Acid-Base Balance
AnS 536
Spring 2015
•The properties of water are essential
to life
•The properties of water are based on
its polar covalent structure and its
ability to form H-bonds with itself
and other molecules...
d+
d+
d-
Water as an Electric Dipole
Structure of Liquid Water (H2O)280
Its polar covalent structure makes
water a good solvent...
•for large molecules like proteins whose
surfaces are charged
•for other molecules with polar covalent
bonds
•for ionic compounds
Dissolving macromolecules (e.g., proteins):
Water of hydration
Dissolving molecules with polar
covalent bonds:
d+
d+
N
d-
d+
NH3
Dissolving ionic compounds:
Ionic solids dissolve readily in water
d+
dNa+
d-
d-
d+
Cld+
d+
The incomplete ionization of water:
H+
H
H
O
O
H
+
H
hydronium
ion
OH hydroxide
ion
or,
HOH
H+
proton
+
OHhydroxide
ion
The concentration of H+ ions (protons)
in a solution is measured by its pH
In pure water:
[H+] = [H3O+] = [OH-] = 10-7M
NOTE: a 1 M solution contains 1 mole of a substance dissolved in 1 liter of water; a
mole of a substance is its molecular mass in grams
pH = -log[H+] = -log10-7 = 7.0
A pH of 7.0 is defined as neutral
10-7M = 10-7 g/liter
Electrolytes
• Anions and cations distributed throughout the
fluid compartments
– Maintain electrical neutrality (anions MUST EQUAL
cations)
•
•
•
•
Cations: Na, K, Ca, Mg
Anions: Cl, HCO3, S04, proteins, lactic acid
Critical to maintenance of acid/base balance
Influence water retention and water dissociation
(favoring either H+ or OH-)
*electrolytes listed in red are most critical to consider in
diet (dietary electrolyte balance)
Na/K ATPase Pump
Lehninger, 1993
Acid–Base Balance
• Anion-cation balance regulates acid-base
balance
– Cations: Ca2+, Mg2+, Na+, K+
• Alkalosis or basic (increased OH–, increased pH)
– Anions: Cl–, SO42–, proteins, lactic acid (toxic)
• Acidosis or acidic (increased H+, lowered pH)
Stewart (1981)
• Concept of electrolytes as critical
factors in acid/base balance
• Strong ion difference (SID)
– sum of all strong cations minus sum of
all strong anions (NA, K, CL, SO42-)
– anions greater = negative SID = H+ > OH– cations greater = positive SID = OH- > H+
Stewart (1981)
• Balance of SID is maintained by the
dissociation and reassociation of water
The incomplete ionization of water:
H+
H
H
O
O
H
+
H
hydronium
ion
OH hydroxide
ion
or,
HOH
H+
proton
+
OHhydroxide
ion
Dissociation of Salt in Water
Acids and bases ionize in water:
acid
base
HCl
NaOH
Cl-
H+
Dissolved in
positively charged
water (H+), thus
lowering pH
OH-
Na+
Dissolved in negatively
charged water (OH-),
thus raising pH
Dissociation of Electrolytes
Peter Stewart’s Theories of
Balance
Acid-Base
• Based upon three variables that contribute to hydrogen
ion concentration [H+]
– Strong ion difference
– Total weak acids
– Partial pressure of carbon dioxide
• Theory was developed to determine renal contribution
to acid-base homeostasis based upon strong ions
regulated by the kidney
– K+, Na+, Cl-
• Equation specific to kidney’s contribution to
homeostasis
– Kidney does not regulate CO2 or weak acids
H+ = Dependent Variable
• Three independent variables determine the
value of H+:
– SID
– Pco2
• H increases as Pco2 increases
– CO2 acts as an acid
– Total concentration of weak acids (plasma
proteins)
• H increases as weak acids increase
Control of Acid/Base Balance
• Short-term (rapid) control
– Lungs
• During acidosis, more carbon dioxide exhaled, affects bicarbonate
concentrations (an anion)
• Decrease bicarbonate, decrease H+, increase pH
CO2 + H2O  HCO3– + H+  H2CO3
• Chronic (long-term) control
– GI tract – altered absorption of anions and cations
– Kidneys – altered excretion/resorption of anions and
cations
Newborn Acid-Base Balance
• Respiratory component
– Mismatch between CO2
production (tissue decreasing) and excretion
(lung - increasing)
– Carbonic anhydrase activity
increases postnatally
• Bicarbonate increases while
carbon dioxide decreases
– In acidotic neonates,
bicarbonate significantly
lower than unstressed
newborn because decreased
dissociation of carbonic acid
to bicarbonate
• Metabolic component
– Lactate is high (above 10
mmol/L in stressed newborns)
• Gluconeogenesis from lactate
does not occur prenatally;
enzymes in liver triggered
postnatally by increased oxyegn
tension
– Ig uptake in domestic species
slow resolution of acidosis
(partial negative charge)
• Plasma expansion also occurs
– SID decreases initially (1st
hour) and then slowly
increases through first day
Altering Acid Base Balance
• DCAD diets
• Sodium bicarbonate administration
– IV vs GI
– effect of other sodium forms
Dietary Electrolyte Balance
• Dietary electrolyte balance (dEB)
– Na+ + K+ – Cl–
• Diet electrolyte balance can be used to
affect acid-base balance in body
– Acidic conditions increase affinity for receptors
to bind PTH
Dairy rations for dry cows
are difficult to make acidic,
because alfalfa is often used
(high in potassium (a cation)
Weight or Equivalents…?
• Dietary electrolyte balance (dEB) is expressed in
equivalents, why not weight or percent of diet?
– Eq = Molecular weight  valence
Element
Na
Mg
K
Ca
Cl
MW
g/mol
22.99
24.31
39.10
40.08
35.45
Valence
+1
+2
+1
+2
–1
Weight equivalents
mmol/g
43.50
41.14
25.58
24.95
28.20
mEq/g
43.50
82.27
25.58
49.90
28.20
Classical Approaches to Renal Acid-Base
Balance
• Metabolism produces [H+] bi-products
– Hydrogen ions consume equal amounts of bicarbonate
buffer
– [H+] uptake by tubule epithelial cells
– Kidney traps [H+] with ammonia to form ammonium
(excreted as the salt ammonium chloride)
• Kidney is the only organ that can restore bicarbonate
buffer
• Acid-base balance
– Pulmonary component
• Regulates amount of CO2 excretion
– Renal system
• Corrects acid-base imbalances
Classical Approaches to Renal Acid-Base
Balance
• Classical approach
– Evaluates overall contribution to acid and base
concentrations
– Does not isolate specific components of hydrogen ions
– Not compatible with Stewart’s definition
• Neonates
– Ammoniagenesis decreased
– Urinary phosphate best reflects titratable acidity
– Oral ammonium chloride loads excreted more slowly than
adults