Acid-Base Balance
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Objectives
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Explain how the pH of the blood is stabilized by
bicarb buffer and define the terms acidosis and
alkalosis.
Explain how the acid-base balance of the blood is
affected by C02 and HC03-, and describe the roles of
the lungs and kidneys in maintaining acid-base
balance.
Explain how C02 affects blood pH, and
hypoventilation and hyperventilation affect acid-base
balance.
Explain how the interaction between plasma K+ and
H+ concentrations affects the tubular secretion of
these.
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pH
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pH of blood is 7.35 to 7.45
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pH = 6.1 + log
[HCO3-]
0.03 x Pco2
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Types of Acids in the Body
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Volatile acids:
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Can leave solution and enter the
atmosphere.
H2C03 (carbonic acid).
Pco2 is most important factor in pH of body
tissues.
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Types of Acids in the Body
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Fixed Acids:
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Acids that do not leave solution.
Sulfuric and phosphoric acid.
Catabolism of amino acids, nucleic acids,
and phospholipids.
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Types of Acids in the Body
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Organic Acids:
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Byproducts of aerobic metabolism, during
anaerobic metabolism and during
starvation, diabetes.
Lactic acid, ketones.
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Buffer Systems
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Provide or remove H+ and stabilize the
pH.
Include weak acids that can donate H+
and weak bases that can absorb H+.
Does NOT prevent a pH change.
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Chemical Buffers
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Act within fraction of a second.
Protein.
HCO3-.
Phosphate.
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Proteins
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COOH or NH2.
Largest pool of buffers in the body.
pk. close to plasma.
Albumin, globulins such as Hb.
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HCO3
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pk. = 6.1.
Present in large quantities.
Open system.
Respiratory and renal systems act on
this buffer system.
Most important ECF buffer.
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HCO3- Limitations
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Cannot protect ECF from respiratory
problems.
Cannot protect ECF from elevated or
decreased CO2.
Limited by availability of HCO3-.
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Phosphates
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pk. = 6.8.
Low [ ] in ECF, better buffer in ICF,
kidneys, and bone.
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Respiratory System
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2nd line of defense.
Acts within min. maximal in 12-24 hrs.
H2CO3 produced converted to CO2, and
excreted by the lungs.
Alveolar ventilation also increases as pH
decreases (rate and depth).
Coarse , CANNOT eliminate fixed acid.
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Urinary Buffers
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Nephron cannot produce a urine pH <
4.5.
IN order to excrete more H+, the acid
must be buffered.
H+ secreted into the urine tubule and
combines with HPO4-2 or NH3.
HPO4-2 + H+
H2PO4-2
NH3 + H+
NH4+
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Renal Acid-Base Regulation
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Kidneys help regulate blood pH by excreting
H+ and reabsorbing HC03-.
Most of the H+ secretion occurs across the
walls of the PCT in exchange for Na+.
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Antiport mechanism.
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Moves Na+ and H+ in opposite directions.
Normal urine normally is slightly acidic
because the kidneys reabsorb almost all
HC03- and excrete H+.
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Returns blood pH back to normal range.
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Reabsorption of HCO3
Apical membranes of tubule cells are
impermeable to HCO3-.
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When urine is acidic, HCO3- combines with H+
to form H2C03-, which is catalyzed by ca
located in the apical cell membrane of PCT.
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Reabsorption is indirect.
As [C02] increases in the filtrate, C02 diffuses into
tubule cell and forms H2C03.
H2C03 dissociates to HCO3- and H+.
HCO3- generated within tubule cell diffuses into
peritubular capillary.
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Acidification of Urine
Insert fig. 17.28
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Urinary Buffers
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Nephron cannot produce a urine pH
< 4.5.
In order to excrete more H+, the acid
must be buffered.
H+ secreted into the urine tubule and
combines with HPO4-2 or NH3.
HPO4-2 + H+
H2PO4NH3 + H+
NH4+
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Metabolic Acidosis
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Gain of fixed acid or loss of HCO3-.
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Plasma HCO3- decreases.
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PCO2 decreases.
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pH decreases.
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Metabolic Alkalosis
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Loss of fixed acid or gain of HCO3-.
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Plasma HCO3- increases.
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PCO2 increases.
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pH increases.
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Respiratory Acidosis
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PCO2 increases.
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Plasma HCO3- increases.
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pH decreases.
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Respiratory Alkalosis
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PCO2 decreases.
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Plasma HCO3- decreases.
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pH increases.
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Anion Gap
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The difference between [Na+] and the
sum of [HC03-] and [Cl-].
+
 [Na ] – ([HC03 ] + [Cl ]) =
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140 - (24 + 105) = 11
 Normal = 12 + 2
Clinicians use the anion gap to identify
the cause of metabolic acidosis.
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Anion Gap
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Law of electroneutrality:
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The major cation is Na+.
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Blood plasma contains an =
number of + and – charges.
Minor cations are K+, Ca2+ ,
Mg2+.
The major anions are HC03and Cl-.
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(Routinely measured.)
Minor anions include
albumin, phosphate, sulfate
(called unmeasured anions).
Organic acid anions include
lactate and acetoacetate,.
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Anion Gap
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In metabolic acidosis, the
strong acid releases protons
that are buffered primarily
by [HC03].
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This causes plasma [HC03-] to
decrease, shrinking the [HC03-]
on the ionogram.
Anions that remain from the
strong acid, are added to the
plasma.
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If lactic acid is added, the
[lactate] rises.
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Increasing the total [unmeasured
anions].
If HCL is added, the [Cl-] rises.
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Decreasing the [HC03-].
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Anion Gap in Metabolic Acidosis
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Salicylates raise the gap to 20.
Renal failure raises gap to 25.
Diabetic ketoacidosis raises the gap to 35-40.
Lactic acidosis raises the gap to > 35 (>50).
Largest gaps are caused by ketoacidosis and
lactic acidosis.