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Acid-Base Balance
AnS 536Spring 2014
•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 other molecules with polar covalent bonds•for ionic compounds
•for large molecules like proteins whose surfaces are charged
Dissolving macromolecules (e.g., proteins):
Water of hydration
Dissolving molecules with polar covalent bonds:
NH3N
d-
d+d+
d+
Na+
d-
d-
d-
d+ d+
d+
d+
Cl-
Ionic solids dissolve readily in water
Dissolving ionic compounds:
The incomplete ionization of water:
OH
HO
H
H
OH
H
H++ OH -
hydroniumion
hydroxide ion
or,
HOH H + OH+ -proton hydroxide
ion
The concentration of H+ ions (protons) in a solution is measured by its pH
[H+] = [H3O+] = [OH-] = 10-7MIn pure water:
pH = -log[H+] = -log10-7 = 7.0
A pH of 7.0 is defined as neutral
10-7M = 10-7 g/liter
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
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, SO4
2-)– 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:
OH
HO
H
H
OH
H
H++ OH -
hydroniumion
hydroxide ion
or,
HOH H + OH+ -proton hydroxide
ion
Dissociation of Salt in Water
Acids and bases ionize in water:
acid
HCl
Cl-
Dissolved inpositively chargedwater (H+), thus
lowering pH
base
Na+
Dissolved in negatively charged water (OH-),
thus raising pH
NaOH
OH-H+
Dissociation of Electrolytes
Peter Stewart’s Theories of Acid-Base Balance
• 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
• Chronic (long-term) control– GI tract – altered absorption of anions and cations– Kidneys – altered excretion/resorption of anions and
cations
CO2 + H2O HCO3– + H+ H2CO3
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)
g/mol mmol/g mEq/gNa 22.99 +1 43.50 43.50Mg 24.31 +2 41.14 82.27K 39.10 +1 25.58 25.58Ca 40.08 +2 24.95 49.90Cl 35.45 –1 28.20 28.20
Element MW Valence Weight equivalents
Weight or Equivalents…?
• Dietary electrolyte balance (dEB) is expressed in equivalents, why not weight or percent of diet?– Eq = Molecular weight valence
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
