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Acid-Base Balance AnS 536 Spring 2014. The properties of water are essential to life The properties of water are based on its polar covalent structure.

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Presentation on theme: "Acid-Base Balance AnS 536 Spring 2014. The properties of water are essential to life The properties of water are based on its polar covalent structure."— 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... --  

3 Water as an Electric Dipole

4 Structure of Liquid Water (H 2 O) 280

5 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

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

7 Dissolving molecules with polar covalent bonds: NH 3 N --   

8 Na + -- -- -- ++ ++ ++ ++ Cl - Ionic solids dissolve readily in water Dissolving ionic compounds:

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

10 The concentration of H + ions (protons) in a solution is measured by its pH [H + ] = [H 3 O + ] = [OH - ] = M In pure water: pH = -log[H + ] = -log10 -7 = 7.0 A pH of 7.0 is defined as neutral M = 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

11 Electrolytes Anions and cations distributed throughout the fluid compartments – Maintain electrical neutrality (anions MUST EQUAL cations) Cations: Na, K, Ca, Mg Anions: Cl, HCO 3, S0 4, 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

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14 Acid–Base Balance Anion-cation balance regulates acid-base balance – Cations: Ca 2+, Mg 2+, Na +, K + Alkalosis or basic (increased OH –, increased pH) – Anions: Cl –, SO 4 2–, 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, SO 4 2- ) – 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 H O H H O H H H + + OH - hydronium ion hydroxide ion or, HOH H + OH +- proton hydroxide ion

18 Dissociation of Salt in Water

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

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 CO 2 or weak acids

22 H + = Dependent Variable Three independent variables determine the value of H + : – SID – P co2 H increases as P co2 increases – CO 2 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 CO 2 + H 2 O  HCO 3 – + H +  H 2 CO 3

24 Newborn Acid-Base Balance Respiratory component – Mismatch between CO 2 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 (1 st 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 g/molmmol/gmEq/g Na Mg K Ca Cl35.45– ElementMWValenceWeight equivalents Weight or Equivalents…? Dietary electrolyte balance (dEB) is expressed in equivalents, why not weight or percent of diet? – Eq = Molecular weight  valence

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 CO 2 excretion – Renal system Corrects acid-base imbalances

29 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

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