Acid buffering systems in body
Acid Buffering Systems
The body's pH Buffer systems correct both excess acidity and alkalinity,
but here the focus is on acid-buffering systems, since over-acidity in the body
is the "problem of the day"
Overview of Body's Acid- Buffering Systems
The body's first step to counter acidosis.
Try and buffer excess acid with alkaline mineral bicarbonates
in the blood and lungs.
If sufficient alkalizing minerals are unavailable.
The body begins to sweep the extra acids into the tissues, especially
muscles and joints. This is known as lactic acid 'buildup'and is experienced as
pain.
If all else fails.
The body will precipitate acids out of solution in the form of solid crystals
and salts, realized as gallstones, kidney stones, uric acid crystals, plaque, and
cholesterol crystals.
Technical Details of Body's Acid-Buffering Systems
An acid-buffering system is likened to a sponge
which soaks up H+ ions. When an acid is added
to a solution, the pH change can be minimized by the adequate presence of buffers,
and to have this effect, acid buffers have to be a weak acid themselves.
For example, carbonic acid (H2CO3) is an acid buffer:
Since carbon dioxide and water are the principal end products from carbohydrate,
protein and fat breakdown, carbon dioxide (CO2 ) is the most abundant
acid-forming substance produced by the body.CO2 + water (H2 O)
in the blood forms carbonic acid (H2 CO3 ) a weak acid
which ionizes to give H+ (hydrogen ion) and HCO3 -
(bicarbonate ion). The H+ in strong acids are completely dissociated ,
but the H+ in weak acids are only partially dissociated and are efficient
at preventing pH changes.
An acid buffer is made up of a buffering pair:
(a) A weak acid (Capable of donating a H+
and thus lowering pH);
(b) The acid's conjugate base (Capable of
accepting H+ , and thus raising pH)
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 tissue
•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+NaHCO3 →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 H2O
- 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. While 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 CO3 →
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 ).
From 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 normal 7.4)
(Alternatively, over-breathing raises EC pH to ~7.7 in 1 minute)
(3) Renal Mechanisms (Compensates 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: composed of uric acid and crystine. A therapeutic diet would alkalize the
urine.