acid-base balance i
DESCRIPTION
Acid-Base Balance I. Seminar No. 9 Chapter 21, I. part -. Homeostasis = maintenance of constant parameters of internal environment (= ECF). volumes of all body fluids (isovolemia) concentrations of cations/anions in body fluids (isoionia) osmolality of body fluids (isotonia) - PowerPoint PPT PresentationTRANSCRIPT
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Acid-Base Balance I.
Seminar No. 9
- Chapter 21, I. part -
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Homeostasis = maintenance of constant
parameters of internal environment (= ECF)
• volumes of all body fluids (isovolemia)
• concentrations of cations/anions in body fluids (isoionia)
• osmolality of body fluids (isotonia)
• body temperature (isothermia)
• pH of body fluids (isohydria)
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Q.
Which (general biological) factors influence the volume
and distribution of body fluids?
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A.
age
• newborn baby ~ 78 % TBW, adults ~ 60 % TBW
sex
• males 55-70 %, females 45-60 % (more fat in the body)
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Cations and anions in plasma (average concentrations)
CationMolarity (mmol/l)
AnionMolarity (mmol/l)
Cation Pos. charge* Anion Neg. charge*
Na+
K+
Ca2+
Mg2+
142
4
2.5
1.5
142
4
5
3
Cl-
HCO3-
Prot-
HPO42-
SO42-
Org. A
103
25
2
1
0.5
4
103
25
18
2
1
5
Total positive charge: 154 Total negative charge: 154
* Molarity of charge = miliequivalents per liter (mEq/l)
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Compare: Isotonic solution of NaCl
Physiological sol. 0.9 % = 9 g/l = 154 mmol/l
Na+ Cl-
154 mmol/l 154 mmol/l
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Commentary - Cations and anions in plasma
• every body fluid is electroneutral system
• in univalent ionic species
molarity of charge = molarity of ion (Na+, K+, Cl-, HCO3-, lactate-)
• in polyvalent ionic species
molarity of charge = charge × molarity of ion
Mg2+ [pos. charge] = 2 × [Mg2+] = 2 × 1 = 2
SO42- [neg. charge] = 2 × [SO4
2-] = 2 × 0.5 = 1
• proteins (mainly albumin) are at pH 7.40 polyanions
• org. acid anions (OA) – mainly lactate (AA, oxalate, citrate, ascorbate ...)
• charge molarity of proteins + OA is estimated by empirical formulas
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Q.
Compare the ion composition of the plasma and ICF.
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A.
Feature Plasma ICF
Main cation
Main anion
Protein content
Main buffer base
Na+
Cl-
HCO3-
K+
HPO42-
HPO42-
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Q.
What are the main dietary sources
of Na+, K+, Ca2+, Mg2+, Cl- ?
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A.
Ion Main dietary source
Na+
K+
Ca2+
Mg2+
Cl-
common (table) salt, salty products
potatoes, vegetables, dried fruits, soya flour
milk products, (cottage) cheese, mineral waters
green vegetable (chlorophyll)
common (table) salt, salty products
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Q.
Calculate the approximate osmolality of blood plasma if:
[Na+] = 146 mmol/l
[urea] = 4 mmol/l
[glucose] = 5.6 mmol/l
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A.
approximate osmolality is calculated
according to empirical relationship:
2 [Na+] + [urea] + [glucose] =
2×146 + 4 + 5.6 = 301.6 mmol/kg H2O
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Data derived (calculated) from ionogram
SID, AG
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SID (strong ion difference)
• strong ions do not hydrolyze in aqueous solution
• Na+, K+, Cl-
• SID = [Na+] + [K+] - [Cl-] = 142 + 4 – 103 = 43 mmol/l
• physiological range of SID = 39 – 45 mmol/l
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Na+
Cl-
SID
K+
SID = HCO3- + HPO4
2- + Prot-
SID = buffer bases of plasma
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AG (anion gap)
• the extent of unmeasured or unusual anions
• AG = [Na+] + [K+] - [Cl-] - [HCO3-]
• AG = 142 + 4 - 103 - 25 = 18 mmol/l
• physiological range of AG = 12 – 18 mmol/l
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AG
Na+
Cl-
HCO3-
AGK+ AG = HPO4
2- + Prot- + SO42- + OA
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Elevated AG may be caused by various conditions
• kidney insufficiency (↑ HPO42- + ↑ SO4
2-)
• diabetes, starvation (↑ acetoacetate + ↑ β-hydroxybutyrate)
• poisoning by methanol (↑ formate HCOO-)
• lactoacidosis (↑ lactate)
• severe dehydratation (↑ proteinates)
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Metabolic processes produce or consume various acids
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Metabolism of nutrients from acid-base point of view
Food – hydrolysis of nutrients in GIT
metabolic reactions in ICF
CO2
(a potential acid)
OH- H+
acid base reactions in ECF – buffers systems
proton consumption
reactions
proton productive reactions
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Proton consumption reactions
Gluconeogenesis from lactate:
2 lactate- + 2 H+ 1 glucose
anion + proton neutral molecule
• protons are consumed in the synthesis of non-electrolyte
from anion
• proton consumption is equivalent to OH- production
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Proton productive reactions
• anaerobic glycolysis: glucose 2 lactate- + 2 H+
• synthesis of urea:
CO2 + NH4+ + OOC CH
NH3
CH2 COO
CO(NH2)2 + -OOC-CH=CH-COO- + H2O + 2 H+
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Q.
Which compound is the main acid product
of metabolism in human body?
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A.
carbon dioxide CO2
compare daily production of acid equivalents:
CO2 - up to 25 000 mmol/day
H+ as NH4+ and H2PO4
- - up to 80 mmol/day
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Q.
What kind of food leads to an increased
production of OH-?
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A.• strictly vegetarian diet
• contains a lot of potassium citrate/malate
• potassium salts get into blood plasma
• organic anions enter cells and are
metabolized (CAC)
• K+ cations remain in plasma
• to keep electroneutrality of plasma
HCO3- concentration increases
• result: mild physiological alkalosis
Na+
Cl-
HCO3-
other
anionsK+
K+ Org.anion-
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Q.
How is CO2 formed in tissues?
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Endogenous production of CO2
• CO2 is produced in decarboxylation reactions
• oxidative decarboxylation of pyruvate acetyl-CoA
• two decarboxylations in CAC (isocitrate, 2-oxoglutarate)
• decarboxylation of aminoacids biogenous amines
• non-enzymatic decarboxylation of acetoacetate aceton
• catabolism of pyrimidine bases
(cytosine, uracil CO2 + NH3 + β-alanine)
• catabolism of glycine CO2 + NH3 + methylen-THF
main sources of CO2
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Acid products of metabolism - Overview
• aerobic metabolism of nutrients CO2
• anaerobic glycolysis lactic acid
• KB production (starvation) acetoacetic/β-hydroxybutyric acid
• catabolism of cystein (-SH) SO42- + 2 H+
• catabolism of purine bases uric acid
• catabolism of phospholipids HPO42- + H+
R1 OH P
O
O
OHO HHO R2 ++ +2 H2O
P
O
O
OO R2R1
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Buffer systems in blood
Buffer system Relevance Buffer base Buffer acid pKA
Hydrogencarbonate
Proteinsa
Hydrogenphosphate
50 %
45 %
5 %
HCO3-
Protein-His
HPO42-
H2CO3, CO2
Protein-His-H+
H2PO4-
6.1
6.0-8.0b
6.8
a In plasma mainly albumin, in erythrocytes hemoglobin
b The pKA value depends on the type of protein
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Buffer bases in (arterial) plasma
Buffer base mmol/l
HCO3-
Proteins
HPO42-
----------
Total
24
17
1
-------------
42
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Q.
Write a general form of Henderson-Hasselbach equation.
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A.
pH = pKA + log acid]buffer [
base]buffer [
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Q.
What does the buffering capacity depends on?
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A.
• buffering capacity depends on:
• concentration of both components
• the ratio of both components
• the best capacity if: [buffer base] = [buffer acid]
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Hydrogencarbonate (bicarbonate)
buffer
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Carbonic acid in vitro
• weak diprotic acid (pKA1 = 6.37; pKA2 = 10.33)
• does exist only in aq. solution, easily decomposes to CO2 and water
• CO2 predominates 800 in sol. therefore CO2 is included into KA
HO OHC
O
H2O + CO2 H2CO3 HCO3- + H+
800 : 1 : 0.03
]COH[CO
][HCO ][H
322
3effA
KKA eff = effective/overall
dissociation constant
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Carbonic acid in vivo
• formation catalyzed by carbonic anhydrase
• under physiological conditions: pKA1 = 6.10
• CO2 is continually eliminated from body by lungs
• the overall concentration of carbonic acid:
[CO2 + H2CO3] = pCO2 × s = 0.23 pCO2 (kPa)
H2O + CO2 H2CO3 HCO3- + H+
1 : traces : 20
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Compare: CO2 in water and blood
Liquid pH [CO2] : [HCO3-]
Carbonated watera
Bloodb
3.50 – 5.00
7.36 – 7.44
800 : 0.03
1 : 20
!
a Closed system (PET bottle), 25 °C, pKA1 = 6.37
pH ~ pCO2 ~ the pressure of CO2 applied in saturation process
b Open system, 37 °C, pKA1 = 6.10
CO2 continually eliminated, pCO2 in lung alveoli ~ 5.3 kPa,
acid component of bicarbonate buffer
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Q.
Give the Henderson-Hasselbalch equation for
the hydrogencarbonate buffer
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A.
pH = 6.1 + log2
3
pCO23.0
][HCO
kPa
mmol/l
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Q.
Express the changes in the bicarbonate buffer
after adding H+.
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A.
protons are eliminated in the reaction with buffer base
HCO3- + H+ H2CO3 H2O + CO2
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Q.
Express the changes in the bicarbonate buffer
after adding OH-.
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A.
hydroxide ions are eliminated in the reaction with buffer acid
H2CO3 + OH- H2O + HCO3-
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Q. Calculate changes in buffer system
after adding 2 mmol H+ into one liter
Initial status Closed system Open system
[HCO3-] 24 mmol/l
[CO2+H2CO3] 1.2 mmol/l
pH 7.40
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Initial status Closed system Open system
[HCO3-] 24 mmol/l 22
[CO2+H2CO3] 1.2 mmol/l 3.2
pH 7.40 6.94
2 H+ react with buffer base 24 – 2 = 22 HCO3- + 2 CO2
newly formed CO2 remain in the system 1.2 + 2 = 3.2 CO2
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Initial status Closed system Open system
[HCO3-] 24 mmol/l 22 22
[CO2+H2CO3] 1.2 mmol/l 3.2 1.2
pH 7.40 6.94 7.36
2 H+ react with buffer base 24 - 2 = 22 HCO3- + 2 CO2
newly formed CO2 is eliminated by lungs 3.2 - 2 = 1.2 CO2
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Q.
Calculate the ratio of [HCO3-]/[CO2+H2CO3] at physiological pH.
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A.
7.40 = 6.1 + log x
log x = 1.3
x = 101.3 = 20 = 20 : 1 = [HCO3-] : [CO2+H2CO3]
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Q.
Is the bicarbonate buffer more resistant to acids or bases?
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A.
• see previous problem
• [HCO3-] : [CO2+H2CO3] = 20 : 1
• the concentration of buffer base is 20 × higher than
the concentration of buffer acid
• conclusion: bicarbonate buffer is 20 × more resistant to acids
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Hydrogenphosphate buffer
• buffer base: HPO42-
• buffer acid: H2PO4-
• occurs mainly in ICF, bones, urine
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Q.
What is the ratio of plasma phosphates at physiological pH?
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A.
7.40 = 6.80 + log x
log x = 0.6
x = 100.6 = 4 [HPO42-] : [H2PO4
-] = 4 : 1
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Hemoglobin buffer
• hemoglobin (Hb) contains a lot of histidine
N
N
CH2 CH COOH
NH2
H
1
3
histidin (His)histidine (His) non-basic nitrogen
weakly basic nitrogen
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Buffering function of Hb is performed
by side chain of histidine
N
N
His
H
N
N
His
H
H
H
H
imidazol
pKB (His) = 8
imidazolium
pKA (His) = 14 - 8 = 6
pKA (His in proteins) = 6 - 8
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The next seminar May 15- Chapter 21, II. part -