Acid-Base Balance : Basics


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Acid-Base Balance : Basics

  1. 1. Vittal - C.S.N.Vittal
  3. 3. Vittal H+ ion & pH SCALE • H+ ion conc. of plasma: 0.000 000 04 mol/L or 40 nmol/L • pH is the negative logarithm of hydrogen ion conc. Normal : 7.35 – 7.45
  4. 4. Vittal Acid Base Balance Introduction Metabolic processes continually produce acid and, to a lesser degree, base. H+ :  can attach to negatively charged proteins &  in high concentrations, alter their overall charge, configuration, and function.
  5. 5. Vittal Acid Base Balance Introduction To maintain cellular function, the body has elaborate mechanisms that maintain blood H+ concentration within a narrow range—   typically : 37 to 43 nmol/L (pH 7.35 to 7.45), & ideally : 40 nmol/L (pH = 7.4) Disturbances of these mechanisms can have serious clinical consequences.
  6. 6. Vittal Types of acids in the body Volatile acid – Can leave solution and enter the atmosphere (e.g. carbonic acid) – Produced by aerobic metabolism Fixed acids – Acids that do not leave solution (e.g. sulfuric and phosphoric acids) – Generated during catabolism of amino acids Organic acids – Participants in or by-products of aerobic and anaerobic metabolism – Metabolic byproducts such as lactic acid, ketone bodies
  7. 7. Vittal Acid-Base Physiology  Most acid comes from carbohydrate and fat metabolism (15,000 to 20,000 mmol of CO2 daily)  CO2 combines with water (H2O) in the blood to create carbonic acid (H2CO3), which in the presence of the enzyme carbonic anhydrase dissociates into H+ and HCO3−.  The H+ binds with Hb in the blood and is released with oxygenation in the alveoli, the above reaction is reversed, creating H2O and CO2, which is exhaled  Very little metabolic acid is produced - which is eliminated by kidney and liver.
  8. 8. Vittal Acid-Base Physiology  Most base comes from metabolism of anionic amino acids (glutamate and aspartate) and  from oxidation and consumption of organic anions such as lactate and citrate, which produce HCO3−
  9. 9. Vittal
  10. 10. Vittal pH pH : the negative logarithm of the hydrogen ion concentration o a "decrease" in pH means an "increase" in acidity. Standard pH: (Hasselbalch, 1916) the pH under standard conditions: o PCO2=40 mmHg, and 37oC, and saturated with oxygen Arterial pH = 7.4 Venous pH = 7.36
  11. 11. Vittal PaCO 2 PaCO 2 : the partial pressure of carbon dioxide. The normal value in arterial blood is 40 mm Hg (or 5.33 kPa) PaCO2 ∝ CO 2 production + inspired CO 2 Low PaCO2 reflects the rate of CO2 elimination Principal physiological cause of hypocapnia is hyperventilation Intentional, incidental (HFV, ECMO)
  12. 12. Vittal Bicarbonate HCO 3 - : concentration (in mEq/L) of the bicarbonate ion is not measured, it is calculated from the PCO2 and pH Standard Bicarbonate : (Jorgensen and Astrup, 1957) bicarbonate concentration under standard conditions: PCO2=40 mmHg, and 37oC, and saturated with oxygen. an excellent measurement of the metabolic component. = 21-27 mmol/l
  13. 13. Vittal Base Escess (Astrup and Siggard-Andersen, 1958) a better method of measuring the metabolic component. In essence the method calculated the quantity of Acid or Alkali required to return the plasma in-vitro to a normal pH under standard conditions.
  14. 14. Vittal Base Excess & Base Deficit (Astrup and Siggard-Andersen, 1958) Amount of strong acid or base that has to be added to a sample of blood to produce a pH of 7.4 under the specified conditions fro standard bicarbonate. a more accurate in assessing metabolic component of acid-base status. Normal Buffer Base = 48mMol/L (41.8 + 0.4 X Hb in g/dL)
  15. 15. Vittal Base Excess & Base Deficit Base excess – 3 mmol/l : means 3 mmol of strong acid had to be added to each litre of original sample to get a pH of 7.4 while kept at 370C and a PaCO2 of 40 mm Hg. Base deficit – 3 mmol/l : means 3 mmol of strong base had to be added to each litre of original sample to get a pH of 7.4 while kept at 370C and a PaCO2 of 40 mm Hg.
  16. 16. Vittal Base Excess & Base Deficit Normal A base excess below -2.0 mmol/l : Metabolic acidosis A base excess above +2.0 mmol/l : Metabolic alkalosis Range
  17. 17. Vittal Anion Gap the difference between major plasma cations and major plasma anions. Anion gap = ([Na+] +[K+]) - ([Cl--] +[HCO3-]) Gap = Na+ + K+ - Cl- - HCO3[ 15 = 140 + 5 - 105 - 25 mMol/L] Normal Anion Gap Children : 9mo. 19 yrs = 8 + 2 mMol /L Adults : 12 + 2 mMol /L
  18. 18. Vittal Metabolic Acidosis: Types “Normal Anion Gap”, “ Anion Gap” ≡ [Na+] - ([Cl-] + [HCO3-]) Alb- AlbHCO3- AlbHCO3A- HCO3- Na+ Na+ Cl- Na+ Cl- No Anion gap M acidosis Cl- High Anion gap M acidosis
  19. 19. Vittal ACID/BASE BALANCE AND THE BLOOD [OH -] [H+] Acidic Alkaline (Basic) Neutral pH 0 Venous Blood Acidosis 6.8 7 7.4 Normal 7.35-7.45 14 Arterial Blood Alkalosis 8.0
  20. 20. Vittal  Abnormal acid-base balance Acid-base imbalances can be defined as acidosis or alkalosis. Acidosis is a state of excess H+ Acidemia results when the blood pH is < 7.35  Alkalosis is a state of excess HCO3Alkalemia results when the blood pH is > 7.45 You can have acidosis without acidemia but You can not have acidemia without an acidosis!
  21. 21. Vittal Re gulation of ar terial pH Respir ator y Buf fer System Renal
  22. 22. Vittal Acid-Base Homeostasis Lungs Metabolism Input Output Maintenance of Normal [H+] Buffers Kidneys
  23. 23. Vittal CHEMICAL BUFFER SYSTEMS Unbuffered Salt Solution Na+ Add HCl ClCl- Protons taken up as Carbonic Acid H2CO3: HCO3- Buffer HCO3H+ H+ Na+ Cl- H2CO3 All protons are free Add HCl H2CO3 HCO3- + H+
  24. 24. Vittal Buffer
  25. 25. Vittal CHEMICAL BUFFER SYSTEMS Weak acid/salt systems act as a “sponge” for protons As acidity tends to increase they take protons up As acidity tends to decrease they release protons
  26. 26. Vittal CHEMICAL BUFFER SYSTEMS Extracellular Buffers : Carbonic acid/Bicarbonate: Primary buffer against non-carbonic acid changes Serum Proteins (albumin) Ammonia ( in renal tubules) Intracellular Buffers : Hemoglobin Intracellular proteins Phosphates
  27. 27. Vittal
  28. 28. Vittal Handerson Hasselbalch Equation pH = 6.1 + log pK HCO3PaCO2 X 0.0301
  29. 29. Vittal Kassirer and Bleich Equation (Handerson Equation) H + = 24 X pCO2 HCO3- With this formula, any 2 values (usually H+ and Pco2) can be used to calculate the other (usually HCO3 −).
  30. 30. Vittal Saturation of carbonic acid – bicarbonate buffer does not occur because carbonic acid is continuously breaking down into carbon dioxide and water.
  31. 31. Vittal Re gulation of ar terial pH •• Respiratory Respiratory •• Buffer System Buffer System •• Renal Renal Respiratory Control: •The power of the lungs to excrete large quantities of carbon dioxide enables them to compensate rapidly, i.e. metabolic acidosis and metabolic alkalosis normally elicit characteristic partial respiratory compensation almost immediately. • Not so efficient (50%) • Less in preterm babies • Control of respiratory centre • A CO2 conc. of > 9% depresses centers and causes CO2 narcosis
  32. 32. Vittal Re gulation of ar terial pH •• Respiratory Respiratory •• Buffer System Buffer System •• Renal Renal Buffer System: • Act within seconds • Act at cellular level • ¾ of body’s buffering system from intracellular proteins and phosphates.
  33. 33. Vittal Re gulation of ar terial pH •• Respiratory Respiratory •• Buffer System Buffer System •• Renal Renal Renal Control:  HCO3-  Reclamation of almost (80%) all the filtered HCO3- (5000 mEq) Substantial task: 180 L x 24 mmol/L = 4320 mmol bicarbonate filtered/day  Generation of new HCO3- with net secretion of H+ (energy dependant)  H+ (1 - 1.5 mmol/kg/day)  Increased excretion of acid as phosphate buffer and as ammonia  Na+ re-absorption during the formation of H+
  34. 34. Vittal Proximal Convoluted Tube
  35. 35. Vittal Convection
  36. 36. Vittal
  37. 37. Vittal Distal Convoluted Tube
  38. 38. Vittal Distal Convoluted Tube
  39. 39. Vittal Acid-Base in the G-I Tract CO2 + H + HCO 3
  40. 40. Vittal Acid-base and the Liver Dominant site of lactate metabolism Only site of urea synthesis
  41. 41. Vittal Severe Liver Failure NH4+ + oxo-glutarate ---X--> glutamine NH4+ + CO2 --X--> Urea + H+ • metabolic alkalosis • NH4+ toxicity
  42. 42. Vittal Response of body to increase in acid load Overview 1. Induces extra-cellular buffering by HCO32. Within minutes Respiratory Compensation with decrease in pCO2 and H2CO3 [to maintain a ratio of HCO3- : H2CO3 ] of 20 : 1 3. Intracellular buffering – in 1 to 4 hours 4. Renal acid excretion and production of new HCO3- formation : in hours to days
  43. 43. Vittal Acid-base disturbance  • Simple Disorder type Primary change in HCO3 - → Primary change in blood pCO2 → Respiratory disorder • Mixed Metabolic disorder
  44. 44. Vittal
  45. 45. Vittal
  46. 46. Vittal
  47. 47. Vittal
  48. 48. Vittal Abnormal acid-base balances Acid-base imbalance Respiratory acidosis Respiratory alkalosis Metabolic acidosis- Metabolic alkalosis- Plasma pH Primary disturbance Compensation
  49. 49. Vittal Abnormal acid-base balances Acid-base imbalance Plasma pH Primary disturbance Respiratory acidosis Low Increased pCO2 Increased renal net acid excretion with resulting increase in serum bicarbonate Respiratory alkalosis High Decreased pCO2 Decreased renal net acid excretion with resulting decrease in serum bicarbonate Metabolic acidosis- Low Metabolic alkalosis- High Decreased HCO3 Increased HCO3 - - Compensation Hyperventilation with resulting low pCO2 Hypoventilation with resulting increase in pCO2
  50. 50. Vittal Conclusions Acid Base Homeostasis is a Dynamic Process Buffers form the first line of Defence Bicarbonate buffers are by far the most important Lungs, Kidneys and Liver play important role in Acid Base Homeostasis
  51. 51. Vittal  Vittal