ACID AND BASE BALANCE
Acid-base balance
➔ Acid-base balance: precise regulation of free unbound hydrogen ion (H+)
concentration in body fluids where [H+] indicates H+ concentration of chemical
Acids liberate free hydrogen ions
➔ Acids: group of hydrogen-containing substances that dissociate in solution to liberate
free H+ and anions
➔ Strong acid has greater tendency to completely dissociate in solution than weak acid
➔ Hydrochloric acid (HCl) strong acid -> every HCl molecule completely dissociates fully
to free H+ ions and chloride (Cl2) when dissolved in H2O
➔ Carbonic acid (H2CO3) weak acid -> portion of H2CO3 molecules partially dissociates
to free H+ ions and bicarbonate anions (HCO3-) when dissolved in solution
➔ Constant degree of the extent of dissociation for acid expressed by dissociation
constant (K): K = [products i.e. ions]/[reactants] = [H+][HCO3-]/[H2CO3]
o [H+][HCO3-] is the concentration of ions from H2CO3 dissociation
o [H2CO3] is the concentration of intact undissociated H2CO3
➔ Dissociation constant depends and varies on strength i.e. strong or weak acid or base
Bases accept free hydrogen ions
➔ Base: substance that combines with free H+ removing it from solution
➔ Strong base binds H+ more readily than weak base
➔ pH used to express [H+]
Acid-base chemistry and pH
➔ pH of pure water = 7.0 which is chemically neutral
➔ Small proportion of H2O dissociates into hydrogen H+ and hydroxyl OH- ions
➔ Equal number of acidic H+ and basic OH2 formed, H2O is neutral, not acidic or basic
➔ pH solution <7.0 contains higher [H+] than pure H2O therefore acidic
➔ pH solution >7.0 contains lower [H+] than pure H2O therefore basic or alkaline
Acidosis and alkalosis in body
➔ pH of arterial blood = 7.45 and pH of venous blood = 7.35 for average blood pH = 7.4
➔ pH venous blood slightly lower i.e. more acidic than arterial blood as H+ generated by
formation of H2CO3 from CO2 picked up at tissue capillaries
➔ Acidosis when blood pH <7.35 and alkalosis when blood pH >7.45
o CO2 and lactic acid change pH and cells not function
➔ Arterial pH <6.8 or >8.0 not compatible
➔ Death occurs if arterial pH outside range of pH 6.8-8.0 for more than few seconds
therefore [H+] in blood carefully regulated for brain function not shutting down
,pH consideration
pH 0-7 = acidic
Normal body pH 7.4
pH 7-14 = basic
Acidosis lower 7.4 = death
Alkalosis above 7.4 = death
Changes in [H+]
Fluctuations in [H+]
➔ Narrow pH range compatible with life
➔ Small changes in [H+] affecting pH consequences:
o Changes in excitability of nerve and muscle cells among pH abnormalities
o Hydrogen ion concentration exerted marked influence on enzyme activity
Stops cellular processes and functions
o Change in [H+] influences K+ levels in body
Increase [H+] = decrease [K+] with negative effect on body
Sources of H+
H+ is continuously added to body fluid from 3 sources:
1. Carbonic acid formation: CO2 + H2O ⇌ slow ⇌ H2CO3 ⇌ fast ⇌ H+ + HCO3-
Step 1: carbonic anhydrase catalyses formation HCO3- from metabolically produced CO2
i.e. CO2 + OH- ⇌ HCO3- via ca
Step 2: water dissociates forming more OH2 that can be used in step 1 yielding H+ in process
i.e. H2O ⇌ OH- + H+
COLLECTIVELY: CO2 + H2O ⇌ H+ + HCO3- via ca
➔ Reactions are reversible as they proceed in either direction
, ➔ CO2 level in systemic blood increases as metabolically produced CO2 enters from
tissues driving reaction to right (H+ side) producing H+
o CO2 and H2O forms H+ and HCO3- via H2CO3 and ca
➔ Lungs: reaction is reversed where CO2 diffuses from blood flowing through
pulmonary capillaries into alveoli and expired to atmosphere
o Resultant reduction blood CO2 drives reaction to left (CO2 side) producing CO2
o H+ and HCO3- forms CO2 and H2O again
o CO2 is exhaled while hydrogen ions H+ are incorporated into H2O molecule
➔ When respiratory system can keep pace with rate of metabolism = no net gain or loss
of H+ in body fluids from metabolically produced CO2
➔ When rate of CO2 removed by lungs not match rate of CO2 production in tissue =
excess or shortage of free H+ in body fluids
o Too much CO2 expelled = alkalosis = rebreatheCO2 to regulate pH
2. Inorganic acids produced during breakdown of nutrients
➔ Dietary proteins in meat contain large quantity of sulfur and phosphorous
➔ These nutrient molecules are broken down where sulfuric and phosphoric acid
produced as by-product
➔ Moderately strong acid therefore the inorganic acids largely dissociate (fully
dissociate) liberating free H+ ions into body fluids
➔ Acids generated during breakdown of proteins in grains and dairy
➔ Bases generated during breakdown of fruits and vegetables that neutralises acids
derived from meat, grain and dairy protein metabolism
➔ More acids than bases are produced during breakdown of ingested food = excess
acids = contribute to [H+]
3. Organic acids resulting from intermediary metabolism
➔ Numerous organic acids produced during normally intermediary metabolism:
o Fatty acids -> fatty acid metabolism
o Muscles -> lactic acid i.e. lactate (heavy exercise)
o Acids partially dissociate -> yields free H+
➔ Hydrogen ion (H+) generation is continuous due to ongoing metabolic activities
➔ Disease states: additional acids produced that contributes to total body pool H+
o Diabetes -> large quantities keto acids produced by abnormal fat metabolism
o Acid-producing medications -> H+ unceasing, highly variable and unregulated
Defences against changes in [H+]
➔ H+ balance is maintaining normal alkalinity of ECF (pH 7.4)
➔ Generated free H+:
o Largely removed from solution while in body
o Eliminated
➔ pH of body fluids therefore can remain within the narrow range compatible with life
, ➔ Mechanisms exist to compensate rapidly when ECF becomes too alkaline
➔ 3 lines of defence against changes [H+] to maintain [H+] of body fluids at constant
level despite unregulated input:
o Chemical buffer systems
o Respiratory mechanism of pH control
o Renal mechanism of pH control
Chemical buffer systems
➔ Chemical buffer system: mixture in solution of two chemical compounds that
minimises pH changes when acid or base is added or removed from solution
➔ Buffer system: pair of substances involved in reversible reactions where one
substance yields free H+ as [H+] falls and another binds with free H+ as [H+] rises
➔ Carbonic acid-bicarbonate H2CO3:HCO3- buffer pair in reversible reaction
o H2CO3 ⇌ H+ + HCO3-
➔ Strong acid HCl added to unbuffered solution = all dissociated H+ free in solution
o 3 HCl to unbuffered solution = 3 free H+ present
➔ Strong acid HCl added to H2CO3:HCO3- buffer pair solution = HCO3- immediately binds
with free H+ forming H2CO3 therefore less H+ free in solution
o 3 HCl to buffered solution = 1 free H+ present = change in acidity due to HCO3-
binding with the H+ forming H2CO3 decreasing the H+ ion in solution
BUFFER SYSTEMS
1. H2CO3:HCO3- buffer pair -> primary ECF buffer against non-carbonic acid changes
2. Protein buffer system -> primary ICF buffer and buffers ECF
3. Haemoglobin buffer system -> primary buffer against carbonic acid changes
4. Phosphate buffer system -> urinary buffer and buffers ICF
H2CO3:HCO3- buffer pair: primary ECF buffer
➔ H2CO3:HCO3- buffer pair buffers pH changes in ECF brought about by causes other
than fluctuations in CO2 generated H2CO3
➔ Effective ECF buffer system:
o H2CO3 and HCO3- abundant in ECF therefore system readily avaliable to resist
changes in pH
o Each component of buffer pair closely regulated: respiratory system regulates
CO2 generating H2CO3 and kidneys regulate HCO3- for control of changes in pH
➔ H2CO3:HCO3- buffer includes involvement of CO2: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
Acid-base balance
➔ Acid-base balance: precise regulation of free unbound hydrogen ion (H+)
concentration in body fluids where [H+] indicates H+ concentration of chemical
Acids liberate free hydrogen ions
➔ Acids: group of hydrogen-containing substances that dissociate in solution to liberate
free H+ and anions
➔ Strong acid has greater tendency to completely dissociate in solution than weak acid
➔ Hydrochloric acid (HCl) strong acid -> every HCl molecule completely dissociates fully
to free H+ ions and chloride (Cl2) when dissolved in H2O
➔ Carbonic acid (H2CO3) weak acid -> portion of H2CO3 molecules partially dissociates
to free H+ ions and bicarbonate anions (HCO3-) when dissolved in solution
➔ Constant degree of the extent of dissociation for acid expressed by dissociation
constant (K): K = [products i.e. ions]/[reactants] = [H+][HCO3-]/[H2CO3]
o [H+][HCO3-] is the concentration of ions from H2CO3 dissociation
o [H2CO3] is the concentration of intact undissociated H2CO3
➔ Dissociation constant depends and varies on strength i.e. strong or weak acid or base
Bases accept free hydrogen ions
➔ Base: substance that combines with free H+ removing it from solution
➔ Strong base binds H+ more readily than weak base
➔ pH used to express [H+]
Acid-base chemistry and pH
➔ pH of pure water = 7.0 which is chemically neutral
➔ Small proportion of H2O dissociates into hydrogen H+ and hydroxyl OH- ions
➔ Equal number of acidic H+ and basic OH2 formed, H2O is neutral, not acidic or basic
➔ pH solution <7.0 contains higher [H+] than pure H2O therefore acidic
➔ pH solution >7.0 contains lower [H+] than pure H2O therefore basic or alkaline
Acidosis and alkalosis in body
➔ pH of arterial blood = 7.45 and pH of venous blood = 7.35 for average blood pH = 7.4
➔ pH venous blood slightly lower i.e. more acidic than arterial blood as H+ generated by
formation of H2CO3 from CO2 picked up at tissue capillaries
➔ Acidosis when blood pH <7.35 and alkalosis when blood pH >7.45
o CO2 and lactic acid change pH and cells not function
➔ Arterial pH <6.8 or >8.0 not compatible
➔ Death occurs if arterial pH outside range of pH 6.8-8.0 for more than few seconds
therefore [H+] in blood carefully regulated for brain function not shutting down
,pH consideration
pH 0-7 = acidic
Normal body pH 7.4
pH 7-14 = basic
Acidosis lower 7.4 = death
Alkalosis above 7.4 = death
Changes in [H+]
Fluctuations in [H+]
➔ Narrow pH range compatible with life
➔ Small changes in [H+] affecting pH consequences:
o Changes in excitability of nerve and muscle cells among pH abnormalities
o Hydrogen ion concentration exerted marked influence on enzyme activity
Stops cellular processes and functions
o Change in [H+] influences K+ levels in body
Increase [H+] = decrease [K+] with negative effect on body
Sources of H+
H+ is continuously added to body fluid from 3 sources:
1. Carbonic acid formation: CO2 + H2O ⇌ slow ⇌ H2CO3 ⇌ fast ⇌ H+ + HCO3-
Step 1: carbonic anhydrase catalyses formation HCO3- from metabolically produced CO2
i.e. CO2 + OH- ⇌ HCO3- via ca
Step 2: water dissociates forming more OH2 that can be used in step 1 yielding H+ in process
i.e. H2O ⇌ OH- + H+
COLLECTIVELY: CO2 + H2O ⇌ H+ + HCO3- via ca
➔ Reactions are reversible as they proceed in either direction
, ➔ CO2 level in systemic blood increases as metabolically produced CO2 enters from
tissues driving reaction to right (H+ side) producing H+
o CO2 and H2O forms H+ and HCO3- via H2CO3 and ca
➔ Lungs: reaction is reversed where CO2 diffuses from blood flowing through
pulmonary capillaries into alveoli and expired to atmosphere
o Resultant reduction blood CO2 drives reaction to left (CO2 side) producing CO2
o H+ and HCO3- forms CO2 and H2O again
o CO2 is exhaled while hydrogen ions H+ are incorporated into H2O molecule
➔ When respiratory system can keep pace with rate of metabolism = no net gain or loss
of H+ in body fluids from metabolically produced CO2
➔ When rate of CO2 removed by lungs not match rate of CO2 production in tissue =
excess or shortage of free H+ in body fluids
o Too much CO2 expelled = alkalosis = rebreatheCO2 to regulate pH
2. Inorganic acids produced during breakdown of nutrients
➔ Dietary proteins in meat contain large quantity of sulfur and phosphorous
➔ These nutrient molecules are broken down where sulfuric and phosphoric acid
produced as by-product
➔ Moderately strong acid therefore the inorganic acids largely dissociate (fully
dissociate) liberating free H+ ions into body fluids
➔ Acids generated during breakdown of proteins in grains and dairy
➔ Bases generated during breakdown of fruits and vegetables that neutralises acids
derived from meat, grain and dairy protein metabolism
➔ More acids than bases are produced during breakdown of ingested food = excess
acids = contribute to [H+]
3. Organic acids resulting from intermediary metabolism
➔ Numerous organic acids produced during normally intermediary metabolism:
o Fatty acids -> fatty acid metabolism
o Muscles -> lactic acid i.e. lactate (heavy exercise)
o Acids partially dissociate -> yields free H+
➔ Hydrogen ion (H+) generation is continuous due to ongoing metabolic activities
➔ Disease states: additional acids produced that contributes to total body pool H+
o Diabetes -> large quantities keto acids produced by abnormal fat metabolism
o Acid-producing medications -> H+ unceasing, highly variable and unregulated
Defences against changes in [H+]
➔ H+ balance is maintaining normal alkalinity of ECF (pH 7.4)
➔ Generated free H+:
o Largely removed from solution while in body
o Eliminated
➔ pH of body fluids therefore can remain within the narrow range compatible with life
, ➔ Mechanisms exist to compensate rapidly when ECF becomes too alkaline
➔ 3 lines of defence against changes [H+] to maintain [H+] of body fluids at constant
level despite unregulated input:
o Chemical buffer systems
o Respiratory mechanism of pH control
o Renal mechanism of pH control
Chemical buffer systems
➔ Chemical buffer system: mixture in solution of two chemical compounds that
minimises pH changes when acid or base is added or removed from solution
➔ Buffer system: pair of substances involved in reversible reactions where one
substance yields free H+ as [H+] falls and another binds with free H+ as [H+] rises
➔ Carbonic acid-bicarbonate H2CO3:HCO3- buffer pair in reversible reaction
o H2CO3 ⇌ H+ + HCO3-
➔ Strong acid HCl added to unbuffered solution = all dissociated H+ free in solution
o 3 HCl to unbuffered solution = 3 free H+ present
➔ Strong acid HCl added to H2CO3:HCO3- buffer pair solution = HCO3- immediately binds
with free H+ forming H2CO3 therefore less H+ free in solution
o 3 HCl to buffered solution = 1 free H+ present = change in acidity due to HCO3-
binding with the H+ forming H2CO3 decreasing the H+ ion in solution
BUFFER SYSTEMS
1. H2CO3:HCO3- buffer pair -> primary ECF buffer against non-carbonic acid changes
2. Protein buffer system -> primary ICF buffer and buffers ECF
3. Haemoglobin buffer system -> primary buffer against carbonic acid changes
4. Phosphate buffer system -> urinary buffer and buffers ICF
H2CO3:HCO3- buffer pair: primary ECF buffer
➔ H2CO3:HCO3- buffer pair buffers pH changes in ECF brought about by causes other
than fluctuations in CO2 generated H2CO3
➔ Effective ECF buffer system:
o H2CO3 and HCO3- abundant in ECF therefore system readily avaliable to resist
changes in pH
o Each component of buffer pair closely regulated: respiratory system regulates
CO2 generating H2CO3 and kidneys regulate HCO3- for control of changes in pH
➔ H2CO3:HCO3- buffer includes involvement of CO2: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
