Download Acid-Base Balance

Acid-Base Balance
Acid - Base Balance
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1.
2.
3.
Blood - normal pH of 7.2 – 7.45
< 7.2 = acidosis
> 7.45 =
alkalosis
3 buffer systems to maintain
normal blood pH
Buffers
Removal of CO2 by lungs
Removal of H+ ions by kidneys
The Negative logarithmic relationship between [H+] and pH
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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,
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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 = 7.4
Blood = 7.35 – 7.45
< 6.8 or > 8.0 death occurs
Acidosis (acidemia) below 7.35
Alkalosis (alkalemia) above 7.45
Small changes in pH can produce
major disturbances
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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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Acids take in with foods
Acids produced by metabolism of
lipids and proteins
Cellular metabolism produces CO2.
CO2 + H20 ↔ H2CO3 ↔ H+ +
HCO3-
Control of Acids
1.
Buffer systems
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
Bicarbonate buffer
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Sodium Bicarbonate (NaHCO3) and
carbonic acid (H2CO3)
Maintain a 20:1 ratio : HCO3- :
H2CO3
HCl + NaHCO3 ↔ H2CO3 + NaCl
NaOH + H2CO3 ↔ NaHCO3 + H2O
Phosphate buffer
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Major intracellular buffer
H+ + HPO42- ↔ H2PO4-
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OH- + H2PO4- ↔ H2O + H2PO42-
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Protein Buffers
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Includes hemoglobin, work in blood and ISF
Carboxyl group gives up H+
Amino Group accepts H+
Side chains that can buffer H+ are present on
27 amino acids.
2. 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
3. Kidney excretion
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Can eliminate large amounts of acid
Can also excrete base
Can conserve and produce bicarb
ions
Most effective regulator of pH
If kidneys fail, pH balance fails
Hydrogen Ion Excretion in Kidney
Buffering of hydrogen ions in urine
Factors that increase or decrease H secretion and HCO3
Reabsorption by renal tubules :
Increase in H ion
secretion and HCO3 ion
reabsorption
PCO2
H ,
Decrease H ion secretion
and HCO3 ion
reabsorption
PCO2
HCO3
H ,
HCO3
ECF volume
ECF volume
Angiotensin II
Angiotensin II
Aldosterone
Aldosterone
Hypokalemia
Hyperkalemia
Rates of correction
Buffers function almost
instantaneously
 Respiratory mechanisms take
several minutes to hours
 Renal mechanisms may take
several hours to days
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Buffers
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Protein Buffer Systems
Amino Acid buffers
Hemoglobin buffers
Plasma Protein buffers
Phosphate Buffer Systems
Carbonic Acid – Bicarbonate Buffer
System
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Buffer systems are used to keep the body in pH balance (homeostasis)
 It consists of a weak acid (H+)and its dissociation products (an anion)
 3 major buffer systems in human
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Protein buffer system (includes hemoglobin buffer system)
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Regulates ICF & ECF (both plasma & interstitial fluid)
Most important in ICF & hemoglobin
 Hemoglobin buffer system = carbonic anhydrase in RBC
* it absorbs CO2 from ECF & get immediate effect
Amino acids have carboxyl group (gives up H+) and
Amino acids have amino group(can accept H+)
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Carbonic acid-bicarbonate buffer system
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Important in ECF
Lots of carbon dioxide from metabolic acids
It mixes with water & get carbonic acid which dissociates into
H+ & HCO3Metabolic acids have H+ ; Our body has “bicarbonate reserve”
 Bicarbonate reserve = ample supply of bicarb in ECF
 These combine to form CO2 + H2O
 CO2 excreted via lungs
Think of CO2 as an acid since it readily combines with water to
become carbonic acid
Maintenance of Acid-Base Balance
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Respiratory System: removal of CO2 by
lungs – stabilizes the ECF, has direct
effect on Carbonic Acid – Bicarbonate
Buffer System
Urinary System: removal of H+ ions by
kidneys
Regulation of blood pH by the
respiratory system
Kidney excretion of H+
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Metabolic reactions produce nonvolatile acids
One way to eliminate this huge load is to excrete H+ in
urine
In the proximal convoluted tubule, Na+/H+ antiporters
secrete H+ as they reabsorb Na+
Intercalated cells of collecting duct include proton pumps
that secrete H+ into tubule fluid; reabsorb K+ and HCO3Urine can be up to 1000 times more acidic than blood
2 other buffers can combine with H+ in collecting duct
 HPO42- and NH3
Secretion of H+ by intercalated cells in the collecting
duct