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NORTH BIOL& 242 - Chapter 26 Balance Part 2. Acid/Base Balance

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1Chapter 26BalancePart 2. Acid/Base BalanceAcid–Base Balance• Precisely balances production and loss of hydrogen ions (pH)• The body generates acids during normal metabolism, tends to reduce pH•Kidneys:– Secrete hydrogen ions into urine– Generate buffers that enter bloodstream in distal segments of distal convoluted tubule (DCT) and collecting system• Lungs: affect pH balance through elimination of carbon dioxideAcid–Base Balance•pHof body fluids is altered by the introduction of acids or bases• Acids and bases may be strong or weak• Strong acids dissociate completely (only HCl is relevant physiologically)• Weak acids do not dissociate completely and thus affect the pH less (e.g. carbonic acid)pH Imbalances• Acidosis: physiological state resulting from abnormally low plasma pH • Alkalosis: physiological state resulting from abnormally high plasma pH • Both are dangerous but acidosis is more common because normal cellular activities generate acids• Why is pH so important?Carbonic Acid• Carbon Dioxide in solution in peripheral tissues interacts with water to form carbonic acid• Carbonic Anhydrase (CA) catalyzes dissociation of carbonic acid into H+ and HCO3-• Found in:– cytoplasm of red blood cells– liver and kidney cells– parietal cells of stomach– many other cellsCO2and pH•Most CO2in solution converts to carbonic acid and most carbonic acid dissociates (but not all because it is a weak acid)•PCO2is the most important factor affecting pH in body tissues•PCO2and pH are inversely related• Loss of CO2at the lungs increases blood pH2Hydrogen Ions (H+) •Are gained:– at digestive tract– through cellular metabolic activities• Are eliminated:– at kidneys and in urine– at lungs (as CO2 + H2O)– must be neutralized in blood and urine to avoid tissue damage• Acids produced in normal metabolic activity are temporarily neutralized by buffers in body fluidsBuffers• Dissolved compounds that stabilize pH by providing or removing H+• Weak acids or weak bases that absorb or release H+ are buffersBuffer Systems• Buffer System: consists of a combination of a weak acid and the anion released by its dissociation (its conjugate base)• The anion functions as a weak base:H2CO3(acid) ↔ H++ HCO3-(base)• In solution, molecules of weak acid exist in equilibrium with its dissociation products (meaning you have all three around in plasma)Buffer Systems in Body FluidsFigure 27–73 Major Buffer Systems1. Protein buffer systems:– help regulate pH in ECF and ICF– interact extensively with other buffer systems2. Carbonic acid–bicarbonate buffer system:– most important in ECF3. Phosphate buffer system:– buffers pH of ICF and urineProtein Buffer Systems• Depend on free and terminal amino acids• Respond to pH changes by accepting or releasing H+• If pH rises:– carboxyl group of amino acid dissociates, acting as weak acid, releasing a hydrogen ion• If pH drops:– carboxylate ion and amino group act as weak bases– accept H+– form carboxyl group and amino ion • Proteins that contribute to buffering capabilities:– plasma proteins – proteins in interstitial fluid – proteins in ICF3Amino Acids in Protein Buffer SystemsFigure 27–8The Hemoglobin Buffer System•CO2diffuses across RBC membrane:– no transport mechanism required• As carbonic acid dissociates:– bicarbonate ions diffuse into plasma– in exchange for chloride ions (chloride shift)• Hydrogen ions are buffered by hemoglobin molecules• the only intracellular buffer system with an immediate effect on ECF pH • Helps prevent major changes in pH when plasma PCO2is rising or fallingThe Carbonic Acid–Bicarbonate Buffer System• Formed by carbonic acid and its dissociation products • Prevents changes in pH caused by organic acids and fixed acids in ECF• H+ generated by acid production combines with bicarbonate in the plasma• This forms carbonic acid, which dissociates into CO2 which is breathed outThe Carbonic Acid–Bicarbonate Buffer SystemFigure 27–9Limitations of the Carbonic Acid Buffer System 1. Cannot protect ECF from changes in pH that result from elevated or depressed levels of CO2 (because CO2 is part of it)2. Functions only when respiratory system and respiratory control centers are working normally3. Ability to buffer acids is limited by availability of bicarbonate ionsThe Phosphate Buffer System• Consists of anion H2PO4—(a weak acid)• Works like the carbonic acid–bicarbonate buffer system• Is important in buffering pH of ICF4Problems with Buffer Systems • Provide only temporary solution to acid–base imbalance• Do not eliminate H+ions • Supply of buffer molecules is limitedMaintenance of Acid–Base Balance• Requires balancing H+gains and losses• For homeostasis to be preserved, captured H+must either be:– permanently tied up in water molecules through CO2removal at lungs OR– removed from body fluids through secretion at kidney• Thus, problems with either of these organs cause problems with acid/base balance• Coordinates actions of buffer systems with:– respiratory mechanisms– renal mechanismsRespiratory Compensation • Is a change in respiratory rate that helps stabilize pH of ECF • Occurs whenever body pH moves outside normal limits• Directly affects carbonic acid–bicarbonate buffer systemRespiratory CompensationH2CO3(acid) ↔ H++ HCO3-(base)• Increasing or decreasing the rate of respiration alters pH by lowering or raising the PCO2– When PCO2rises, pH falls as addition of CO2drives buffer system to the right (adding H+)– When PCO2falls, pH rises as removal of CO2drives buffer system to the left (removing H+)Renal MechanismsSupport buffer systems by:1. secreting or absorbing H+or HCO3-2. controlling excretion of acids and bases3. generating additional buffersRenal Compensation• Is a change in rates of H+and HCO3—secretion or reabsorption by kidneys in response to changes in plasma pH• Kidneys assist lungs by eliminating any CO2that enters renal tubules during filtration or that diffuses into tubular fluid en route to renal pelvis• Hydrogen ions are secreted into tubular fluid along:– proximal convoluted tubule (PCT)– distal convoluted tubule (DCT)– collecting system5Buffers in Urine• The ability to eliminate large numbers of H+in a normal volume of urine depends on the presence of buffers in urine (without them, we’d need to dilute the H+ with like 1000x


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NORTH BIOL& 242 - Chapter 26 Balance Part 2. Acid/Base Balance

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