Lecture # 5Second class/ 2015
Acid-Base Balance
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
Terms
Acidosis pH less than 7.35
Alkalosis pH greater than 7.45
Note: Normal pH is 7.35-7.45
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 andbases
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 cellularprocesses are not removed or buffered,the body’s pH would drop
Significant drops in pH interferes with cellenzyme 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 highconcentration in plasma
Buffer both hydrogen ions and carbondioxide
Blood Buffering Systems
Hemoglobin Buffer System Roles Binds CO2
Binds and transports hydrogen andoxygen
Participates in the chloride shift Maintains blood pH as hemoglobin
changes from oxyhemoglobin todeoxyhemoglobin
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 buffersystem Function almost instantaneously Cells that are utilizing O2, produce CO2, which
builds up. Thus, more CO2 is found in the tissuecells than in nearby blood cells. This results in apressure (pCO2).
Diffusion occurs, the CO2 leaves the tissuethrough the interstitial fluid into the capillaryblood
Bicarbonate/Carbonic Acid Buffer
Carbonicacid
Bicarbonate
Conjugatebase
Excreted inurine
Excretedby 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 formcarbaminohemoglobin
92-95% of CO2 will enter the RBC, and underthe following reaction CO2 + H20 H+ + HCO3
-
Once bicarbonate formed, exchanged forchloride
Henderson-Hasselbalch Equation
Relationship between pH and thebicarbonate-carbonic acid buffersystem 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 HCO3H2CO3 ( PCO2 x 0.0301)
Henderson-Hasselbalch Equation
1. pH= pK+ log HHA
2. The pCO2 and the HCO3 are read or derived from the blood gas analyzerpCO2= 40 mmHgHCO3
-= 24 mEq/L
3. Convert the pCO2 to make the units the samepCO2= 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 formulapH = pK + log 24 mEq/L
1.2 mEq/LpH = pK + log 20pH= pK+ 1.30pH= 6.1+1.30pH= 7.40
The Ratio….
Normal is : 20 = Kidney = metabolic1 Lungs 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
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
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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Acid-Base Balance• Electrolytes that ionize in water and release
hydrogen ions are acids; those that combine withhydrogen ions are bases.
• Sources of Hydrogen Ions– Most hydrogen ions originate as by-products of
metabolic processes, including: the aerobic andanaerobic respiration of glucose, incomplete oxidationof fatty acids, oxidation of amino acids containingsulfur, and the breakdown of phosphoproteins andnucleic acids.
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Acid-Base Imbalances• Acidosis
– Two major types of acidosis are respiratory andmetabolic acidosis.
• Respiratory acidosis results from an increase of carbonicacid caused by respiratory center injury, air passageobstructions, or problems with gas exchange.
• Metabolic acidosis is due to either an accumulation of acidsor a loss of bases and has many causes including kidneydisease, vomiting, diarrhea, and diabetes mellitus.
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Acid-Base Imbalances– Increasing respiratory rate or the amount of hydrogen
ions released by the kidney can help compensate foracidosis.
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Acid-Base Imbalances• Alkalosis
– Alkalosis also has respiratory and metabolic causes.• Respiratory alkalosis results from hyperventilation causing
an excessive loss of carbon dioxide.• Metabolic alkalosis is caused by a great loss of hydrogen
ions or a gain in base perhaps from vomiting or use ofdrugs.
The Bohr effect
The Bohr effect
Learning outcome: To describe and explain the effects of raised
carbon dioxide concentrations on thehaemoglobin dissociation curve.
To learn how carbon dioxide is transported inblood.
What determines the loading andunloading of oxygen by haemoglobin?The amount of oxygen thathaemoglobin carries is affectedby:
1) The partial pressure of oxygenand
2) The partial pressure of carbondioxide
The presence of a high partial pressure of carbon dioxidecauses haemoglobin to release oxygen.
This is called the Bohr effect
HighpC02
Haemo-globinreleasesoxygen
The Bohr effect1. During respiration, CO2 is produced.
This diffuses into the blood plasmaand into the red blood cells.
2. Inside the red blood cells are manymolecules of an enzyme calledcarbonic anhydrase *.
3. It catalyses the reaction betweenCO2 and H2O.
Red cell
plasma
CO2 H2CO3
HCO3- + H+.CO2 + H2O H2CO3
carbondioxide
water carbonicacid
4. The resulting carbonic acid thendissociates into HCO3
- + H+.(Both reactions are reversible).
*H2O
HCO3-
The Bohr effect (continued)
5. Haemoglobin very readily combines withhydrogen ions forming haemoglobinic acid.
6. As a consequence haemoglobin releasessome of the oxygen it is carrying.
7. By removing hydrogen ions from thesolution, haemoglobin helps to maintain thepH of the blood close to neutral. It is actingas a buffer.
The Bohr effect
Three Oxygen Dissociationcurves illustrating the BohrEffect.
Increased carbon dioxide inthe blood causes a right-shiftin the curves, such that thehaemoglobin more easilyunloads the oxygen it iscarrying.
Oxygen Dissociation Curve
Curve B: Normalcurve
Curve A: Increasedaffinity for hgb, sooxygen keep close
Curve C: Decreasedaffinity for hgb, sooxygen released totissues
Bohr Effect
It all aboutoxygenaffinity!
Why is the Bohr effect useful?
High concentrations of carbon dioxide arefound in actively respiring tissues, whichneed oxygen. Due to the Bohr effect, thesehigh carbon dioxide concentrations causehaemoglobin to release its oxygen even morereadily than it would do otherwise.
How is carbon dioxide transported?
Carbon dioxide is mostly carried ashydrogencarbonate ions in bloodplasma, but also in combination withhaemoglobin in red blood cells(carbamino-haemoglobin) anddissolved as carbon dioxide moleculesin blood plasma.
Carbon dioxide transport
About 5% of the CO2produced simply dissolves inthe blood plasma.
Some CO2 diffuses into the redblood cells but instead of formingcarbonic acid, attaches directly ontothe haemoglobin molecules to formcarbaminohaemoglobin.
Since the CO2 doesn’t bind to thehaem groups the Haemoglobin isstill able to pick up O2.
About 85% of the CO2 produced byrespiration diffuses into the red blood cellsand forms carbonic acid under the controlof carbonic anhydrase.
The carbonic acid dissociates to producehydrogencarbonate ions (HCO3
-)
The HCO3- diffuses out of the red bloodcell into the plasma
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 andInterpretation. Denver, Colorado: Colorado Association forContinuing Medical Laboratory Education, Inc.
Sunheimer, R., & Graves, L. (2010). Clinical LaboratoryChemistry. Upper Saddle River: Pearson .
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