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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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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
Bicarbonate
Conjugate base
Excreted in urine
Excreted by lungs
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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. pH= pK+ log H HA2. 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. Lets determine the pH:4. Plug in pK of 6.1
5. Put the data in the formula pH = pK + log 24 mEq/L
1.2 mEq/LpH = pK + log 20pH= pK+ 1.30pH= 6.1+1.30pH= 7.40
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The Ratio….Normal is : 20 = Kidney = metabolic
1 Lungs respiratory
The ratio of HCO3- (salt) to H2CO3 ( acid) is normally 20:1
Allows blood pH of 7.40The pH falls (acidosis) as bicarbonate decreases in relation to
carbonic acidThe 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
