1

Metabolic processes producing and consuming H+.

Buffer systems in the body.

Acid-base balance in the body and its control.

2011 (E.T.)

2

The production and regulation of hydrogen ions in the body

Metabolic processes

CO2 OH- H+

ICT

CO2 + H2O ⇔ H2CO3 ⇔ HCO3- + H+ ⇔ bufferYECT

Lung

CO2

~ 20 mol /d

Kidneys

1 mmol/d

HCO3- NH4

+, H2PO4-, SO4

2-

40-80 mmol/d

Liquids, foodAcids are continuously produced in the body and threaten the normal pH of the body fluids

3

Sources of acids in metabolism :1/ volatile carbonic acid: ● the source is CO2 - from decarboxylations ● CO2 with water gives weak volatile carbonic acid ● the exchange of CO2 ( 15 – 25 mol . d-1 ) between the blood and the external environment secure the lungs

2/ nonvolatile acids: ● sulfuric acid - from sulfur-containing amino acids (Cys + Met) ● phosphoric acid - from phosphorus-containing compounds ● carboxylic acids (e.g. lactate, acetoacetate, 3-hydroxybutyrate), unless they are completely oxidized to CO2

and water ● nonvolatile acids cannot be removed through the lungs, they are excreted by the kidney into the urine ( 40 – 80 mmol . d-1 )

4

The limit value of pH (blood)

pH = 7,40 [H+] ≅ 40 nmol . l-1

pH = 6,80[H+] ≅ 160 nmol . l-1

pH = 7,70[H+] ≅ 20 nmol . l-1

Although there is a large production of acidic metabolites in the body,concentrations of H+ ions in biological fluids are maintained in the verynarrow range:

The human body is more tolerant of acidaemia (acidosis) than of alkalaemia (alkalosis).

Steep decrease or increase of pH may be life-threatening

5

~ 5642 ± 3(BBp)

48 ± 3 (BBb)

Concentration of buffer systems (mmol/l)

org. phosphates

anorg. phosphates

3 % (org.)1 % (anorg.)

1 %(anorg.)5 %HPO4

2−/H2PO4−

proteins–27 %18 %45 %Protein−/HProtein

HCO3−HCO3

−17 %33 %50 %HCO3−/H2CO3 + CO2

erytrocytesplazmablood

ICFISFIVF

Buffering system

Buffer systems in organism

6

Hydrogen carbonate buffer - the most important buffer in blood

[ ]ef32

3

322

3)CO(H COH

][HCOlog6,1]COH[CO

][HCOlogppH32

−−

+=+

+= K

[CO2 + H2CO3] = 0,23 x pCO2

0,23 is the coefficient of solubility of CO2 for pCO2 in kPa

Effective concentration of carbonic acid

Physiological values:

pCO2 5,3 kPa ± 0,5 kPa. HCO3- 24 ± 2 mmol/l

Buffer systems in blood

CO2 + H2O H2CO3 H+ + HCO3–

In these equations, the concentrations [HCO3-] and [CO2+H2CO3] are expressed in

mmol/l, not in the basal SI unit mol/l !

7

[ ]ef32

3

COH][HCOlog6,1pH

−

+=

Respiratory component

Metabolic component

Hydrogen carbonate buffer

8

The ratio HCO3- / H2CO3 in blood at pH 7,4

120

2,124

22,0.2

3 ==−

pCOHCO

pCO2 5,3 kPa ± 0,5 kPa. HCO3- 24 ± 2 mmol/l

4,7120log1,6

22,0.log1,6

2

3 =+=+=−

pCOHCOpH

9

The other buffer systems in organismThe protein buffers – mainly albumin

H-proteinn− H+ + protein(n+1)− (based on dissociation of histidine

In blood ∼ 7%, important intracellulary

Hemoglobin buffer

HHb Hb− + H+

HHbO2 HbO2− + H+

pKA ~ 7,8

pKA ~ 6,2

O2 O2

Phosphate buffer

H2PO4− HPO4

2− + H+ pKA2 = 6,8

It contributes to intracellular buffering, important for buffering of urine. In plasma only ∼5%.

10

Blood in tissues

Transport of O2 and CO2 between the tissues and lungs – cooperation of hydrogencarbonate and hemoglobine

O2

HHbHbO2

−

H+

HCO3−

H2CO3

CO2

H2OCA

Cl− Cl-

Blood in lungs

Erc

O2

CO2CO2

HCO3−

O2

HHbHbO2

−

H+

HCO3−

H2CO3

CO2

H2OCA

Erc

O2

HCO3−

CO2 CO2

O2 O2

Chloride shift in venous blood

11

LungpCO2 in alveols is lower than in venous blood, CO2 diffuses into alveols.

HCO3- in red blood cells binds H+, that is released at reoxygenation of Hb and CO2 is

formed.

Concentration of HCO3- in ercs decreases, exchange of bicarbonate for chloride in red

blood cells flushes the bicarbonate from the blood and increases the rate of gas exchange

TissuespCO2 in arterial blood ∼5,3 kPa

CO2 difunding from cells increases pCO2 up to ∼6,3 kPa

The amount of CO2 dissolved in plasma increases, CO2 difuses into ercs

HCO3- is formed in red blood cells by the action of carboanhydrase, it partially binds to

Hb (carbaminohemoglobin)

H+ ions are buffered by Hb in ercs

Concentration of HCO3- in ercs is higher than in plasma, HCO3

- diffuses out of the red cells, Cl- diffuses into the red cell to maintain electroneutrality. (Hamburgers shift)

12

Forms of CO2 transport in blood

~ 5Physically dissolved

~ 10Carbamino proteins

~ 85HCO3−

Occurence in plasma (%)

Form of CO2

13

The respiratory system regulates acid base balance by controlling the rate of CO2 removal

Peripheral chemoreceptors in arterial walls and central chemoreceptors in brain

Increase of [H+] in arterias at metabolic disturbances, or ↑of pCO2

in CNS activates medullary respiratory center that stimulates increased ventilation promoting elimination of CO2

Conversely, the peripheral chemoreceptors reflexely suppres respiratory activity in response to a fall in arterial H+ concentration resulting from non-respiratory causes.

14

Tubular cell Tubular lumen

Gln

GluNH4

+

H2O

H2CO3

HCO3−

HCO3−

Cl-

CO2

H+

carboanhydrase

H+

H2CO3

H2OCO2

Na+

Na+

NH4+

HCO3− HPO4

2−

H2PO4−

HPO42−

A−

A−

H+

2-OG

H+

Na+

acidosis

+ acidosis

Activation at alkalemia

Competition with K+

~30 mmol/day

Na+

+

Kidneys function in maintaning acid-base balance

ATPH+

15

Kidneys control the pH of body fluids by adjusting three interrelated factors

• H+ excretion

• HCO3- excretion

• ammonia secretion from tubular cells

16

H+ secretion in proximal tubulus

H+-ATPase

Antiport with Na+

H+ secretion in distal tubulus and collecting duct

Type A intercalated cells:

Active secretion of H+ into urine – H+ -ATPase, H+-K+ -ATPase

HCO3- resorption

Type B intercalated cells:

HCO3- secretion

H+

H+

ATP

Na+

H+

lumenTub.cell

17

Kidneys and HCO3-

HCO3-

H+

Reabsorption of HCO3-

H2CO3

CO2

H2O

CO2

H2O

HCO3-

OH-

CO2

H2O

OH-

H+

ca

ca – carboanhydrase

ca

Reabsorption of HCO3-

occurs in proximal tubulus and A type intercalated cells

Secretion of H+ from three sources:

• CO2 from plasma

• CO2 from tubular fluid

•CO2 produced within the tubular cell

CO2

18

Kidney and NH3

gln

glu

2-oxoglu

NH3

NH3

H+

NH4

NH4

H+

Na+

Production of NH3 from glutamate and glutamin in tubular cells is increased at acidosis.

NH3 difuses into the tubular fluid and buffers H+ ions that are secreted from tubular cells

ATP

19

Urinary buffers

H+ transporters in tubular cells and collecting duct can secrete H+

against the concentration gradient until the tubular fluid becomes 800 times more acidic than plasma. At this point, further secretion stops, because the gradient becomes too great for the secretory process to continue. The corresponding pH value is 4,5.

H+ ions are buffered by:

HPO4- (filtered from blood)

NH3 secreted from tubular cells

If more buffer base is available in the urine, more H+ can be secreted before the limiting gradient is reached.

H+

ATP

Na+

H+

lumenTub.cellHPO4

-

H2PO4-

NH3 NH4

20

Summary of renal responses to acidosis and alkalosis

Acidification to normal

alkaline↑↓↓↓Alkalosis

Alkalinization to normal

acidic-↑↑↑Acidosis

Change of pH in plasma

pH of urine

HCO3- excretion

HCO3-

resorptionH+ excretion

H+ secretion

abnormality

Kidneys requires hours to days to compensate for changes in body fluid pH (compared to the immediate responses of the body buffers and the few minute delay before the respiratory systém responds)

However, kidneys are the most potent acid-base regulatory system

21

Contribution of liver to maintenance of acid base balance

NH3

AK

CO2 + H2O H2CO3 H+ + HCO3−

2NH4+

Gln

urea

2-OG

acidemia +

Liver KidneysGln

acidemie

urea

NH4+

+NH4

+

URINE

H+

Two ways of NH3 elimination in the liver

urea synthesis (connected with release of 2H+ - acidifying process)

glutamin synthesis (without release of H+)

Higher synthesis of glutamin is stimulated at acidosis, synthesis of urea is potentiated at alkalosis.

22

Main indicators of acid-base state

Measured parameters

pH 7,40 ± 0,04

pCO2 5,3 ± 0,5 kPa

(pO2, Hb, HbO2, COHb, MetHb)

Measuring by means of acid-base analyzers

23

Derived (calculated) parameters

Actual HCO3− concentration 24 ± 3 mmol/l

is the concentration of bicarbonate (hydrogen carbonate) in the plasma of the sample. It is calculated using the measured pH and pCO2 values.

Base excess (BE, base excess) 0 ± 3 mmol/l is the concentration of titratable base when the blood is titrated with a strong base or acid to a plasma pH of 7.40 at a pCO2 of 5.3 kPa and 37 °C at the actual oxygen saturation.

It is calculated for plasma, blood or extracelular fluid. Arterial oxygen saturation (sO2) 0,94–0,99

is defined as the ratio between the concentrations of O2Hb and HHb +

O2Hb Information about contration of the main electrolytes (Na+, K+, Cl−, Pi) and albumin are also important

24

Dependent and independent variables of acid base balance

• Dependent variables: pH, BE a HCO3-

These variable are not subject to independent alteration. Their concentrations are governed by concentrations of other ions and molecules.

• Independent variables: pCO2, SID, weak nonvolatile acids Atot

the concentration of each of the dependent variables is uniquely and independently determined by these three independent variables

(primary changes in concentrations of some cations (mainly Na+) and anions (Cl−, albumin, phosfphate and unmeasured ions) triggers consequently the changes of acid-base parameters).

25

[Na+] + [K+] + [Ca2+] + [Mg2+] = ([Cl-] + [HCO3-] + [albx-] + [Piy-] + [UA-])

The complementary calculations are derived from principle of plasma electroneutrality

UA- - unmeasured anions (see later)

26

Some complementary calculations

Anion gap

AG = [Na+] + [K+] − ([Cl−] + [HCO3−])

16 ± 2 mmol/l

Higher value of AG indicates the presence of extra unmeasured anions e.g. lactate, acetoacetate, 3-hydroxybutyrate.The value is often corrected on serum albumin concentration:*AGkorig=AG + 0.25 x ([Alb]norm- [Alb]zjišt

100

150

50

Cl-Na+

K+

Ca2+

Mg2+Piy-

UA-

Cl-

Albx-

HCO3-

AG

* Information about empirical formulas are given only for ilustration, students need not to know them

27

Albumin charge Albx-

It is calcultaed from albumin concentration (g/l) and pH

11,2 mmol/l at pH =7,4 a [alb]= 40 g/l

Phosphate charge Piy-

It is calculated from pH and concentration of phosphates

1,8 mmol/l at pH =7,4 a [Pi]=1 mmol/l

Corrected chloride ion concentrationcorrecting the chloride concentration for changes in Na+

[Cl]kor = [Cl]zjišt.x [Nanorm.] / [Nazjišt]

Some complementary calculations

28

Unmeasured anions

[UA] = ([Na+] + [K+] + [Ca2+] + [Mg2+]) – ([Cl−] + [HCO3−] +[Albx-] +[Piy-] )

6-10 mmol/l

UA expresses the concentration of other anions that are not included in the equation of electroneutrality (e.g.lactate, keton bodie, glycolate at poisonning with ethylene glycol, formiate at poisonning with methanol, salicylates etc.)Increased value of UA is compensated by decrease of concetration of other anions or mainly HCO3

-

100

150

50

Cl-Na+

K+

Ca2+

Mg2+Piy-

UA-

Cl-

Albx-

HCO3-

Some complementary calculations

29

SID (strong ion difference)

SIDeff = [Na+] + [K+] + [Ca2+] + [Mg2+] – ([Cl-] + [UA-])

38–40 mmol/l

100

150

50

Cl-Na+

K+

Ca2+

Mg2+Piy-

UA-

Cl-

Albx-

HCO3-

SID

Accurate measurement of SID is complicated by difficulties with determination of UA (unmeasured anions), empirical relation is therefore used

SIDeff = [HCO3−] + 0,28∙[albumin] + 1,8∙[Pi] [Pi] and [HCO3-] are in mmol/l and albumin in g/l

Some complementary calculations

30

[HCO3-]

pH = pK + log [CO2 + H2CO3 ]

H2CO3

„acidosis“ (pH < 7,36) metabolic disorder

„alkalosis“ (pH > 7,44) respiratory disorder

Classification of acid-base disorders

Disturbances are often combined.

31

Classification of acid-base disorders according to the primary cause

Respiratory disorderthe primary change in pCO2 due to low pulmonary ventilation

or a disproportion between ventilation and perfusion of the lung. Metabolic disorder

the primary change in buffer base concentration (not only HCO3

–, but also due to changes in protein, phosphate, and strong ions concentrations).

Quite pure (isolated) forms of respiratory or metabolic disorders don't exist in fact, because of rapid initiation of compensatory mechanisms; however, full stabilization of the disorder may settle in the course of hours or days.

32

4,7120log1,6

22,0.log1,6

2

3 =+=+=−

pCOHCOpH

Metab. alkalosis

Metab. acidosis

Resp. alkalosis

Resp.acidosis

Typ acute disturbance

↑

-

↓

-

-

↓

-

↑

Change of parameter

HCO3-

concentration

↑↑pCO2

HCO3-

concentration

↓↓pCO2

Change of HCO3-

concentration

↑↑pCO2

HCO3-

concentration

↓↓pCO2

Change of pH

Change of the ratio HCO3

-/pCO2

33

The classification of A-B disorders according to time manifestation

acute (uncompensated) stabilized (compensated)

- simple metabolic disorders or simple respiratory disorders practically do not exist, because the compensation processes begin nearly immediately, however the stabilization can also take some days (in dependence on the type of disorder)

34

Compensatory processes

Compensation

The secondary, physiological process occurring in response to a primary disturbance in one component of acid/base equilibrium whereby

the component not primarily affected changes in such a direction as to restore blood pH towards normal.

Metabolic disorders of acid-base balance are modified by respiratory compensation and oppositely

Correction

The secondary, physiological process occurring in response to a primary disturbance whereby the component that is primarily affected is restored to normal. .

35

Time course of regulatory responses

dayshours/daysDevelopment of compenzation

h/daysmin/hoursimmediately

Full efectivity

kidneyliverlungboneICTECT

OrganBuffer systems

The primary respiratory disorder leads to a compensatory change in HCO3–

reabsorption by the kidney, which reaches its maximal effectivity in 5 – 7 days.In the primary metabolic disorder, a change in blood pH evokes a rapid changein the pulmonary ventilation rate (during 2 – 12 hours).

36

Acid-base balance graph

→ overall evaluation (pH, pCO2 a BE) of acid base balance

BE (mmol/l)

Ac acidosis, Alk alkalosis; M metabolic, R respiratory; a accute, u stabilized

7,1 7,2 7,3 7,37 7,43

7,5

7,6

-10 0 10 20 30-20

pCO2 (kPa)

5,3

6,7

8,0

9,8

10,6

12,0

4,0

2,7

1,3

aMAlk

uMAlk

uRAc

aRAc

aMAc

aRAlkuRAlk

uMAc

pH

normalvalues

37

Example of acid-base balance disturbance

Metabolic acidosis (MAc)Causes of MAc• Increased production of H+ - lactacidosis

- ketoacidosis (starvation, non-compensated DM)- acidosis from retention of non volatile acids in renal failure

2. Exogenous gain of H+ - metabolites at intoxication with methanol, ethylen glycol, - overdosing with acetylsalicylic acid

- NH4Cl infusion at the treatment of MAlk

3. Loss of HCO3- - diarhea, burns, renal disturbances

4. Relative dilution of plasma – excessive infusion of isotonic solutions

38

Correction and compensation of MAc

1. Effect of buffers : H+ + HCO3- → H2CO3 → H2O + CO2

HCO3- ↓

120

22,0.2

3 <−

pCOHCO

2. respiratory compensation – increase of pulmonary ventilation

pCO2 ↓ 120

22,0.2

3 ≈−

pCOHCO

pH approches to 7,4, but concentrations of HCO3- and pCO2 are non physiological

3. Renal correction (development during 2–3 days) - acidic urine is excreted.

Excretion of H+ is accompanied by excretion of the given anion (A−) (lactate, acetacetate, 3-hydroxybutyrate). HCO3

− consumed during buffering reaction is regenerated in renal tubuli.

pH <7.4

↑H+

39

7,1 7,2 7,3 7,37 7,43

7,5

7,6

-10 0 10 20 30-20

pCO2 (kPa)

5,3

6,7

8,0

9,8

10,6

12,0

4,0

2,7

1,3

aMAlk

uMAlk

uRAc

aRAc

aMAc

aRAlkuRAlk

uMAc

pH

normalvalues

Grafical description of changes during compensation and correction of MAc

1

BE (mmol/l)

2

3

40

Changes of electrolyte parameters during acid-base disturbances (example)

Acidosis

The cause: loss of HCO3-

(e.g.diarrhea)

Loss of HCO3- is replaced

by Cl- → hyperchloremic acidosis

50SID ↓

Cl-Na+

K+

Ca2+

Mg2+Piy-

UA-

Cl-

Albx-

HCO3-

AG not changedUA not changed

41

Acidosis

cause – production of lactate, keton bodies, formiate, salicylate etc.

Due to buffering reaction concentration of [HCO3

-], event. Albx- a Piy- is decreased

Concentration of unmeasured anions (UA) increases

Concentration of chlorides is not changed – normochloremic acidosis

50SID ↓

Cl-Na+

K+

Ca2+

Mg2+Piy-

UA-

Cl-

Albx-

HCO3-

AG ↑

150

UA ↑

42

Dilution acidosis – consequence of plasma dilution

50SID ↓

Cl-Na+

K+

Ca2+

Mg2+Piy-

UA-

Cl-

Albx-

HCO3-

By the dilution the concentration of buffer bases falls

AG ↓UA ↓

150

43

SIDSID [UA[UA--]]

unidentified anions (excess)

[Alb-] [Pi

-] [Alb-] [Pi

-]

2. weak nonvolatile acids a/ serum albumin b/ inorganic phosphate

SIDSID [Cl[Cl--]]

SIDSID [Cl[Cl--]]

b/ imbalance of strong anions chlorides (excess/deficit)

SID [Na+]

SID [Na+]

II. metabolic (nonrespiratory) 1. abnormal SID a/ water (excess/deficit)

pCOpCO22 pCO2I. respiratoryalkalosisacidosisDisorder

The classification of acid-base disorders :

44

The procedure of evaluation of AB-balance parameters :

45

Evaluation of acid –base parameters (1)1/ pH, pCO2, BE – type of disturbance, measure of compensation

pH= 7,367, pCO2 = 5,25 kPa, BE = - 2,5 mmol.l-1

46

2/ recalculation of laboratory results• calculation of [Albx-] and [Pi

y-]• calculation of unmeasured anions [UA-]• correction of Cl- to actual content of water

Evaluation of acid –base parameters (2)

47

the deviations of patient values from the reference values arefiled to the columns „acidosis“ / „alkalosis“ (according to their signs: „+“ for increase, „−“ for decrease)

Evaluation of acid –base parameters (2)

−−− −+alkalosis

++++−

acidosispatient

12[Alb-]2[Pi

-]8[UA-]correc

100[Cl-]correc

140[Na+]mmol . l-1

48

3/ quantitative evaluation

Evaluation of acid –base parameters (1)

− 10,1− 0,3−−+alkalosis

+++ 1+ 11− 11acidosis

1,91,79

111129

patient

12[Alb-]2[Pi

-]8[UA-]correc

100[Cl-]correc

140[Na+]mmol . l-1

pH= 7,367, pCO2 = 5,25 kPa, BE = - 2,5 mmol.l-1

= combined metabolic disorder with normal ABE parameters

„hypoalbuminemic MAlk+ hyponatremic MAc“