DR. AKIF AHSAN Assistant Professor

Dept. of Biochemistry JNMC, AMU, Aligarh

ACID-BASE REGULATION

OBJECTIVES By the end of this session you should be able to:

Define buffers & explain how they act?. 1.

Explain how proteins act as buffer. 2.

Describe the effect of pH change on the rate of breathing

3.

Explain the role of hemoglobin in iso-hydric transport of CO2.

4.

Describe the 4 mechanisms by which kidneys regulates acid-base balance.

5.

ACIDs & BASEs

Acids: Donate protons (H+)

Bases: Accept protons (H+)

Strong acids & Weak acids

Strong Acids: Dissociates to a greater extent.

Weak Acids: Dissociates to a lesser extent.

Physiological pH = 7.38 - 7.42

Principal mechanisms for regulation of pH:

Blood Buffers: 1st line of defense

Respiratory regulation: 2nd line of defense

Renal regulation: 3rd line of defense

BUFFERS

Solutions which prevent or resist a change in pH on addition of an acid or base.

Weak ACID + Its salt

Weak BASE + Its salt

pH of a buffer can be calculated by Henderson-Hasselbalch equation.

Major BUFFERS in the Body

Bicarbonate buffer (H2CO3 / NaHCO3)

Proteins

Phosphate buffer (NaH2PO4 / Na2HPO4)

pH of Bicarbonate Buffer [H2CO3 / HCO3-]

𝐩𝐩 = 𝐩𝐩𝐩 + 𝐥𝐥𝐥[𝐒𝐩𝐥𝐒][𝐀𝐀𝐀𝐀]

𝐩𝐩 = 𝐩𝐩𝐩 𝐥𝐨 𝐩𝐇𝐇𝐇𝐇 + 𝐥𝐥𝐥[𝐩𝐇𝐇𝐇

_][𝐩𝐇𝐇𝐇𝐇]

𝐩𝐩 = 𝟔.𝟏 + 𝐥𝐥𝐥 𝐇𝟐𝟏.𝐇

𝐩𝐩 = 𝟔.𝟏 + 𝟏.𝐇 = 𝟕.𝟐

pH of Phosphate Buffer [H2PO4- / HPO4

2-]

𝐩𝐩 = 𝐩𝐩𝐩 + 𝐥𝐥𝐥[𝐒𝐩𝐥𝐒][𝐀𝐀𝐀𝐀]

𝐩𝐩 = 𝐩𝐩𝐩 𝐥𝐨 𝐩𝐇𝐇𝐇𝟐 − + 𝐥𝐥𝐥[𝐩𝐇𝐇𝟐

𝐇_]

[𝐩𝐇𝐇𝐇𝟐_]

𝐩𝐩 = 𝟔.𝟖 + 𝐥𝐥𝐥 𝟐

𝐩𝐩 = 𝟔.𝟖 + 𝟎.𝟔 = 𝟕.𝟐

How Does The Buffer Acts ?

H2CO3 NaHCO3

NaOH H2O + NaHCO3

Strong Base is converted into salt

H2CO3 +

How Does The Buffer Acts ?

H2CO3

NaHCO3 HCl NaCl + H2CO3

Strong Acid is converted into Weak Acid

+

NaHCO3

pH of the BUFFER depends on ?

𝐩𝐩 = 𝐩𝐩𝐩 + 𝐥𝐥𝐥[𝐒𝐩𝐥𝐒][𝐀𝐀𝐀𝐀]

Ratio of Salt and Acid

H2CO3

NaHCO3

= 20 Bicarbonate Buffer

Proteins (as Buffer)

Acts as buffer due to its AMPHOTERIC nature

Amino acid residues with pKa close to 7.4 are most effective in buffering

HISTIDINE (pKa = 6.1): Maximum buffering capacity

COOH - CH -

NH2

CH2

COOH - CH -

NH2

CH2 H+ Ionized Histidine (acts as Acid)

H+

H+ H+

H+

H+

H+

H+ H+

H+

If pH of plasma FALLS

H+

H+ H+

If pH of plasma RISES

Un-Ionized Histidine (acts as Base)

RESPIRATORY REGULATION

H2CO3

CO2 + H2O

Lungs regulate the elimination of H2CO3 in

the form of CO2

HYPER-VENTILATION

H2CO3

CO2 + H2O

↓ pH / ↑ CO2

Chemoreceptors (in the aortic arch & carotid sinus) detect these

changes

STIMULATES the Respiratory Center (in

Medulla) CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2

HYPO-VENTILATION

H2CO3

CO2 + H2O

↑ pH / ↓ CO2

Chemoreceptors detect these changes

INHIBITS the Respiratory Center

CO2 CO2 CO2 CO2 CO2

O2

CO2 CO2 CO2 CO2

O2

Hemoglobin acts as Buffer…

CO2

O2

Hb O2

CO2

H2O

H2CO3

CA

HCO3-

H+

Cl-

CO2

Hb H+

Cl-

HCO3-

O2

H2CO3

CO2 + H2O

CA

HCO3- H+

Renal Regu lat io n

Renal Regulation of pH

Prevents change in pH by excreting H+ ions and reabsorbing HCO3

- ions.

Glomerulus

Loop of Henle

P C T

D C T

Collecting Duct

Bowman’s Capsule

1 2

3

1 = Filtration

2 = Reabsorption

3 = Secretion

Proximal Convoluted Tubular Cell

Peritubular Space

Tubular Lumen

CO2 + H2O

H2CO3

CA

Na+

H+ HCO3-

Na+

HCO3-

Na+

H+

Hydrogen ions are excreted

Alkali is recovered

Na+/H+ antiporter Na+/HCO3-

symporter

Proximal Convoluted Tubular Cell

Peritubular Space

Tubular Lumen

H2CO3

CA

Na+

HCO3-

CO2 + H2O

H2CO3

HCO3-

H+

H+

CO2 + H2O

NaHCO3

Na+

HCO3-

Na+

Alkali is recovered

Na+/H+ antiporter Na+/HCO3-

symporter

Distal Convoluted Tubular Cell

Peritubular Space

Tubular Lumen

Na+

HCO3-

Na2HPO4

H2CO3

CA

CO2 + H2O

NaHPO4-

NaH2PO4-

H+

H+ excreted as titrable acid.

Na+

HCO3-

Na+

Alkali is recovered

H+

Na+/Cl - symporter Na+/HCO3-

symporter H+ pumps

Distal Convoluted Tubular Cell

Peritubular Space

Tubular Lumen

Alkali is recovered

HCO3-

NH3

NH4+ H+

H+ is trapped & excreted.

CO2 + H2O CA

(Glutaminase) Glutamine Glutamate

Na+

NH3

H+

HCO3-

Na+

Na+/HCO3-

symporter H+ pumps

Renal Regulation

Excretion of H+

Reabsorption of HCO3-

Excretion of Titratable acids

Excretion of Ammonium ions

MCQ

a) Bicarbonate buffer b) Phosphate buffer c) Protein buffer d) All of the above

The pH of the body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer system at physiological pH ?

[KEY: d]

MCQ

a) Lysine b) Histidine c) Aspartic acid d) Leucine

The greatest buffering capacity at physiological pH would be provided by a protein rich in which of the

following amino acids?

[KEY: b]

MCQ

a) Deoxy hemoglobin is a weak base b) Oxyhemoglobin is a relatively strong acid c) The buffering capacity of Hb is lesser than

plasma protein d) The buffering capacity of Hb is due to

histidine residues.

Choose the incorrect statement out of the followings

[KEY: c]

MCQ

a) ↓ pH b) ↑ pH c) ↓ CO2

d) ↑ O2

Respiratory center will be stimulated in which of the following conditions?

[KEY: a]

MCQ

a) Excretion of H+

b) Reabsorption of HCO3-

c) Reabsorption of titratable acid d) Excretion of Ammonium ions

Kidneys regulate acid-base balance by all the following mechanisms, except:

[KEY: c]

CAN YOU ??? Define buffers & explain how they act?. 1.

Explain how proteins act as buffer. 2.

Describe the effect of pH change on the rate of breathing

3.

Explain the role of hemoglobin in iso-hydric transport of CO2.

4.

Describe the 4 mechanisms by which kidneys regulates acid-base balance.

5.