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+
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
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
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
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
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.
Blood Buffer Systems
Four Major Buffer Systems Protein Buffer systems
Amino acids Hemoglobin Buffer system
Phosphate Buffer system Bicarbonate-carbonic acid Buffer system
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
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
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
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 –
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• Essential within the erythrocytes
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
Bicarbonate/Carbonic Acid Buffer
Carbonic acid
Bicarbonate
Conjugate base
Excreted in urine
Excreted by lungs
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
Henderson-Hasselbalch Equation
Relationship between pH and the bicarbonate-carbonic acid buffer system in plasma
Allows us to calculate pH
Henderson-Hasselbalch Equation
General Equation
pH = pK + log A-
HA
Bicarbonate/Carbonic Acid system
o pH= pK + log HCO3
H2CO3 ( PCO2 x 0.0301)
Henderson-Hasselbalch Equation
1. pH= pK+ log H
HA
2. 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/L
pH = pK + log 20
pH= pK+ 1.30
pH= 6.1+1.30
pH= 7.40
The Ratio….
Normal is : 20 = Bicarbonate = Kidney = metabolic 1 carbonic acid Lungs respiratory
The ratio of HCO3- (salt/bicarbonate) to H2CO3
(acid/carbonic 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
Physiologic Buffer Systems
Lungs/respiratory Quickest way to respond, takes minutes
to hours to correct pH by adjusting carbonic acid
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
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
References
Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry: Techniques, principles, Correlations. Baltimore: Wolters Kluwer Lippincott Williams & Wilkins.
Carreiro-Lewandowski, E. (2008). Blood Gas Analysis and Interpretation. Denver, Colorado: Colorado Association for Continuing Medical Laboratory Education, Inc.
Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson .
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