GSE
Body's Acid ▲ Alkaline balance
Acid-Alkaline buffering systems in body
Body's Acid-Alkaline Buffering Systems
3 main buffer systems
In functional equilibrium with each other, there
are three main buffer systems contributing to the regulation of the acid-base
balance :
(1)
Chemical Buffer Systems In blood, lymph, and intra/extracellular fluids;
(2)
Respiratory Compensation
(Gaseous exchange in the lungs) Breathing out CO2 deals with much of our acid excess.
(3)
Renal Mechanisms (Excretory
functions of the kidneys) The kidneys serve primarily to excrete
protons created during the breakdown of different acids. This excretory system
is needed because the typical diet tends to present more H+ ions (protons) than
alkalizing substances that might neutralize them.
(1) Chemical
Buffer Systems
First line of defense against acid or base
additions to body fluids. All body fluids contain acid-alkaline
buffers. These buffers are chemicals that readily combine with an acid (or base)
that function to prevent drastic changes in the pH of a body fluid when a strong
acid (or base) is added. There are seven types of chemical buffers
maintaining healthy body pH levels. These buffers (located in ECF, intracellular
fluid and bone) neutralize, bind or dilute strong acid solutions in:
• Blood, lymph and tissues
• Lungs
• Kidneys
The 3 major chemical buffers are carbonic
acid-bicarbonate, phosphate and protein .
1) Carbonic Acid - Bicarbonate Buffers
(in blood, lymph and tissues)
This is the most important
system, since if this system is stable, then the other systems are
stable
Interacts directly with respiration through the
formation of carbon dioxide. Also, carbonic acid is created from hydrogen carbonate, which
breaks down into water and carbon dioxide, which is breathed out. Blood pH
regulation is carried out by different buffer systems consisting of a weak acid
and its corresponding base in a particular ratio;
This system controls pH instantaneously
This
system is a mixture of carbonic acid (H2 CO3 ) and the bicarbonate
ion (HCO3 - ) present as Salt HCO3
(Salt
can be sodium, potassium, or magnesium).
- When the H+ ions of a
strong acid are added to this mixture, it immediately combines with the
bicarbonate ion to form carbonic acid (a very weak acid). And then the pH is only slightl y
decreased.
- Example of Acid buffer:
HCl + NaHCO 3 → NaCl +
H2 CO3
strong acid +
weak base → salt +
weak acid
The carbonic acid-bicarbonate buffer is the only
significant buffer for keeping the extra-cellular bicarbonate to carbonic acid
ratio at the required 20:1
Bicarbonate ions are generated in the RBCs or
stomach's parietal cells from CO2 and H2 O
- RBCs. RBCs contain enzyme carbonic anhydrase which speeds up the
reaction of CO2 and water by 5000 times.CO2 + H2 O
→ H2 CO3
+ H+ + HCO3 - . Hemoglobin absorbs the H+ ions and the
bicarbonate ions enter
blood stream (this removes 70% of
CO2 from blood; 7%
of CO2 is dissolved in plasma; remaining 23% of CO2
combines directly with hemoglobin and is exhaled through the lungs).
- Stomach's Parietal Cells.
Whilst
producing gastric acid, the
parietal cells of the stomach
simultaneously generate
bicarbonate ions from CO2
+ H2 O, which diffuse
into the blood stream.
Bicarbonate ions are used to neutralize H+ ions
in blood, lymph, tissue fluids, and kidneys
Bicarbonate neutralizes H+ to form CO2.
CO2 is
exhaled through the lungs via the respiratory buffering system. E.g. H2SO4 + 2NaHCO3 →Na2SO4 + 2H2CO3 →2CO2 + 2H2O
+ Na2SO4.
Bicarbonate is also active in the kidney
buffering system - where it is either
reabsorbed to lower blood acidity or excreted with bound acids to maintain a
balanced pH.
In an example of an alkaline-buffering
system the chemical equation looks like this:
NaOH + H2 CO 3 →
H2 O +
NaHCO3
strong base +
weak acid →
water +
weak base
When a strong base is added to the mixture, it combines with the carbonic acid
to form water and its neutral bicarbonate salt.
(2) Phosphate and Ammonia Buffers
(Primarily for blood passing through kidneys and
intracellular fluid)
Phosphates can also work weakly in the blood and
lymph. These buffers are
especially important in intracellular fluids, where their concentration far
outweighs bicarbonate buffers.
Acid Buffering:
HCl + Na2 HPO4 → NaCl + NaH2 PO4
strong acid + weak base → salt + weak acid
Na2 HPO4 is actually the "salt" in the following dissociation
reaction:
H2 PO4 - <=> H+ + HPO4
-2
Alkaline Buffering:
NaOH +
NaH2 PO4 → H2 O + Na2 HPO4
strong base +
weak
acid
→ water +
weak base
Phosphate. Acid blood (H+ ions) passing through the kidneys is buffered with
salts of sodium dihydrogen phosphate(Na2 HPO4 ) , creating H2 PO4 - ,
a weak acid, which is then excreted in urine ( H2 PO4 - â†→H+ + HPO4 -2 ).In the
process, sodium is exchanged for an H+ ion removed from the
extracellular fluid (ECF), forming a bicarbonate ion ,
which is then releasedinto the ECF. The kidney thus reduces the
degree of acidosis in the body fluids.
Ammonia (NH3 ). Fermentation of amino acids reacts with the H+
ions to form ammonium ions, which are excreted into the urine, again
increasing the bicarbonate concentration
in ECF. By removing specific amounts of H+ ions from the blood and
secreting them into the filtrate, the kidney can keep the pH of the blood at a
constant level of 7.365.
(3) Protein Buffers (15%)
(intracellular fluid, lymph, blood)
The most plentiful buffers
inside cells and in plasma. Proteins have negative charges and serve as
both acidic (H+) and alkaline buffers.
E.g.
histidin, glutathione, methionine, cysteine, taurine, hemoglobin.Most action is to bind or neutralize acids inside cells.
(4) Electrolyte Buffers
(in blood, lymph, extracellular / intracellular
fluids)
The "X CO3's ". Alkaline minerals ("X") work to bind acids, which are then removed through the
urine. The "Big 3"alkaline minerals are sodium, calcium and potassium, which
are recycled by the kidneys back into the blood and lymph by binding them to CO2
(provided by 70% of the output from the body's cellular energy-producing
fermentation processes). In chronic acidosis, alkaline minerals are drawn from
the bones to maintain life-sustaining blood pH.
E.g. The strong sulphuric and phosphoric acids (from the oxidation of
sulphur-containing amino acids (e.g. in meat, eggs) and the oxidation of
phospholipids) can be neutralized by alkaline minerals and then excreted by the
kidneys.
MINERALS
Examples
Compounds
ACID
-ve charge
Chlorine (Cl- )
Sulfur (S- )
Phosphorus (P- )
Attracted to the H+ ion
Hydrochloric acid (HCl)
Sulfuric acid (H2 SO4 )
Phosphoric acid (H3 PO4 )
ALKALINE
+ve charge
Calcium (Ca++ )
Potassium (K+ )
Sodium (Na+ )
Magnesium (Mg++ )
Attracted to the negatively charged hydroxyl ion (OH-)
E.g. CaCO3 +
H2 SO4 →CaSO4 + H2 O + CO2
(5) Low density Lipoproteins of Fat Buffers
(in the blood, lymph, and extracellular fluids)
LDL and fat (especially electron-rich polyunsatured fats)
bind acids, which are then excreted via the urine - If elimination is
compromised, these fat-bound acids are moved into the body cavities, hips,
thighs, stomach, etc. i.e. obesity.
(6) Hormonal Buffers
Two kidney hormones especially help the kidneys
maintain alkalinity and reduce excess acidity:
- ADH (antidiruretic hormone). Regulates rate of water excretion or retention
- ALDOSTERONE. Regulates the level of sodium ions (Na+ )
and potassium ions (K+ ) in the blood.
(7) Water
(in the blood, lymph, intracellular and
extracellular fluids)
Water helps to maintain alkalinity in the blood,
lymph, intracellular and extracellular fluids -
by
diluting excess acidity .
(2) Respiratory Compensation (Gaseous exchange in the lungs)
- Buffers
CO2 in blood
CO2 is constantly being produced in tissues and then transported via the blood
to be expelled by the lungs - This provides the
body with the
biggest pH-buffering job -
to maintain equilibrium in the
concentrations of carbon dioxide,
carbonic acid and bicarbonate resulting from their continuous
formation and elimination. This is not usually a problem since CO2 is
easily eliminated via the lungs, and
pH (H+
ion concentration)
can be adjusted by altering the number of breaths / minute
to increase or decrease exhalation of acidic CO2 .
Hydrogen ion Concentration Controls Breathing
Rate
• An extracellular (EC) acid pH (high
H+ concentration) stimulates the respiratory center in the
medulla of the brain to speed up breathing rate
(removing CO2 and thus carbonic acid, and so increasing pH by
removing H+ ions)
• An EC alkaline pH
(low H+ concentration)
stimulates the respiratory center to slow down breathing rate
Consider the following reaction:CO2 + H2 O < -- > H2 CO3
< -- > H+ + HCO3 -
In the tissues where carbon dioxide is abundant, the reaction is shifted to the
right:
CO2 + H2 O→H2 CO3 →H+ +HCO3 -
In the lungs where H+ ions are liberated from hemoglobin, the
reaction is shifted to the left:
CO2 + H2 Oâ†H2 CO3 â†H+ +HCO3 -
Hemoglobin absorbs H+ ions - This protein respiratory pigment of the red blood cells is a
VERY IMPORTANT BUFFER in RBCs ,
particularly in carbonic acid buffering.
If breathing decreases below normal:
→ Increases CO2 → increases H2 CO3
→ increases H+ ions → increases
acidity .
E.g. If breathing stops for
1 minute, extracellular pH → 7.1 (from its normal 7.4)
(Alternatively, over-breathing raises extracellular pH to ~7.7 in 1 minute)
(3) Renal Mechanisms - Compensate for acid pH by excretion of H+ by producing acidic urine
Normal Renal Mechanisms vs. Renal Compensation
- Normal Renal Mechanism.
Distal tubules of the kidneys secrete H+ ions directly
into the filtrate so that urine is acidified and the H+ ions are lost
from the body.This is a normal process that occurs at a normal rate.
- Renal Compensation.
Occurs when other
mechanisms for acid-base balance fail and the
rate of kidney excretion of H+ ions increases above normal ,which
are then eliminated from the body.
The kidneys are the ultimate acid-base regulatory organs -
Although the body's buffer systems can resist pH changes, by tying
up excess H+ ions,the constantly
generated H+ must eventually be eliminated from the body.The lungs can eliminate only carbonic acid (by eliminating carbon
dioxide), but only the kidneys can rid the body of metabolic acids (phosphoric,
uric, and lactic acids, and ketones) and prevent metabolic acidosis.
(1) By eliminating H+ ions
or retaining bicarbonate. The kidneys
control H+ ion concentration in extra-cellular fluids by
either -
(a)
H+ ion secretion - Excess H+ ions (secreted by kidney's nephron tubule cells) combine with urinary buffers to be excreted as acids
(i.e. acidifies the urine)
and ammonium.
or (b) Bicarbonate ( HCO3-)
ion reabsorption. Excess, secreted H+ ions and bicarbonate ions [filtered into
the glomerular filtrate] combine in the kidney tubules to form carbon dioxide
[which is exhaled out] and water [which is urinated out].
E.g.HCl + NaHCO3 ↔ NaCl + H2CO3 ↔ CO2 + H2O + NaCl
Or (2) By
exchanging H+ ions for Na+ or K+ ions.
Another
compensatory method for acidosis is to exchange H+ ions for sodium or
potassium ions in the kidney tubules, producing a sodium/potassium imbalance and
possible hyperkalemia;
- Effective
blood buffering by the kidneys produces a urine pH of 4.6 - 8;
Note: the kidney can
correct states of excess but not states of deficiency -
Correction of an acidosis due to a loss
of or insufficient base (Na+ or K+
+HCO3- ), e.g. by diarrhea, requires the administration of Na
or K salts from which Na+ or K+ +HCO3- can be formed.
Kidney Stones
Formed in overly-alkaline conditions: are
composed of magnesium and calcium phosphates, carbonates and oxalates, which are
insoluble in alkaline fluids. A diet
that acidifies the urine is desireable (e.g. cranberry juice) to reduce
these type of stones.
Formed in overly-acid
conditions: are composed of uric acid and crystine. A therapeutic diet would
alkalize the urine.