Buffer in the blood

Buffers in Blood.
Acidosis and Alkalosis.

Chemistry Department of UB

1

Buffers in the Blood
 The pH of blood is 7.35 – 7.45
 Changes in pH below 6.8 and above 8.0 may result in
death
 The major buffer system in the body fluid is H2CO3/HCO3-
 Some CO2, the end product of cellular metabolism, is
carried to the lungs for elimination, and the rest dissolves
in body fluids, forming carbonic acid that dissociates to
produce bicarbonate (HCO3-) and hydronium (H3O+) ions.
 More of the HCO3- is supplied by the kidneys.
 CO2 + H2O ↔ H2CO3
 H2CO3 + H2O ↔ H3O+ + HCO3-

2

Carbonate buffer
H2CO3 + H2O ↔ H3O+ + HCO3-
 Excess acid (H3O+) in the body is
neutralized by HCO3-
 H2CO3 + H2O ← H3O+ + HCO3-
 Equilibrium shifts left
 Excess base (OH-) reacts with the carbonic
acid (H2CO3)
 H2CO3 + OH- → H2O + HCO3-
 Equilibrium shifts right
3

pH of the blood buffer
 The concentrations in the blood of H2CO3 and
HCO3- are 0.0024M and 0.024 respectively
 H2CO3/ HCO3- = 1/10 is needed to maintain the
normal blood pH (7.35 – 7.45) −
[ H 3O + ][ HCO3 ]
Ka =
[ H 2CO3 ]
[ H 2CO3 ]
[ H 3O + ] = K a −
=
[ HCO3 ]
0.0024
= 4.3 x10 −7 x = 4.3 x10 −7 x0.10 = 4.3 x10 −8
0.024
pH = − log(4.3 x10 −8 ) = 7.37

4

The effectiveness of the blood buffer
 If the pH of 100 mL of distilled water is 7.35 and
one drop of 0.05 M HCl is added, the pH will
change to 7.00.
 To change 100 mL of “normal” blood from pH of
7.35 to 7.00, approximately 25 mL of 0.05 M HCl
is needed.
 With 5.5 L of blood in the average body, more
than 1300 mL of HCl would be required to make
the same change in pH.
5

Regulation of blood pH
 The lungs and kidneys play important role in
regulating blood pH.
 The lungs regulate pH through retention or
elimination of CO2 by changing the rate and
volume of ventilation.
 The kidneys regulate pH by excreting acid,
primarily in the ammonium ion (NH4+), and by
reclaiming HCO3- from the glomerular filtrate (and
adding it back to the blood).
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Importance of the bicarbonate-
carbonic acid buffering system
1. H2CO3 dissociates into CO2 and H2O,
allowing H3O+ to be eliminated as CO2 by
the lungs
2. Changes in PCO2 modify the ventilation
rate
3. HCO3- concentration can be altered by
kidneys

7

Other important buffers
 The phosphate buffer system (HPO42-/H2PO4-) plays
a role in plasma and erythrocytes.
 H2PO4- + H2O ↔ H3O+ + HPO42-
 Any acid reacts with monohydrogen phosphate to
form dihydrogen phosphate
dihydrogen phosphate monohydrogen phosphate
 H2PO4- + H2O ← HPO42- + H3O+
 The base is neutralized by dihydrogen phosphate
dihydrogen phosphate monohydrogen phosphate
 H2PO4- + OH- → HPO42- + H3O+
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Proteins act as a third type of blood
buffer
 Proteins contain – COO- groups, which, like acetate ions
(CH3COO-), can act as proton acceptors.
 Proteins also contain – NH3+ groups, which, like
ammonium ions (NH4+), can donate protons.
 If acid comes into blood, hydronium ions can be
neutralized by the – COO- groups
 - COO- + H3O+ → - COOH + H2O
 If base is added, it can be neutralized by the – NH3+
groups
 - NH3+ + OH- → - NH2 + H2O

9

Normal Values for Blood Buffer in
Arterial Blood.
 The following values are determined by
blood gas analyzer:
 pH 7.35 – 7.45
 PCO 2 35 – 45 mm Hg
 H2CO3 2.4 mmoles/L of plasma
 HCO3- 24 mmoles/L of plasma
 PO 2 80 – 110 mm Hg

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Blood Gases
 In the body, cells use up O2 and give off CO2.
 O2 flows into the tissues because the partial
pressure of O2 is higher (100 mm Hg) in
oxygenated blood, and lower (<30 mm Hg) in
the tissues.
 CO2 flows out of the tissues because the
partial pressure of CO2 is higher (>50 mm
Hg) in the tissues and lower (40 mm Hg) in
the blood.
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Blood Gases
 In the lungs, O2
enters the blood,
while CO2 from
the blood is
released.
 In the tissues, O2
enters the cells,
which release
CO2 into the
blood. 12

Interpretation of Clinical Blood Gas Data
H 2O
CO2 + H2O ↔ H2CO3 ↔ H3O+ + HCO3-

 The concentration of carbonic acid in the body is
associated with the partial pressure of CO2.
 When CO2 level rises, producing more H2CO3, the
equilibrium produces more H3O+, which lowers the
pH – acidosis.
 Decreasing of CO2 level due to a hyperventilation,
which expels large amounts of CO2, leads to a
lowering in the partial pressure of CO2 below
normal and the shift of the equilibrium from
H2CO3 to CO2 and H2O. This shift decreases H3O+
and raises blood pH – alkalosis.
13

Respiratory Acidosis: CO2 ↑ pH ↓
 Symptoms: Failue to ventilate, suppression of
breathing, disorientation, weakness, coma
 Causes: Lung disease blocking gas diffusion (e.g.,
emphysema, pneumonia, bronchitis, and asthma);
depression of respiratory center by drugs,
cardiopulmonary arrest, stroke, poliomyelitis, or
nervous system disorders
 Treatment: Correction of disorder, infusion of
bicarbonate
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Respiratory Alkalosis: CO2 ↓ pH ↑
 Symptoms: Increased rate and depth of breathing,
numbness, light-headedness, tetany
 Causes: hyperventilation due to anxiety, hysteria,
fever, exercise; reaction to drugs such as
salicylate, quinine, and antihistamines; conditions
causing hypoxia (e.g., pneumonia, pulmonary
edema, and heart disease)
 Treatment: Elimination of anxiety producing state,
rebreathing into a paper bag
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Metabolic (Nonrespiratory)
Acidosis: H+ ↑ pH ↓
 Symptoms: Increased ventilation, fatigue,
confusion
 Causes: Renal disease, including hepatitis and
cirrhosis; increased acid production in diabetes
mellitus, hyperthyroidism, alcoholism, and
starvation; loss of alkali in diarrhea; acid retention
in renal failure
 Treatment: Sodium bicarbonate given orally,
dialysis for renal failure, insulin treatment for
diabetic ketosis
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Metabolic (Nonrespiratory)
Alkalosis: H+ ↓ pH ↑
 Symptoms: Depressed breathing, apathy,
confusion
 Causes: Vomiting, diseases of the adrenal
glands, ingestions of access alkali
 Treatment: Infusion of saline solution,
treatment of underlying diseases

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Buffer in the blood

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Buffer in the blood