: eg S-containing amino acids = H2SO4 and phosphoric acid is produced from phospholipids
b) organic acids: fatty acids, lactic acid. On a normal diet, there is a net gain to the body of 50-100 mmoles H+ per day
Major source of alkali is oxidation of what?
organic anions such as citrate
what do buffers do?
Minimize changes in pH when H+ ions are added or removed
what equation is this:
pH = pK + log[A-]/[HA]
Henderson-Hasselbalch equation
what is the Henderson-Hasselbalch equation?
defines the pH in terms of the ratio of [A-]/[HA] NOT the absolute amounts
what is the most important extracellular buffer? and what is the equation?
The most important is the bicarbonate buffer system
H2CO3 <=> H+ + HCO3-
pH = pK + log [HC03-]/[H2CO3]
pK = 6.1 so in plasma at pH 7.4, 7.4 = 6.1 + log [HC03-]/[H2CO3]
1.3 = log 20, so the ratio of [HCO3-]/[H2CO3] in blood at 7.4 is 20:1, Need 20 times more bicarbonate
The quantity of H2CO3 depends on the amount of CO2 dissolved in plasma, depends on solubility of CO2 and Pco2
how do you calculate the standard biocarbonate?
The quantity of H2CO3 depends on the amount of CO2 dissolved in plasma, depends on solubility of CO2 and Pco2
Solubility of CO2 in blood at 37°C = 0.03 mmoles/l/mmHg, PCO2 = 0.225 mmoles/l/kPa PCO2
So, at a normal Pco2 of 40mmHg, 5.3kPa, [H2CO3]= 40 x 0.03 mmoles/l or 5.3 x 0.225mmoles/l =1.2mmoles/l
Since the ratio of [HCO3-]/[H2CO3] in blood at 7.4 is 20:1, [HCO3-] = 24mmoles/l = “Standard bicarbonate”
what is the normal value and ranges for pH?
pH =7.4 Range 7.37-7.43
Range of pH compatible with life: 6.8-7.8 (US), 7.0-7.6(UK)
what is the normal value and rages for pCO2?
pCO2 = 5.3kPa Range 4.8- 5.9
= 40mmHg 36-44
what si the normal value and range of [HCO3-]?
[HCO3-] = 24mmoles/l Range 22-26
The Henderson-Hasselbalch equation can be more simply written as what?
pH = [HCO3-]/PCO2
The unique importance of the bicarbonate buffer system is crucial:
increased H+ + HCO3- <=> H2CO3 <=> H2O + CO2
describe what is happening?
Basic mechanism by which it acts as a buffer is evident, an increased ECF H+ drives the reaction to the right, so that some of the additional increased H+ ions are removed from solution and therefore a change in pH is reduced
increased H+ + HCO3- <=> H2CO3 <=> H2O + CO2
If this was an ordinary buffer system then increasedH+ drives the reaction to the right and what would happen?
the increased products would begin to push the reaction back to reach a new equilibrium position, where only some of the additional H+ ions are buffered
In any normal buffer system the products would stop the reaction happening and make it go slower and make it reverse back the other way

is the bicarbonate an ordinary buffer system?
no
how is the bicarbnate buffering system not an ordinary buffering system?
the reaction is pulled to the right, greatly increasing the buffering capacity of the bicarbonate
Ventilation removes the product of CO2 and cant participate in the backwards reaction any more and increases capacity of this buffer system dramatically

does the bicarbonate buffering system eliminate the H+?
Important to note that H+ has NOT been eliminated from the body. Instead the HCO3- has buffered the H+ and the respiratory compensation has greatly increased the buffering capacity so that free H+ions are prevented from contributing to the pH
Elimination of H+ from the body is by the _______ and this excretion is coupled to the regulation of plasma _______
kidneys
[HCO3-]
pH = [HCO3-] = renal regulation
Pco2 = respiratory regulation
what are other buffers in the ECF?
Plasma proteins Pr- + H+ <=> HPr
Dibasic phosphate HPO42- + H+ <=> H2PO4- monobasic phosphate
what are intracellular buffers?
Primary intracellular buffers are proteins, organic and inorganic phosphates and, in the erythrocytes, haemoglobin
Buffering of H+ ions by ICF buffers cause changes in plasma electrolytes, since to maintain electrochemical neutrality, movement of H+ must be accompanied by Cl- as in red cells or exchanged for a cation, K+
















