MedChem 6 1

Buffer Systems of the BodyBuffer Systems of the Body

Dr. M. Sasvári: Medical Chemistry LecturesDr. M. Sasvári: Medical Chemistry Lectures 6. 6.

MedChem 6 2

Extracellular fluid (Blood plasma + interstitial fluids)

Intracellular fluids

Extracellular fluid (Blood plasma + interstitial fluids)

Intracellular fluids

The major body water components The major body water components

pH of the blood-plasma pH of the blood-plasma

pH = 7.4pH = 7.47.38

ACIDOSIS

7.42

ALKALOSIS

MedChem 6 3

The most important buffer systems in our body 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)

MedChem 6 4

The phosphate buffer system: The most important intracellular buffer system

The phosphate buffer system: The most important intracellular buffer system

pKa=7.2H2PO4- HPO4

2- + H+

Henderson-Hasselbalch equation:Henderson-Hasselbalch equation:

pH = 7.2 + log [HPO4

2-]

[H2PO4-]

MedChem 6 5

Forms of intracellular phosphatesForms of intracellular phosphates

O

O -

H O - P - O - Pi

Inorganic phosphate(phosphoric acid)

O O

O - O -

- O - P - O - P - O - PPi

pyrophosphate

O

O -

- P - O - -PO32-

phosphoryl (phosphate) group

Phosphate group is present: on ATP on the intracellular sugars and their catabolic products

MedChem 6 6

What is the average charge of Pi 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 [HPO4

2-]

[H2PO4-]

Average charge of phosphoric acid (Pi)Average charge of phosphoric acid (Pi)

[HPO42-] : [H2PO4

-] = 1 : 2

MedChem 6 7

The bicarbonate/carbonic acid system The bicarbonate/carbonic acid system

Carbonic acidCarbonic acid

CO2 + H2O H2CO3

H2CO3 HCO3- + H+

CO2 HCO3- + H+

carbondioxide bicarbonate

Henderson-Hasselbalch equation:Henderson-Hasselbalch equation:

[HCO3-]

pH = pK + log [CO2]a1

MedChem 6 8

pH = 6.1+ log 24

1.2= 7.4

Normal values Normal values

[CO2] = 1.2 meqv/L [CO2]=0.03 pCO 2where p= partial pressure

[HCO3-]= 24 meqv/L

Henderson-Hasselbalch equation:Henderson-Hasselbalch equation:

[HCO3-]

[CO2]= 24

1.2= 20

1 ?

MedChem 6 9

(HCO3-) >>>> (CO2)

Acid /base capacity?

H2O + CO2 HCO3- + H+

24 meqv/L1.2 meqv/L

Acid capacity is larger than base capacity!

0

5

10

0 2 4 6 8 10

ml NaOH

pH

1:1

acid capacity B.C.more salt

a.c. base capacity

more acid

0

5

10

0 2 4 6 8 10

ml NaOH

pH

0

5

10

0 2 4 6 8 10

ml NaOH

pH

1:11:1

acid capacity B.C.acid capacity B.C.more salt

a.c. base capacitya.c. base capacity

more acid

MedChem 6 10

Why is the bicarbonate buffer a super-buffer for the blood?Why is the bicarbonate buffer a super-buffer for the blood?

H2O + CO2

H2CO3 HCO3- + H+

24 meqv

1.2 meqv

H+

x meqv24 - x

1.2 + x

pH = 6.1+ log 24 – 12

1.2 + 12= 6.06 ?e.g. x=12 meqv

MedChem 6 11

CO2 concentration is stabilized by ventilationACIDOSIS: Hyperventilation

ALKALOSIS: Hypoventilation

Effect of VentilationEffect of Ventilation

H2CO3 HCO3- + H+

H2O + CO2

24 meqv

1.2 meqv

24 - x

pH = 6.1+ log 24 - 12

1.2 + 0= 7.1

H+

x meqv

MedChem 6 12

Stabilization of [HCO3-] by the RBCStabilization of [HCO3

-] by the RBC

CO2 + H2O H2CO3

H+ + HCO3-

HbH+

Hb-

HCO3-

Cl-

Cl-

CO2

MedChem 6 13

0

5

10

15

20

25

30

35

40

6,8 7,0 7,2 7,4 7,6 7,8 pH

[bicarbonate]

The pH-bicarbonate diagramThe pH-bicarbonate diagram

ALKALOSIS

(hypoventilation)ACIDOSIS

(hyperventilation)

Constant CO2 concentration(normal ventilation)[CO2]=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

MedChem 6 14

Sources of acids and basesSources 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)

HCO3- depletion (sever diarrhea)HCO3

- 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)Abnormal loss of acids (prolonged vomiting)

MedChem 6 15

Final removal of acids: KidneyFinal removal of acids: Kidney

pH of the urine ~ 6range: 4.4 < pH > 8.0

pH of the urine ~ 6range: 4.4 < pH > 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 4.450 meqv [H+] excretion would need

Forms of H+ excretionHPO4

2- + H+ H2PO42- pKa= 7.2

NH3 + H+ NH4+ pKa= 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 ????????

MedChem 6 16

Total acidity of the urineTotal acidity of the urine

Titrable acidity[H+]

(Titration back to pH = 7.4)

NH4+ content

major urinary acid(could reach 500 meqv/day

in acidosis)

Final removal of basis in alkalosis: HCO3

- secretion (kidney)

Final removal of basis in alkalosis: HCO3

- secretion (kidney)

MedChem 6 17

Role of the kidneyRole of the kidney

BLOOD KIDNEY URINE

HCO3-

Excretionof H+ (NH4

+)

Re-absorption of bicarbonate ion

H+

H2CO3

NH3Amino acids NH4

+

MedChem 6 18

The super-buffer of the bloodThe super-buffer of the blood

Ventilation

Proteins(Hb)H2CO3

H+

Kidney

Final excretion of acids

CO2 + H2O HCO3- + H+

Temporal pH stabilization

by red blood cells

MedChem 6 19

+ Hb- HbH

Problemwith

ventilation

Respiratory acidosisRespiratory acidosis

H2O + CO2

H2CO3 HCO3- + H+

Increase in HCO3-

with slight decrease in pH

Increase in HCO3-

with slight decrease in pH

MedChem 6 20

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