1. MedChem 61 Buffer Systems of the Body Dr. M. Sasvári: Medical Chemistry Lectures 6.

2. MedChem 62  Extracellular fluid (Blood plasma + interstitial fluids)  Intracellular fluids  Extracellular fluid (Blood plasma + interstitial fluids)  Intracellular fluids The major body water components  pH of the blood-plasma pH = 7.4 7.38 ACIDOSIS 7.42 ALKALOSIS

3. MedChem 63 The most important buffer systems in our body  bicarbonate/carbonic acid (extracellular)   bicarbonate/carbonic acid (extracellular)   the Histidine side chain of proteins (Hemoglobin in RBC) phosphate buffer (intracellular)

4. MedChem 64 The phosphate buffer system: The most important intracellular buffer system  pK a =7.2 H 2 PO 4 - HPO 4 2- + H + Henderson-Hasselbalch equation: pH = 7.2 + log [HPO 4 2- ] [H 2 PO 4 - ]

5. MedChem 65 Forms of intracellular phosphates O O - H O - P - O - PiPi Inorganic phosphate (phosphoric acid) OO O - - O - P - O - P - O - PP i pyrophosphate O O - - P - O - -PO 3 2- phosphoryl (phosphate) group Phosphate group is present:  on ATP  on the intracellular sugars  and their catabolic products

6. MedChem 66 What is the average charge of P i at pH=6.9 ? pK values of phosphoric acid are: 2.1, 7.2 and 12.3. 6.9 – 7.2 = -0.3= + log (x) x =0.5 = 1/2 average charge: [1(-2)+2(-1)]/3=1.33 (-) 6.9=7.2 + log [HPO 4 2- ] [H 2 PO 4 - ] Average charge of phosphoric acid (Pi) [HPO 4 2- ] : [H 2 PO 4 - ] = 1 : 2

7. MedChem 67 The bicarbonate/carbonic acid system Carbonic acid CO 2 + H 2 O H 2 CO 3 H 2 CO 3 HCO 3 - + H + CO 2 HCO 3 - + H + carbondioxidebicarbonate Henderson-Hasselbalch equation: [HCO 3 - ] pH = pK + log [CO 2 ] a1

8. MedChem 68 pH = 6.1+ log 24 1.2 = 7.4 Normal values [CO 2 ] = 1.2 meqv/L [CO 2 ]=0.03 p CO 2 where p= partial pressure [HCO 3 - ]= 24 meqv/L Henderson-Hasselbalch equation: [HCO 3 - ] [CO 2 ] = 24 1.2 = 20 1 ?

9. MedChem 69 (HCO 3 - ) >>>> (CO 2 ) Acid /base capacity? H 2 O + CO 2 HCO 3 - + H + 24 meqv/L 1.2 meqv/L Acid capacity is larger than base capacity!

10. MedChem 610 Why is the bicarbonate buffer a super-buffer for the blood? H 2 O + CO 2 H 2 CO 3 HCO 3 - + H + 24 meqv 1.2 meqv H+H+ x meqv 24 - x 1.2 + x pH = 6.1+ log 24 – 12 1.2 + 12 = 6.06 ? e.g. x=12 meqv

11. MedChem 611 CO 2 concentration is stabilized by ventilation ACIDOSIS: Hyperventilation ALKALOSIS: Hypoventilation Effect of Ventilation H 2 CO 3 HCO 3 - + H + H 2 O + CO 2 24 meqv 1.2 meqv 24 - x pH = 6.1+ log 24 - 12 1.2 + 0 = 7.1 H + x meqv

12. MedChem 612 Stabilization of [HCO 3 - ] by the RBC CO 2 + H 2 OH 2 CO 3 H + + HCO 3 - HbH + Hb - HCO 3 - Cl - CO 2

13. MedChem 613 The pH-bicarbonate diagram ALKALOSIS (hypoventilation) ACIDOSIS (hyperventilation) Constant CO 2 concentration (normal ventilation) [CO 2 ]=1.2 meqv (p = 40 Hgmm) Clinical test: Assay of [bicarbonate] in the serum  pH from the graph Clinical test: Assay of [bicarbonate] in the serum  pH from the graph 24 meqv

14. MedChem 614 Sources of acids and bases Metabolic acidosis: normal metabolism:  lactic acid (exercise)  acetoacetic acid (starvation)  phosphoric acid (energy)  diet (~60 meqv acid / day) Metabolic acidosis: normal metabolism:  lactic acid (exercise)  acetoacetic acid (starvation)  phosphoric acid (energy)  diet (~60 meqv acid / day) HCO 3 - depletion (sever diarrhea) Metabolic alkalosis: normal metabolism: - diet: Na-salts of weak acids (fruits, vegetables) Metabolic alkalosis: normal metabolism: - diet: Na-salts of weak acids (fruits, vegetables) Abnormal loss of acids (prolonged vomiting)

15. MedChem 615 Final removal of acids: Kidney pH of the urine ~ 6 range: 4.4 8.0 pH of the urine ~ 6 range: 4.4 8.0 Extractable amount of acids: approximately 50 meqv / day (normal) Extractable amount of acids: approximately 50 meqv / day (normal) Calculation: The lowest pH of the urine is 4.4 50 meqv [H + ] excretion would need Forms of H + excretion HPO 4 2- + H + H 2 PO 4 2- pK a = 7.2 NH 3 + H + NH 4 + pK a = 9.3 Diabetic ketoacidosis (pH of the urine is ~4.4) acetoacetic acid pK=3.6  -OH-butyric acid pK=4.7 1250 L volume of urine ????????

16. MedChem 616 Total acidity of the urine Titrable acidity [H + ] (Titration back to pH = 7.4) NH 4 + content major urinary acid (could reach 500 meqv/day in acidosis) Final removal of basis in alkalosis: HCO 3 - secretion (kidney) Final removal of basis in alkalosis: HCO 3 - secretion (kidney)

17. MedChem 617 Role of the kidney BLOODKIDNEYURINE HCO 3 - Excretion of H + (NH 4 + ) Re-absorption of bicarbonate ion H+H+ H 2 CO 3 NH 3 Amino acids NH 4 +

18. MedChem 618 The super-buffer of the blood Ventilation Proteins (Hb) H 2 CO 3 H+H+ Kidney Final excretion of acids CO 2 + H 2 O HCO 3 - + H + Temporal pH stabilization by red blood cells

19. MedChem 619 + Hb -  HbH Problem with ventilation Respiratory acidosis H 2 O + CO 2 H 2 CO 3 HCO 3 - + H + Increase in HCO 3 - with slight decrease in pH Increase in HCO 3 - with slight decrease in pH

20. MedChem 620 SUMMARY Buffer systems of the body Important terms: Intracellular and extracellular buffer systems of the body, average charge of phosphoric acid at various pH, components of the bicarbonate buffer in the blood, role of the ventilation in pH stabilization role of the red blood cells in pH stabilization, role of the kidney in pH stabilization, metabolic acidosis, metabolic alkalosis, respiratory acidosis, total acidity of the urine