5 Its polar covalent structure makes water a good solvent... for large molecules like proteins whosesurfaces are chargedfor other molecules with polar covalentbondsfor ionic compounds
6 Dissolving macromolecules (e.g., proteins): Water of hydration
7 Dissolving molecules with polar covalent bonds: NH3d-Figure: 2.15abCaption:(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 waterd+d-d-d+Na+Cl-d+d-d+Figure: 2.18dCaption:(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: +OH-OH+OHhydroxideionhydroniumionor,HOHH OH+-protonhydroxideion
10 A pH of 7.0 is defined as neutral The concentration of H+ ions (protons) in a solution is measured by its pHIn pure water:[H+] = [H3O+] = [OH-] = 10-7MNOTE: 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 gramspH = -log[H+] = -log10-7 = 7.0A pH of 7.0 is defined as neutral10-7M = 10-7 g/liter
11 ElectrolytesAnions and cations distributed throughout the fluid compartmentsMaintain electrical neutrality (anions MUST EQUAL cations)Cations: Na, K, Ca, MgAnions: Cl, HCO3, S04, proteins, lactic acidCritical to maintenance of acid/base balanceInfluence water retention and water dissociation (favoring either H+ or OH-)*electrolytes listed in red are most critical to consider in diet (dietary electrolyte balance)
15 Stewart (1981)Concept of electrolytes as critical factors in acid/base balanceStrong 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: +OH-OH+OHhydroxideionhydroniumionor,HOHH OH+-protonhydroxideion
21 Peter Stewart’s Theories of Acid-Base Balance Based upon three variables that contribute to hydrogen ion concentration [H+]Strong ion differenceTotal weak acidsPartial pressure of carbon dioxideTheory was developed to determine renal contribution to acid-base homeostasis based upon strong ions regulated by the kidneyK+, Na+, Cl-Equation specific to kidney’s contribution to homeostasisKidney does not regulate CO2 or weak acids
22 H+ = Dependent Variable Three independent variables determine the value of H+:SIDPco2H increases as Pco2 increasesCO2 acts as an acidTotal concentration of weak acids (plasma proteins)H increases as weak acids increase
23 Control of Acid/Base Balance Short-term (rapid) controlLungsDuring acidosis, more carbon dioxide exhaled, affects bicarbonate concentrations (an anion)Decrease bicarbonate, decrease H+, increase pHChronic (long-term) controlGI tract – altered absorption of anions and cationsKidneys – altered excretion/resorption of anions and cationsCO2 + H2O HCO3– + H+ H2CO3
24 Newborn Acid-Base Balance Respiratory componentMismatch between CO2 production (tissue - decreasing) and excretion (lung - increasing)Carbonic anhydrase activity increases postnatallyBicarbonate increases while carbon dioxide decreasesIn acidotic neonates, bicarbonate significantly lower than unstressed newborn because decreased dissociation of carbonic acid to bicarbonateMetabolic componentLactate is high (above 10 mmol/L in stressed newborns)Gluconeogenesis from lactate does not occur prenatally; enzymes in liver triggered postnatally by increased oxyegn tensionIg uptake in domestic species slow resolution of acidosis (partial negative charge)Plasma expansion also occursSID decreases initially (1st hour) and then slowly increases through first day
25 Altering Acid Base Balance DCAD dietsSodium bicarbonate administrationIV vs GIeffect 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 bodyAcidic conditions increase affinity for receptors to bind PTHDairy 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 valenceg/mol mmol/g mEq/gNaMgKCaCl –Element MW Valence Weight equivalents
28 Classical Approaches to Renal Acid-Base Balance Metabolism produces [H+] bi-productsHydrogen ions consume equal amounts of bicarbonate buffer[H+] uptake by tubule epithelial cellsKidney traps [H+] with ammonia to form ammonium (excreted as the salt ammonium chloride)Kidney is the only organ that can restore bicarbonate bufferAcid-base balancePulmonary componentRegulates amount of CO2 excretionRenal systemCorrects acid-base imbalances
29 Classical Approaches to Renal Acid-Base Balance Evaluates overall contribution to acid and base concentrationsDoes not isolate specific components of hydrogen ionsNot compatible with Stewart’s definitionNeonatesAmmoniagenesis decreasedUrinary phosphate best reflects titratable acidityOral ammonium chloride loads excreted more slowly than adults
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