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Acid-Base Balance AnS 536 Spring 2014.

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Presentation on theme: "Acid-Base Balance AnS 536 Spring 2014."— Presentation transcript:

1 Acid-Base Balance AnS 536 Spring 2014

2 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+

3 Water as an Electric Dipole

4 Structure of Liquid Water (H2O)280

5 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

6 Dissolving macromolecules (e.g., proteins):
Water of hydration

7 Dissolving molecules with polar covalent bonds:
NH3 d- Figure: 2.15ab Caption: (a) Because of oxygen’s high electronegativity, the electrons that are shared when hydrogen and oxygen form a covalent bond are pulled toward the oxygen nucleus. The electrons spend more time close to the oxygen nucleus, so the oxygen atom has a slight negative charge and the hydrogen atom a partial positive charge. (b) The electrical attraction that occurs between the partial positive and negative charges on water molecules forms a hydrogen bond.  Exercise Label the hydrogen bond in part (b).

8 Dissolving ionic compounds:
Ionic solids dissolve readily in water d+ d- d- d+ Na+ Cl- d+ d- d+ Figure: 2.18d Caption: (d) Ionic solids dissolve readily in water. Why?  Exercise Ionic bonding can be thought of in terms of reduction and oxidation.

9 The incomplete ionization of water:
+ O H - O H + OH hydroxide ion hydronium ion or, HOH H OH + - proton hydroxide ion

10 A pH of 7.0 is defined as neutral
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

11 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)

12 Na/K ATPase Pump Lehninger, 1993


14 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)

15 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+

16 Stewart (1981) Balance of SID is maintained by the dissociation and reassociation of water

17 The incomplete ionization of water:
+ O H - O H + OH hydroxide ion hydronium ion or, HOH H OH + - proton hydroxide ion

18 Dissociation of Salt in Water

19 Acids and bases ionize in water:
HCl NaOH Cl- Na+ H+ OH- Dissolved in positively charged water (H+), thus lowering pH Dissolved in negatively charged water (OH-), thus raising pH

20 Dissociation of Electrolytes

21 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

22 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

23 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

24 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

25 Altering Acid Base Balance
DCAD diets Sodium bicarbonate administration IV vs GI effect of other sodium forms

26 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)

27 Weight or Equivalents…?
Dietary electrolyte balance (dEB) is expressed in equivalents, why not weight or percent of diet? Eq = Molecular weight  valence g/mol mmol/g mEq/g Na Mg K Ca Cl – Element MW Valence Weight equivalents

28 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

29 Classical Approaches to Renal Acid-Base Balance
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


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