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Physiology
Blood Buffer System
Behrouz Mahmoudi
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Terms
• Acid
– Any substance that can yield a hydrogen ion (H+) or
hydronium ion when dissolved in water
– Release of proton or H+
• Base
– Substance that can yield hydroxyl ions (OH-)
– Accept protons or H+
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Terms
• pK/ pKa
– Negative log of the ionization constant of an acid
– Strong acids would have a pK <3
– Strong base would have a pK >9
• pH
– Negative log of the hydrogen ion concentration
– pH= pK + log([base]/[acid])
– Represents the hydrogen concentration
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Terms
• Buffer
– Combination of a weak acid and /or a weak base
and its salt
– What does it do?
• Resists changes in pH
– Effectiveness depends on
• pK of buffering system
• pH of environment in which it is placed
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Terms
• Acidosis
– pH less than 7.35
• Alkalosis
– pH greater than 7.45
• Note: Normal pH is 7.35-7.45
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Acid-Base Balance
• Function
– Maintains pH homeostasis
– Maintenance of H+ concentration
• Potential Problems of Acid-Base balance
– Increased H+ concentration yields decreased pH
– Decreased H+ concentration yields increased pH
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Regulation of pH
• Direct relation of the production and retention of acids and bases
• Systems
– Respiratory Center and Lungs
– Kidneys
– Buffers
• Found in all body fluids
• Weak acids good buffers since they can tilt a reaction in the other
direction
• Strong acids are poor buffers because they make the system more
acid
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Blood Buffer Systems
• Why do we need them?
– If the acids produced in the body from the
catabolism of food and other cellular processes
are not removed or buffered, the body’s pH would
drop
– Significant drops in pH interferes with cell enzyme
systems.
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Blood Buffer Systems
• Four Major Buffer Systems
– Protein Buffer systems
• Amino acids
• Hemoglobin Buffer system
– Phosphate Buffer system
– Bicarbonate-carbonic acid Buffer system
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Blood Buffer Systems
• Protein Buffer System
– Originates from amino acids
• ALBUMIN- primary protein due to high concentration in
plasma
– Buffer both hydrogen ions and carbon dioxide
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Blood Buffering Systems
• Hemoglobin Buffer System
– Roles
• Binds CO2
• Binds and transports hydrogen and oxygen
• Participates in the chloride shift
• Maintains blood pH as hemoglobin changes
from oxyhemoglobin to deoxyhemoglobin
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Oxygen Dissociation Curve
Curve B: Normal
curve
Curve A: Increased
affinity for hgb, so
oxygen keep close
Curve C: Decreased
affinity for hgb, so
oxygen released to
tissues
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Bohr Effect
• It all about
oxygen
affinity!
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Blood Buffer Systems
• Phosphate Buffer System
• Has a major role in the elimination of H+ via the
kidney
• Assists in the exchange of sodium for hydrogen
• It participates in the following reaction
• HPO-24 + H+
H2PO – 4
• Essential within the erythrocytes
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Blood Buffer Systems
• Bicarbonate/carbonic acid buffer system
– Function almost instantaneously
– Cells that are utilizing O2, produce CO2, which builds up.
Thus, more CO2 is found in the tissue cells than in nearby
blood cells. This results in a pressure (pCO2).
– Diffusion occurs, the CO2 leaves the tissue through the
interstitial fluid into the capillary blood
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Bicarbonate/Carbonic Acid Buffer
Carbonic
acid
Excreted
by lungs
Conjugate
base
Bicarbonate
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Excreted in
urine
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Bicarbonate/carbonic acid buffer system
• How is CO2 transported?
– 5-8% transported in dissolved form
– A small amount of the CO2 combines directly with the
hemoglobin to form carbaminohemoglobin
– 92-95% of CO2 will enter the RBC, and under the
following reaction
• CO2 + H20
H+ + HCO3-
– Once bicarbonate formed, exchanged for chloride
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Henderson-Hasselbalch Equation
• Relationship between pH and the
bicarbonate-carbonic acid buffer system in
plasma
• Allows us to calculate pH
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Henderson-Hasselbalch Equation
 General Equation
 pH = pK + log A-
HA
 Bicarbonate/Carbonic Acid system
o
pH= pK + log HCO3
H2CO3 ( PCO2 x 0.0301)
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Henderson-Hasselbalch Equation
1.
2.
pH= pK+ log H
HA
The pCO2 and the HCO3 are read or derived from the blood gas analyzer
pCO2= 40 mmHg
HCO3-= 24 mEq/L
3.
Convert the pCO2 to make the units the same
pCO2= 40 mmHg * 0.03= 1.2 mEq/L
3.
4.
Lets determine the pH:
Plug in pK of 6.1
5.
Put the data in the formula
pH = pK + log 24 mEq/L
1.2 mEq/L
pH = pK + log 20
pH= pK+ 1.30
pH= 6.1+1.30
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pH= 7.40
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The Ratio….
Normal is : 20 = Kidney
1
Lungs
= metabolic
respiratory
 The ratio of HCO3- (salt) to H2CO3 ( acid) is normally 20:1
 Allows blood pH of 7.40
The pH falls (acidosis) as bicarbonate decreases in relation
to carbonic acid
The pH rises (alkalosis) as bicarbonate increases in relation
to carbonic acid
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Physiologic Buffer Systems
• Lungs/respiratory
– Quickest way to respond, takes minutes to hours
to correct pH
– Eliminate volatile respiratory acids
such as
CO2
– Doesn’t affect fixed acids like lactic acid
– Body pH can be adjusted by changing rate and
depth of breathing “blowing off”
– Provide O2 to cells and remove CO2
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Physiologic Buffer Systems
• Kidney/Metabolic
–
–
–
–
Can eliminate large amounts of acid
Can excrete base as well
Can take several hours to days to correct pH
Most effective regulator of pH
– If kidney fails, pH balance fails
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