Transcript L9&10-Acid and Base

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ACID BASE BALANCE
Dr. Eman El Eter
pH Review
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pH = - log [H+]
H+ is really a proton
Range is from 0 - 14
If [H+] is high, the solution is acidic; pH < 7
If [H+] is low, the solution is basic or alkaline ; pH >
7
pH Review, cont………….
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Acids are H+ donors.
Bases are H+ acceptors, or give up OH- in solution.
Acids and bases can be:
 Strong
– dissociate completely in solution
HCl, NaOH
 Weak – dissociate only partially in solution
Lactic acid, carbonic acid
The Body and pH
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Homeostasis of pH is tightly controlled
Extracellular fluid pH= 7.4
Arterial Blood pH= 7.35 – 7.45
Venous blood is more acidic than arterial?
Because it contains more CO2 than arterial blood.
< 6.8 or > 8.0 death occurs
Acidosis (acidemia) below 7.35
Alkalosis (alkalemia) above 7.45
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Why Small changes in pH is a serious
condition?
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It can produce major disturbances:
Most enzymes function only with narrow pH
ranges.
Acid-base balance can also affect
electrolytes (Na+, K+, Cl-).
Can also affect hormones.
The body produces more acids than bases
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Sources of acids:
 Acids take in with foods
 Acids produced by metabolism of lipids and
proteins
 Cellular metabolism produces CO2.
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CO2 + H20 ↔ H2CO3 ↔
H+ + HCO3-
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Body defense against changes in
Hydrogen ion concentration
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Rates of correction
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Buffers function almost instantaneously
Respiratory mechanisms take several minutes to
hours
Renal mechanisms may take several hours to days
A- buffer systems of body fluids:
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Take up H+ or release H+ as conditions
change
Buffer pairs – weak acid and a base
Exchange a strong acid or base for a weak
one
Results in a much smaller pH change
1- Bicarbonate buffer
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Sodium Bicarbonate (NaHCO3) and carbonic acid (H2CO3). Most
important extracellular buffer why?
Because the components of the system are closely regulated by the
lungs and kidneys.
It acts both extracellular and intracellular.
Its concentration in blood equals = 27mEq/L and is called alkali
reserve.
Maintain a 20:1 ratio : HCO3- : H2CO3
 CO2 + H2O
H2CO3
HCl + NaHCO3 ↔ H2CO3 (week acid)+ NaCl
NaOH + H2CO3 ↔ NaHCO3 (weak base) + H2O.
Bicarbonate buffer system, cont…….
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The relationship of bicarbonate and carbon dioxide
to pH is given by Handerson-Hasselbalch equation:
pH = 6.1 + log
HCO3-
0.03 X PCO2
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CO2: is the acid element as it binds with water to
form H2CO3.
HCO3-: is the basic element.
2- Phosphate buffer
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Important intracellular buffer& renal tubular fluid.
It is an important buffer in renal tubules.
H+ + HPO42- ↔ H2PO4-
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OH- + H2PO4- ↔ H2O + H2PO42-
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3- Protein Buffers
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Includes hemoglobin, work in red blood cells.
Carboxyl group gives up H+
Amino Group accepts H+
Plasma proteins.
Intracellular proteins.
Most important buffer systems in the body are the
proteins of the cells
b. Respiratory mechanisms
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Exhalation of carbon dioxide.
Powerful, but only works with volatile acids
Doesn’t affect fixed acids like lactic acid
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3Body pH can be adjusted by changing rate and depth
of breathing. Works within seconds to minutes and acts
as a second line of defense.
Hyperventilation wash out excess CO2.
Hypoventilation retain CO2.
This process is controlled by chemoreceptor.
C. Renal control of acid base balance
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Can eliminate large amounts of acid
Can also excrete base
Can conserve and produce HCO3- ions
Most effective regulator of pH
Adjust H+ ion concentration to normal
It is a slow mechanism that works over a period of
hours to several days, but a powerful mechanism.
If kidneys fail, pH balance fails.
C. Renal control of acid base balance, cont……
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Renal regulation of acid base occurs through 3 basic
mechanisms:
1- Secretion of H+ and urine acidification at
pH 5.0
2- Reabsorption of filtered HCO33- Production of new HCO3-
Secretion of H+ & Reabsorption of filtered HCO320
This mechanism of
HCO3reabsorption occurs
in:
PCT
Thick ascending
loop of Henle
DCT
Where H+ is
secreted into tubular
fluid by Na+-H+
countertransport.
Normally > 99% of
filtered HCO-3 is
reabsorbed by renal
tubules
Reabsorption of filtered HCO321
Secretion of H+ & Reabsorption of filtered HCO3-,
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In distal and collecting tubules H+ is secreted by
primary active transport.
These segments can increase H+ secretion by 900fold
pH of tubular fluid thus can be reduced to 4.5. This is
the lower limit that can be achieved in normal
kidneys.
For the kidney to continue secretion of H+, it has to
be buffered by two main urinary buffers:
1- phosphate buffer
2- Ammonia: most important urinary buffer in acidosis.
Reabsorption of new of HCO3- by distal &
collecting tubules
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Cell
metabolism
As sodium salt
Under normal condition H+ secretion= 4400 mEq/day,
Rate of HCO3- Filtration = 4320 mEq/day
H+ and HCO3- normally titrate each other in the tubules.
For each H+ excreted with phosphate buffer, a new HCO3- is generated and
reabsorbed. It represents a net gain of HCO3- by the blood
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Under normal conditions , 75%of filtered phosphate
is reabsorbed, only 30-40 mEq/day is available
for buffering H+.
That is why much of the buffering of excess H+ in
the tubular fluid in severe acidosis occurs through
ammonia buffer system.
Ammonia buffers H+ and form Ammonium
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new
2
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How is ammonia formed? Where?
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Ammonia buffer system:
Ammonia
(NH3)/ammonium (NH4+)
Synthesized from
glutamine which is actively
transported into the cells
of:
PCT
Thick ascending limb of
Henle
Distal tubules.
Ammonia is a product of
glutamine metabolism.
Glutamine is actively transported
into renal cells.
Once inside, each molecule is
metabolized to give two NH3
and two HCO3NH3 is secreted into tubular
lumen.
HCO3+ moves across the
basolateral membrane. This is a
new bicarbonate added to the
blood.
Ammonia formation
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